STRATIGRAPHY AND STRUCTURE OF THE PALEN FORMATION,...

STRATIGRAPHY AND STRUCTURE OF THE PALEN FORMATION,PALEN MOUNTAINS, SOUTHEASTERN CALIFORNIA

Richard A. LeVequeUnion Oil Company2323 Knoll Drive

Ventura, California 93006

ABSTRACT

The Palen Formation is comprised of threemembers: lithofeldspathic arenite, polymicticconglomerate and quartzose arenite, depositedprior to the development of at least a portion ofa Jurassic magmatic arc that existed in southeastern California and western Arizona. The composition of the three members of the Palen Formation defines a distinct feldspar to quartz-richtrend upward in the section. Gradational contactsbetween each member imply that sedimentation wasnot interrupted and that drastic changes in sedimentary environments did not occur. Beddingcharacteristics suggest that the depositionalenvironments record a transition from subaqueousto subaerial deposition in a basin that shoaledthrough time.

Intense deformation of the Palen Formationproduced thrust and strike-slip faults, tight toisoclinal south-vergent folds and a penetrativecleavage. This deformation occurred during thesouthward movement of upper plate Paleozoic andpossible Mesozoic metasedimentary rocks along thePalen Pass thrust over a lower plate consisting ofPalen Formation and an intrusive rhyodaciteporphry. Fold axes in the lower plate stratatrend roughly E-W, plunge gently to the westnorthwest and are parallel to the Palen Pass thrustfault. The similarity of structural fabric datafrom the upper and lower plate rocks with deformedrocks to the east in the Big Maria Mountainssuggests that this ~eformation occurred in theearly Late Cretaceous (90-100 m.y.b.p.).

INTRODUCT ION

The Mesozoic sedimentary history of southeastern California is obscure largely because ofthe scarcity of rocks of this age and type.Regional correlations of sedimentary strata ofknown Mesozoic age are difficult to make due tothe disruption of sedimentary environments byigneous activity that occurred both during andafter sedimentation.

Pre-Jurassic, post-Paleozoic sedimentaryrocks are exposed at scattered localities in theeastern and southern Mojave desert. These strataprovide the only paleogeographic details of thetransition in regional geological environmentsfrom cratonal sedimentation in the Paleozoic toarc magmatism in the Jurassic. The Palen Formationis one such sequence of Early Jurassic metamorphosed sandstone and conglomerate that outcrop insoutheastern California. This paper describestheir occurrence, stratigraphy and structure andpresents speculative paleogeographic interpretations based on this data.

267

The Palen Formation is known to occur only inthe Palen Mountains of southeastern California(Fi g. 1). It has been intruded by a porphyriti crhyodacite dated by K-Ar methods at 175 m.y.b.p.and by quartz monzonites dated at 66 m.y.b.p.(K-Ar; Pelka, 1973). The Palen Mountains areabout 43 km northwest of Blythe, California and areseparated from the Granite Mountains to the northby Palen Pass. Access to the study area is byCalifornia State Route 177 (Rice Road) north fromU.S. Interstate 10 approximately 25.6 km (16 mi.)and then east via an unimproved road to the PalenPass area. The U.S. Geological Survey PalenMountains 15' quadrangle encompasses the study area.

STRATIGRAPHY

The Palen Formation was first studied andnamed by Pelka (1973). He chose to divide theformation into three members based on litholigiccharacteristics. In ascending stratigraphicorder. the three members are: 1) lithofeldspathicarenite, 2) polymictic conglomerate, and 3)feldspathic to quartzose arenite (fig. 2; classification based on Crook, 1960). The base of thesection is not exposed and the uppermost memberhas been intruded (see Fi g. 3). The absenceof a complete section combined with the degree ofdeformation precludes an accurate estimate of thethickness of the formation.

Member Descriptions

Lithofeldspathic Arenite

The lowermost member is gray-green to olivegreen in color and comprised of framework grains ofangular plagioclase feldspar embedded in a matrixof epidote, green biotite and chlorite. Thepercentage of matrix ranges from approximately 20%to almost 70%. In a number of samples lithicfragments of volcanic and quartzitic rocks arepresent. although small in size and relativelylow in abundance (20% maximum).

The bedding characteristics of the lowermostmember are not prominently displayed. Most ofthe exposures of this member are massive anddifficult to interpret. Where observed, beddingis represented by thin, wavy laminations thatrange from 1-3 mm to 1-1.5 cm in thickness. Thisappears to be the most common style of beddingalthough both low-angle crossbeds and rare gradedbeds occur. Texturally this member is poorlysorted and contains angular to subangular frameworkgrains.

These observations (abundant plagioclase,lithic fragments, poor sorting and massive bedding)suggest that this member was rapidly deposited

115

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34

33

Figure 1 - Location map of study area. A = Arica Mountains; BM = Big Maria Mountains;CH = Chuckwalla Mountains; C = Coxcomb Mountains; D = Dome Rock Mountains; E = EagleMountains; G = Granite Mountains; H = Hexie Mountains; LM - Little Maria Mountains;MC = McCoy Mountains; M= Mule Mountains; P = Palen Mountains; PP = Palen Pass; PI =Pinto Mountains; R = Riverside Mountains.

Quartzite is the dominant clast type (range40-100%), followed by leucogranite (0-50%),carbonate (0-30%) and minor resedimented cobblesof the lower member of the Palen Formation.Volcanic clasts are extremely rare within thismember. Most of the clasts are well roundedalthough angular clasts do occur throughout thesection. The most pronounced aspect of thismember relative to the lowermost member is the

with only minor reworking. The sedimentary characteristics of this member are similar to the peliticarenaceous facies II of Mutti and Ricci-Luchi'smodel of turbidite sedimentation (1978 Englishtranslation of the original 1972 Italian paper byT. Nilsen). In their model sediments of thisfacies are deposited on the outer fan apron wherechanneling is of less importance for the distribution of turbidity flows.

Polymictic Conglomerate

The middle member of the Palen Formation iscomprised of polymictic conglomerate that is

268

interbedded withspathic arenite.lithologic typesarenite.

thin, discontinuous beds of feldRelative percentages of the two

are 75% conglomerate. and 25%

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Figure 2 - Generalized QFR diagram for arenites; from Crook, 1960.

increase in the abundance of quartz, both asseparate detrital grains and as polycrystallinequartzite fragments.

It is apparent that these sediments underwentdeposition with minor consequent sorting or redistribution into graded sequences. Deposition in analluvial-fan system or within the inner-fan of asubsea canyon are plausible alternatives for theenvironment of deposition of this member. Aninner-fan environment is favored because of itslogical succession to the inferred outer fanenvironment described for the first member. Itmay be that although the conglomerates in thelower part of this member were deposited in asubaqueous environment, deposition of the conglomerates in the upper part of the member (and upinto the transition zone between this and theupper member) occurred in a subaerial alluvial fanenvironment. Identification of those 'partsdeposited subaqueously cannot be made in theabsence of marine fossils. Such fans are described by Rust (1980) as "coastal alluvial fans".

Quartzose Arenite

The uppermost member of the Palen Formationranges from subarkose to quartzose arenite incomposition. The greatest amount of feldspar isfound above and within the transition zone betweenthe middle and upper members. Original texturaldetails of the quartz grains have been obliteratedby deformation. An examination by luminoscope ofthree samples entirely composed of quartz revealedthat the quartz population is essentially homogeneous and that appreciable recrystallization hasprobably occurred.

The more feldspar-rich sands in the lowermost

269

part of this member are massively bedded. Actualbedding traces in the lower part of this member maybe obscurred by the strongly developed cleavage.The middle(?) to upper parts of this member aredramatically trough cross-bedded on a very largescale. Single sets of some cross-strata are atleast 10 Min height. These structures are bestexposed on the east side of the Palen Mountains,topographically below the ridge of rhyodaciteporphry that is intruded into this uppermostmember. The morphology of these sets (tabular,wedge-shaped) is difficult to determine becausemost of these beds are vertical and exposed alonga steep slope.

The transitional contact between the uppermostand middle members of the Palen Formation suggeststhat a significant hiatus in deposition did notoccur between them. A complex mixed environmentthat incorporated waning-stage coastal alluvialfan sedimentation with encroaching eolian sanddeposition may have existed during the depositionof the uppermost middle member and the lowermostupper member. This interpretation is based uponthe assumption that depositional environmentswere logically successive.

Age of the Palen Formation

The intrusive contact with the rhyodaciteporphry provides the only direct constraint on theage of the Palen Formation. Although an isotopicage of 175 m.y.b.p. for the rhyodacite was derivedby K-Ar analysis (Pelka, 1973), regional considerations suggest that this age may be lower thanits true value. Work by Anderson and Silver (1978),Haxel and others (1980) and Wright, Haxel and May(1981) document the presence of the prophyriticrhyodacite arc terrane through south-central

Figure 3 - Geologic map of the northern Palen Mountains, Riverside County, Southeastern California.

270

Figure 3 - Map explanation.

EXPLANATION

62 CLEAVAGE-A- 90

Strike and dip of inclined and verticalteavoge

STRUCTURE

The Palen Pass thrust fault is the dominantstructural feature in the area (Fig. 3). Itstrikes approximately east-west and dips from 20to 40 degrees to the north. Along the westernend of the thrust, in both plates, the rocks areextremely fractured and lack stratification. Thisbrecciation zone extends across the entire PalenPass area, separating gypsiferous schists in theupper plate from chloritically-altered metasandstones in the lower plate. Along the eastern endof the thrust fault highly-deformed limestones ofpresumed Kaibab Limestone affinity are clearlyresting atop inverted patches of the middle andupper members of the Palen Formation. The eastwest trending ridge that extends westerly from themass of porphyritic rhyodacite (just south of BM1472 in Palen Pass) is the topographic expressionof imbricate fault splays of the thrust. Withinthe imbricated slices, upper and lower-plate strataare strongly cleaved parallel to the thrust fault.

Fold axes of the major folds in the lower plateplunge to the northwest and have an average axialtrend of N57W; axial planes have an average strikeof N71W and dip of 42N (Fig. 4). The axes of allfolds are sinuous and commonly truncated by strikeslip faults that strike approximately normal to thefold axes. The offset continuations of these foldsare not present across these faults. This fact andthe areal pattern indicate that the strike-slipfaults separate the area into discrete structuralblocks in which shortening as expressed by thefolds has occurred independantly of the adjoiningblocks.

The Palen Formation has been intensely deformedinto south-vergent, tight to isoclinal folds, mostof which are overturned to the south. Attendantwith the folding was the development of penetrativeaxial-planar cleavage and reverse and strike-slipfaults. All of the structures possess a commonfabric that most likely formed during the emplacement of an upper-plate of Paleozoic and possibleMesozoic metasedimentary rocks exposed in PalenPass over a lower plate consisting of the rhyodaciteporphry and the Palen Formation along the PalenPass thrust fault (LeVeque, 1981).

similar to the Paleozoic rocks exposed in PalenPass. This would indicate that the Palen Formationis post-Paleozoic in age, an inference favoredhere, Therefore, the age of the Palen Formation isconsidered to be post-Paleozoic and pre-175 m.y.b.p.

\gradational

-------

FAULTS

BEDDING

CONTACTS

Dashed where uncertain

20-'-

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CfILJ Landslide unit - cansish of angular claete ofJP2 and Jp31n mud matrix; clasts ae largeae 5M In size

0iJ Granite - Includes twa phases; Il porphyriticquartz monzonite and 2) leucacratlc quartzmonzonite; K-Ar age of 66 my (Pelka,1973)

[TI£] intrusive porphyry- hypabyssal rhyodaciteporphyry consisting of light gray aphaniticgroundmasa with phenocrysts of feldsparand quartz; K-Ar age of 176 my(Pelka,1973)

Q£iJ Upper member - tan, vitreous feldspathlc toquartzose arenite; prominent high-angietrough crossbeds

~ QP.LJ.£ Middle member - light ton to green polymictic~ conglomerate; Includes clash of quartzite,l; carbonate and leucogranite; minor Interbede

~ of arkose

QeLJ I Lower member-medium to olive green IIthofeldspathlc arenite; consist of angular pIaglaclase feldspar In matrix of epldate,chklr-

FAULT it. and green biotite

CD Paleozoic rocks undlvlded- Includes probable Supai Formation, Coconino SandstolHl,Kalbab Limestone and unnamed gypliferoulschists (possibly Melozoic)

FOLDS-+- --+-- --A- ----Y-

Axial trace of plunging anticline, syncline, overturnedsyncline and anticline showing plunge of fold axis

~ ~~T"""'''''''Thrust faults; dashed where uncertain

---.l...Lo ----High - angle faults showing dip and relative movement(U on upthrawn side); dashed where uncer taln

Arizona and into northern Sonora, Mexico. Uraniumlead isotopic ages for these rocks range from 170to 194 m.y.b.p., suggesting that the developmentof the magmatic arc occurred primarily in theEarly Jurassic. Additionally, the bedding characteristics of the upper member and its relativechronological position (pre-175 m.y.b.p.) suggesta tentative correlation of these sands with theLower Jurassic (Peterson and Pipiringos, 1979)Aztec Sandstone. The middle and lowermost membersof the Palen Formation possess less distinctivelithologic Characteristics than the uppermostmember and therefore provide little regionalchronostratigraphic information. It may be thatthe clasts of quartzite and carbonate in themiddle member were derived from a cratonal sequence

Axial-planar cleavage is ubiquitous and penetrative on all scales. Detailed examination of thecleavage planes reveals that movement along themhas produced much of the fold form. This movementreaches a maximum where reverse faults occur. Thetrends of the reverse faults parallel axes of themajor folds. These faults dip uniformly to thenorth and are commonly terminated by the strikeslip faults that truncate the major folds.

The structural features of the upper platestrata (Demaree, 1981) are similar to those of thelower plate (south-vergent, tight to isoclinal,west-northwest plunging folds). Additionally, theorientation and style of folding displayed by thePaleozoic rocks in Palen Pass is identical to that

271

FA N58W,5NAP N62W,37N



FA N67W,.5NAP N66W,42N

Figure 4 - Equal-area, lower hemisphere plot showing orientation offold axes (FA) and axial planes (AP) for major folds in the PalenFormation. All data points plot in one quadrant because the foldsare isoclinal.

seen in the Little and Big Maria Mountains to theeast (Emerson, 1981; Ellis 1981). In the BigMaria Mountains these folds were syntectonicallyintruded by a porphyritic granodiorite that hasbeen dated at about 90-100 m.y. (Rb-Sr, Ellis andothers, 1981). A possible correlative diorite ispresent within the Palen Pass area that has afoliation that is subparallel to the thrust fault.The folds in the Big Maria Mountains that aresimilar to those in Palen Pass are cut by pegmatites that have been dated at 90 m.y. (Rb-Sr;D. Krummenacher, personal communication, 1981).If folding in the Palen Pass area was simultaneouswith thrust faulting it would be appropriate toplace an early Late Cretaceous age to this deformation.

DISCUSSION

The stratigraphic position and paleogeographicsignificance of the Palen Formation must be derivedfrom regional stratigraphic and temporal associations. Sedimentation in the region of southeasternCalifornia and western Arizona during the earlyMesozoic is represented by limestones, shales andsiltstones of the Lower Triassic Moenkopi Formation,which crop out in the Soda Mountains (Grose, 1959),Old Dad Mountains (Dunne, 1972 in Novitsky-Evans,1978, p. 31) in the Mojave Desert, and also possibly

272

in the Little Maria Mountains east of Palen Pass.East of the Soda Mountains the Moenkopi Formationis overlain by sandstones and conglomerates of theUpper Triassic Shinarump and Chinle Formations andto the west by volcanic flows, conglomerates andvolcaniclastics of the Soda Mountains Formation,or its suggested equivalent, the informally namedlower volcanic unit in the Mojave Desert (Grose,1959; Novitsky-Evans, 1978). Both the SodaMountains Formation and the lower volcanic unitoccupy a stratigraphic position above the Lowel'Triassic Moenkopi Formation and below the LowerJurassic Aztec Sandstone. This relation suggeststhat the volcanic sequences may be Middle to LateTraissic in age. The prevalence of volcanic stratawest of the Soda Mountains implies that arcvolcanism was active in this region during thelatter part of the Triassic.

Late Triassic to Early Jurassic volcanism maynot have developed synchronously along the axis ofthe arc. Therefore, sedimentation in the PalenMountains area may have occurred at the same timethat volcanism was active in the region of theMojave Desert. Thus, the lower member of the PalenFormation may represent the erosion of volcanicrocks similar to the Soda Mountains Formation andthe lower volcanic unit. The lack of volcanicmaterial in the middle and upper members of the

1978, Jurass i cGeol. Soc.10, no. 7, p.

and PalenPh. D.

Santa

Palen Formation suggests that arc activity wasvariable in space and time during the Early Jurassicin southeastern California and western Arizona.

Quartz sandstones interbedded with Jurassicvolcanic rocks are found in both the Mojave Desertand to the southeast in the Santa Rita Mountains ofsouth-central Arizona (Miller and Carr, 1978;Bilodeau and Keith, 1981). The close spatialassociation of the upper member quartzose areniteof the Palen Formation with the porphyriticrhyodacite provides a link between these localities.Additionally, this relationship of sandstones andinterbedded volcanic rocks is consistent with theinterpretation of Bilodeau and Keith (1981) thatlarge dune fields existed on the craton behind theJurassic arc and that a mixing of these differentrock types occurred along their common boundary.

In summary, the Palen Formation probablyrepresents sedimentation within a shallow basinthat shoaled through time. Litholigic contrastsand bedding characteristics in these unfossiliferous strata suggest that the depositional environments for these sediments were unique, althoughsuccessive to one another without major depositionalhiatus. Deposition of the entire Palen Formationoccurred synchronously with and prior to theearliest development of the Jurassic arc terranein southeastern California. The compositionalvariation of the Palen Formation on the local scaleand the temporal disparities in volcanism betweenthis area and Mojave Desert region suggest thatthe development of the Jurassic arc was intermittentin time and space. The association of quartz sandstones and silicic volcanic rocks in the PalenMountains provides a link between similar associations to the northwest in the Mojave Desert andto the southeast in southcentral Arizona.

Acknowledgements

I would like to thank W. R. Dickinson, P. J.Coney and G. B. Haxel for their advice andassistance with this study. Discussions andcorrespondence with S. M. Richard, L. E. Harding,D. Krummenacher, K. A. Howard and R. G. Demareehelped clarify my understanding of the geology ofthis area. G. Blake, J. Ellis and J. Grover readearly versions of this paper and made suggestionsthat improved the text. Thanks also to P. Brannenfor typing the manuscript.

REFERENCES CITED

Anderson, T. H. and Silver, L. T.,magmatism in Sonora, Mexico:America, Abst. w/Programs, v.359.

Bilodeau, W. L. and Keith S. B., 1981, Intercalcated volcanics and eolian "Aztec-NavajoLike" sandstones in southeast Arizona: anotherclue to the Jurassic-Triassic paleotectonicpuzzle of the southwestern U.S.: Geol. Soc.America, Cordilleran Sec. Field Trip Guide,no. 12, p. 93-111.

Ellis, M. J., 1981, Structural analysis and regionalsignificance of complex deformational eventsin the Big Maria ~10untains, Riverside County,California: M.S. Thesis, San Diego StateUn i v., 108 p.

273

Ellis, M. J., Frost E. G. and Krummenacher, D.,1981, Structural analysis of multiple deformational events in the Big Maria Mountains,Riverside County, California: Geol. Soc.America, Abst. w/Programs, v. 13, no. 2,p. 54.

Emerson, W. S. and Krummenacher, D., 1981, Geo1ogical and deformational characteristics of theLittle Maria Mountains, Riverside County,California: Geol. Soc. America, Abst. w/Programs, v. 13, no. 2, p. 64.

Grose, L. T.. 1959, Structure and petrology of thenortheast part of the Soda Mountains, SanBernardino County, California: Geol. Soc.America Bull., v. 70, p. 1509.

Haxel, G., May. D. J., Wright, J. E., and Tosdal,R. M., 1980, Reconnaissance geology of theMesozoic and lower Cenozoic rocks of thesouthern Papago Indian Reservation, Arizona:a preliminary report: Ariz. Geol. Soc.Di ges t, v. 12, p. 17-30.

LeVeque, R. A., 1981, Stratigraphy and structureof the Palen Formation, Palen Mountains,Southeastern California: M.S. Thesis,University of Arizona, Tucson, 58 p.

Miller, E. L. and Carr, M. D., 1978, Recognition ofpossible Aztec-equivalent sandstones andassociated Mesozoic metasedimentary depositswith the Mesozoic magmatic arc in the southwestern Mojave Desert. California, in Howell,D. G. and McDougall, K. A. (eds.), Mesozoicpaleogeography of the western United States,Pac. Coast Paleogeog. Symp. 2: Soc. Econ.Paleon. Mineral., Pac. Sec., p. 283-289.

Novitsky-Evans, J. M., 1978, Geology of the CowholeMountains: structural, petrologic and geochemical studies: Ph.D. Thesis, Rice University, Houston, Texas, 128 p.

Pelka, G. J., 1973, Geology of the ~kCoy

Mountains, southeastern California:Thesis, University of California atBarbara, 162 p.

Peterson, F. and Pipiringos, G. N., 1979, Stratigraphic relations of the Navajo sandstone toMiddle Jurassic formations, southern Utahand northern Arizona: U. S. Geol. SurveyProf. Paper 1035-B, 43 p.

Rust, B. R., 1980, Coarse alluvial deposits, inFacies Models, Walker R. (ed.): Geol. Assoc. Canada, p. 9-22.

Wright, J. E., Haxel G., and May, D. J., 1981,Early Jurassic uranium-lead isotopic ages forMeSozoic supracrustal sequences, PapagoIndian Reservation, southern Arizona: Geol.Soc. America, Abst. w/Programs, v. 13, no. 2,p. 115.

SYNOROGENIC EVOLUTION OF TIlE COPPER BASIN FOmlATION IN TIlEEASTERN HllIPPLE HOUNTAINS, SAN BERNARDINO COUNTY, CALIFORNIA

Derrick B. Teel and Eric G. FrostDepartment of Geological Sciences

San Diego State UniversitySan Diego, CA 92182

AB~;TRACT

The Copper Basin Formation in the easternHhipple Hountains of southeastern Californiaprovides a synorogenic record of the :liocenetectonic developwent of that region. Development ofthe h'hipple Hountain antifonls and normal faultsassociated with the regional detachment faultresulted in the establishment of relatively highsediment-source terranes and numerous linear depositional basins during Miocene time. The Copper BasinFormation is composed of bedded mudstone, arkosicsandstone, and paraconglomerate that likely reflectthe deposition of a progradational alluvial-fansequence during the Hiocene. Flow-regime relatedbedforms are uncommon, nevertheless, scarce paleocurrent indicators suggest a sediment transportdirection consistent with the trend of suspecteddepositional troughs (nortlmest-southeast).Although a thorough, detailed examination of theunit is incomplete at present, preliminary fieldinvestigations and petrographic examination suggesta mixed plutonic-volcanic source terrane for theCopper Basin sediments.

INTlWDUCTION

A significant aspect of Cenozoic tectonicactivity in southeastern California and westernArizona has been the development of a major,regional low-angle normal fault, or detachment fault,during mid-Tertiary time. This detachment fault andassociated upper-crustal folding both appear toreflect regional crustal extension that occurredduring passage of the mid-Tertiary arc through thisarea. Because detachment faulting and upper-crustalfolding appear to have occurred over at least aten-million year period (~23 to -13 m.y.B.P.), theirdevelopment is recorded by the stratigraphic recordthat accumulated during this time interval. Bylooking at this stratigraphic tape recording, thedevelopment of detachment-related deformation andsedimentation can be interrelated and collectivelydeciphered (Frost, 1979, 1981, 1982).

In southeastern California the Copper BasinFormation in the eastern Hhipple Hountains containsan excellent synorogenic record of the Hiocenetectonic development of this region. The stratigraphy, depositional environments, paleocurrentdirections, petrography, and history of the formation are directly related to the tectonics of thelfuipp Ie Houn tains area.

The locality under examination is situated duenorth of Parker, Arizona, on the California side ofthe Colorado River in the lfuipple Hountains. Accessto Copper Basin and surrounding regions is providedby roads controlled by the !letropolitan HaterDistrict of Southern California (Figures 1, 2 and 3).

275

TECTONIC SETTING

The Copper Basin Formation in the lfuippleNountains is broken into a series of irregular,elongate blocks by high-angle northwest-strikingnormal faults (Figures 1, 4c) associated with theregional detachment faulting (Davis and others, 1977,1979, 1980). It is this episode of normal faultingthat is partially responsible for the development ofrelative highlands (source terranes) and 10ldands(depositional basins) that allo",ed for the establishment of an alluvial-fan sedimentation regime duringNiocene time (Figures 4b, c). Normal faulting thatoccurred simultaneously with the developing lfuippleantiforms (Frost, 1981; Frost and others, 1981)resulted in the development of numerous sub-parallelsediment-catchment troughs. Continued periodicmovement of the faults and progressive uplift duringHiocene time stimulated fan progradation and burialof fine, distal deposits by coarser, proximaldetritus.

The Copper Basin Formation as exposed in CopperBasin, is a spectacular array of hematite-stained,fine mudstone, coarse arkosic sandstone, and paraconglomerate approaching nearly 600 meters in totalthickness and dipping uniformly to the south",est(Figure 5) at an average of 40 0 (Kemnitzer, 1937;Davis and others, 1979, 1980). Along strike ",ithinthe same and adjacent fault blocks, the clasticrocks of the Copper Basin Formation intertongueIvith andesitic volcanic flo",s and agglomerate (Davisand others, 1979, 1980). In the Hhipple Hash areanorth of Copper Basin, the formation consistspredominantly of these volcanic units as it does inthe Ivestern lfuipples. The Copper Basin Formation,therefore, can be dated fairly ",ell using theseinterlayered volcanic rocks (Davis and others, 1979,1980, this volume; Hartin and others, 1980).

The Copper Basin section unconformably overliesthe late Oligocene to early Hiocene Gene CanyonFormation in some areas (Figure 6). The Gene CanyonFormation represents playa and fanglomerate-debrisflow deposition prior to and during early developmental stages of the Ifhipple anti forms (Figure 4a, b)and normal faults (Davis and others, 1979, 1980;Frost, 1979, 1981). The Gene Canyon sediments andvolcanic rocks are exposed only in the majorsynforms ",ithin the Ifhipple Hountains region. Fromthe orientation of bedding within the Gene CanyonFormation, it appears that this restricted exposureto the synforms is due both to the preservation ofthe unit lvithin the structural troughs and controlof the sedimentation by the same troughs. Deformation of the Copper Basin Formation across the risingantiforms and synforms, thus preserved the GeneCanyon Formation in the synforms, ",ith the CopperBasin Formation directly overlying upper-platecrystalline rocks across the intervening antiforms(Figure 7). Subsequent folding of the Copper BasinFormation has also resulted in its preservation in

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MIO.-PLIO. OSBORNE WASH FM

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ROCKS

LOWER - PLATE MYLONITIC ROCKS

Figure 1. Geologic map of the central and eastern Whipple 11ountains, San Bernardino County, California.Note the position of the antiformal highs and adjacent synformal lows. Hodified from Davis and others (1980).

'"--J--J

Figure 2. Landsat image of the same area as Figure 1, showing the detachment fault separating dark, upperplate rocks from light, lower-plate rocks. The antiformal-synformal geometry of the range is clearly visible.

Figure 3. View to the ,.,est across the eastern \Jhipple Hountains shm.,ing the lithologic contacts betweenthe Copper Basin Formation (Tcb), Gene Canyon Formation (Tgc), and the upper-plate crystalline rocks (upxln)of the area. Note the linear configuration of the consecutive topographic ridges, formed by repetition of thesame sequence. Town of Parker Darn, California, is in foreground.

the synformal troughs within the \Jhipple terrane(Frost, 1981).

STRATIFICATION AND SEDHlENTARY STRUCTURES

The depositional sequence of the Copper BasinFormation exhibits a coarsening upward, cyclicdevelopment that may be considered diagnostic of aprograding alluvial-fan sequence (Steel and Hilson,1975). The depositional sequence initially begins,.,ith fine-grained siltstone or mudstone laminaegenerally a centimeter or so in thickness. Theselaminae often display desiccation cracks suggestingdeposition in a subaerial, arid to semi-aridenvironment. The mudstone laminae are in turn overlain by coarse, poorly sorted arkosic sandstones,approximately 20 to 30 centimeters in thickness. Anon-erosive contact between these two layers isindicated by the preservation of upturned desiccatedmud flakes in many places. The sandstones aregenerally conformably overlain by COarse paraconglomerates ranging from less than a meter to severalmeters in thickness.

Although variations in this sedimentation regimeare observable this cyclic repetition of siltstone,sandstone, and conglomerate is remarkably consistent,occurring every meter or so throughout the sectionin Copper Basin (Figure 8).

278

In the Gene Block, hm.,ever, the cyclic repetition of bedding is less well developed. Siltstoneand mudstone laminae tend to occur more randomly,conglomerate beds are fewer in number, thinner onthe average, and the mean clast sizes appearsmaller. Coarse arkosic sandstone is the mostabundant sediment type.

Despite occasional occurrences of sedimentarystructures, a general lack of flow-regime relatedbedforms is apparent. This feature likely indicatesviscous transport of material by sediment gravityflm.,s (debris flm.,s) belm., the flm., regime levels(Harms and Fahnestock, 1965). Likewise, clastswithin the conglomeratic units show no preferentialorientation or imbrication (Figure 9).

Sedimentary structures that have been recognizedinclude poorly defined current ripples and associatedripple cross-lamination, load features, tractivestructures, and scour troughs ,.,ith associated laggravels. Paleocurrent indicators, although scarce,indicate a generally south to southeast (S65°E-S45°W)transport direction, which is reasonably consistentwith the axes of the inferred depositional troughs.

PETROGRAPHY

Texture

Figure 4. Interpretive paleogeographic reconstruction of the l1hipple Hountains region fromlate Oligocene through Hiocene time.

UPxln

Ash Flow ruffs

.... _~ .. "\0.....,;, ••

Playa Lake Deposifsand

Fanglomerafe Debris

Flow

A. UPxln - Upper PlateCrystalline Rocks

WDF - Whipple Detachment

FaullMF - Mylonitic Front

Mxln - Mylonitic CrystallineRocks

A. Late Oligocene pre-deformational stage of the l1hipple tlountains area prior to (?) themajor development of the anti forms and normal faults.

Playa Lake Deposifsand

B. Early formational stage of the !Vhipple Hountains region during late Oligoceneearly Hiocene time. Tectonic highs and lows created by the concomitantdevelopmentof antiforms and normal faults established an alluvial-fan sedimentation regimeduring this period of time.

279

'"CDo

c.

C. Whipple Mountains region during Miocene time. Continued faulting, antiformal uplift anderosion have generated maximum relief, producing alluvial-fan progradation. The resultingsediments (Tcb) are deposited in fault-bounded, catchment troughs.

FlowTuffs

UPxlnWDF

Mxln

Figure 5. Photographs showing the evenly bedded and uniformly dippingnature of the Copper Basin rocks. A) View to the southeast across CopperBasin Reservoir. Copper Basin rocks on far side of the reservoir. B) Aerialview to the south across part of Copper Basin Reservoir. Copper Basin rocksdip to the west at approximately 40°.

Host of the Copper Basin Formation consists ofclastic sedimentary rocks. Rock types range fromcoarse conglomerate to mudstone with gradationalintermediate sizes. Sandstone is the most abundantrock type composing from 50 to 60 percent of theformation. Congomerate and fine-grained sedimentare less abundant and constitute approximatelysubequal parts of the remaining unit. Althoughfine-grained rocks are abundant, fissility is poorand shale is nearly absent.

281

Sorting is poor in the Copper Basin Rocks. Thecomglomerates of the unit (Figure 9) are primarilymatrix-supported conglomerates (paraconglomerates).

Clast sizes range from several centimeters toboulders greater than a meter in diameter.

The Copper Basin sandstones are poorly sortedalso. Authigenic cements constitute between 10 and20 percent of the sandstones with the interstitialmatrix material often partially replaced by carbonat~

Figure 6. Angular unconformity between Gene Canyon Formation (Tgc) andoverlying Copper Basin Formation (Tcb) in the southeastern \fuipple tlountainsin Gene Canyon, just northwest of Parker Dam.

Vigure 7. View to the northwest of Copper Basin showing the nonconformable contact between upper-plate crystalline rocks (upxln) and overlying CopperBasin volcanic rocks (Tcbv).

282

Figure 8. Photographs showing bedded nature of Copper Basin sediments.Note cavities left by preferentially weathered andesitic volcanic clasts.A) Bedded nature of the Copper Basin sediments. Note the layering of themudstone, sandstone, and conglomerate. B) Lensoidal character of conglomerate beds in the Copper Basin Formation.

283

Figure 9. Paraconglomerate of the Copper Basin Formation. Note the randomorientation of the clasts.

The fine-grained rocks include primarilypoorly sorted sandy mudstone, mudstone, and siltstone. Although these rocks lack fissility they aregenerally deposited in discreet laminae a centimeteror so in thickness and, in places, display softsediment load features such as flame structures.

Composition

The composition of frame,,'ork constituents isdescribed according to the sandstone classificationof Folk (1974). The sandstones of the Copper BasinFormation are coarse to fine-grained arkoses tomixed plutonic (metamorphic) - volcanic lithicarkoses. They are mineralogically and texturallyimmature sediments. The sandstones contain between10 and 20 percent matrix material and the frameworkgrains are angular. The fine-grained rocks of theformation are compositionally very similar to thesandstones. Both contain high percentages ofpotassium feldspar and plagioclase (40-50%), asubequal amount of quartz (40-50%), and a mixedvarietv of plutonic and metamorphic rock fragments(10-20%) .

The conglomerates of the Copper Basin Formationcontain at least three identifiable clast population~

upper-plate crystalline rocks, lower-plate myloniticclasts, and vesicular andesitic fragments (Davisand others, 1979, 1980). The crystalline clastsare generally well rounded, probably indicatingmulticyclicity (Figure 9). Contrastingly, vesicularandesitic fragments show preferential weatheringdue to relative chemical instability (Figure 8).

Authigenic carbonate and hematite cementsgenerally compose between 10 and 20 percent of theCopper Basin sediments. The authigenic carbonate isprobably calcite that has replaced some of thematrix material and is actively replacing many ofthe detrital feldspar and quartz grains. Vuggycalcite stringers are also observable. The

284

authigenic hematite cement is likely to be aresult of diagenetic alteration of matrix materialsin the sediments and imparts a classic redbed pigmentation on nearly all of the Copper Basin rocks.

Provenance

One of the fundamental factors controlling thenature of clastic sedimentary rock unit is thecomposition of the source material from which thesediment is derived. Optical characteristics ofquartz grains, relative abundance of feldspars androck fragments can be used to infer sediment sources.

In thin section, the Copper Basin Formationsediments contain both alkali and plagioclasefeldspars, with orthoclase, microcline and albitebeing especially abundant. Quartz occurs primarilyas single or composite grains exhibiting slight tostrongly undulose extinction under crossed nicols.Plutonic (metamorphic and igneous) and volcanicrock fragments are identifiable, as well. Thisinformation considered in conjunction with paleocurrent data, conglomerate clast species, and the

textural and mineralogical immaturity of thesediments suggests that the clastic rocks of theCopper Basin Formation can best be characterized astectonically generated sediments reflecting thecontinuous uplift and erosion of the lfuipple antiforms and the sporadic movement of associatednormal faults during Miocene time. Rugged topo

graphy was initiated and maintained by this tectonicepisode and was consequently incised into byvigorous streams giving rise to rapid erosion onsteep slopes. Large volumes of immature sedimentswere then transported and deposited as thicksections in numerous, fault-bounded, sedimentcatchment troughs (Figure 4c).

SUMMARY AND CONCLUSIONS

The Copper Basin Formation of the eastern

\fuipple llountains reflects significant tectonicevents in that area during liiocene time. Theinternal stratigraphy of the unit represents acoarsening upHard cyclic sedir,lentary sequence thatis probably indicative of a prograding alluvial fan.This alluvial fan appears to have developed inresponse to the rise of the antiformal folds in the\fuipple liountains region. Progressive developmentof these folds provided both the source area andcatchment basins for the unit. Coeval offsetalong normal faults associated Hith the regionaldetachment fault further complicated the basinmorphology and resulted in a groHth fault typecharacter of Tertiary sedimentation and tectonics.The result of this concomitant normal faulting andsedimentation ,vas the grmoith of a thick alluvialfan sequence that is tilted and extended. Thicksequences of tilted redbed and volcanic rocks appearto be a signature of mid-Tertiary detachmentfaulting and related crustal folding. The presenceof such stratigraphic and deforraational indicatorsmay provide a pOHerful clue for determining theoverall extent of detachment-related deformationand ultimately its cause. The genetic relationshipbetHeen sedimentation and extensional tectonicsmay also prove to be a pm,erful tool for decipheringthe basin history of terranes containing hydrocarbonaccumulations.

REFERENCES CITED

Davis, G.A., Evans, K.V., Frost, E.G., Lingrey, S.H.,and Shackelford, T.J., 1977, Engimatic lliocenelow-angle faulting, southeastern California andwest-central Arizona--suprastructural tectonics(?): Geol. Soc. America Abstracts withPrograms, v. 9, no. 7, p. 943-944.

Davis, G.A., Anderson, J.L., Frost, E.G., andShackelford, T.J., 1979, Regional Hiocenedetachment faulting and early Tertiary (?)mylonization, \fuipple-Buckskin-Rmvhide HOllntains, southeastern California and westernArizona, in, Geologic excursions in thesouthern California area, Abbott, P.L., ed.,San Diego State University, San Diego, California.

Davis, G.A., Anderson, J.L., Frost, E.G., andShackelford, T.J., 1980, Hylonization anddetachment faulting in the Ilhipple-BuckskinRaHhide Hountains terrane, southeasternCalifornia and western Arizona, in, Tectonicsignificance of metamorphic core~omplexes ofthe North American Cordillera, Crittenden,H.D., Jr., Coney, P.J., and Davis, G.H., eds.,Geol. Soc. America Hemoir 153.

Folk, R.L., 1974, Petrology of Sedimentary Rocks:

Austin, Texas: Hemphill Publishing Co., 182 p.Frost, E.G., 1979, Growth-fault character of Tertiary

detachment faulting, \fuipple Hountains, southeastern California, and Buckskin Hountains,,oiestern Arizona: Geol. Soc. America Abstractswith Programs, v. 11, no. 7, p. 429.

Frost, E.G., 1981, Hid-Tertiary detachment faultingin the \fuipple Htns., Calif., and Buckskin Htns~

Ariz., and its relationship to the developmentof major antiforms and synforms: Geol. Soc.America, Abstracts with Programs, v. 13, no. 3,p. 57.

Frost, E.G., 1982, Structural style of detachmentfaulting in the \fuipple Hountains, California,and Buckskin Hountains, Arizona: ArizonaGeological Society Digest 15.

285

Frost, E.G., Cameron, T.E., Krummenacher, D., andMartin, D.L., 1981, Possible regional interaction of mid-Tertiary detachment faulting withthe San Andreas fault and the Vincent-Orocopiathrust system, Arizona and California: Geol.Soc. of America Abstracts "ith Programs, v. 13,no. 7.

Harms, J.C., Fahnestock, 1965, Stratification, bedforms and flow phenomena ("ith an example fromthe Rio Grande) in SEPH Spec. Pub. 1112,Primary Sedimentary Structures and TheirHydrodynamic Interpretation, Midddleton, G.V.,(ed.) •

Kemnitzer, L.E., 1937, Structural studies in theHhipple Hountains, southeastern California:Ph.D. dissertation, California Institute ofTechnology, Pasadena, Calif., 150 p.

Hartin, D.L., Barry, H.L., Krummenacher, D., andFrost, E.G., 1980, K-Ar dating of mylonitization and detachment faulting in the \fuippleHountains, San Bernardino County, California,and the Buckskin Hountains, Arizona: Geol.Soc. America Abstracts "ith Programs, vol. 12,no. 3, p. 113.

Steel, R.J., and Hilson, A.C., 1975, Sedimentationand tectonism (?Permo-Triassic) on the marginof the North llinch Basin (Devonian) Norway:Sedimentary response to tectonic events:Geol. Soc. Am. Bull., v. 38, p. 69-83.

THE PETROLOGIC AND TECTONIC EVOLUTION OF VOLCANIC ROCKS IN THE SOUTHERN CALIFORNIA BORDERLAND:A TRANSITIONAL TECTONIC ENVIRONMENT

Richard W. HurstDept. of Geology

Calif. State Univ.Los Angeles, CA 90032

ABSTRACT

Wi lliam R. WoodDept. of Geology


Mary HumeDept. of Geology


Volcanic rocks of the southern California Borderland are believed to be genetically related to the interaction of the East Pacific Rise with the NorthAmerican Plate in mid-Miocene time. Three specificvolcanic suites have been studied from the Santa Monica Mountains (Conejo volcanic suite), Santa Cruz Island (Santa Cruz Island volcanic suite), and CatalinaIsland (Catalina Island volcanic suite) in order toassess the nature and extent of these interactions onthe chemical characteristics of these rocks. Thecrystallization histories of the Conejo and SantaCruz Island volcanic suites are similar to each otherand to the observed fractionation products of tholeiitic suites. Both suites have low values of K

20,

high values of Ti02

and low initial Sr isotopic compositions suggesting mid-ocean ridge affinities forthese two suites. A moderate amount of iron-enrichment is also characteristic of these suites. TheCatalina Island volcanic suite differs from theConejo-Santa Cruz Island volcanic suites in that itscrystallization history resembles that of calc-alkalicisland arc rocks with no observed iron-enrichment.The Catalina volcanic suite resembles the other suitesin its low values of K

20 and higher values of TiO?

relative to those expected in island arc rocks. Amodel is proposed in which a dilational environmentin the proximity of ridge-trench-transform triplejunction results in partial melting of the mantleas the trenchward flank of the mid-ocean ridge is subducted. The seaward flank moves in episodic spurtstoward the trench resulting in a temporary capping ofthe magma chamber below which results in fractionalcrystallization and more siliceous differentiationproducts than normally observed at a ridge. TheConejo-Santa Cruz Island volcanic suites are suggestedto have erupted through oceanic crust while CatalinaIsland may represent a piece of the North AmericanPlate affected by this triple junction volcanogeniczone.

INTRODUCTION

The southern California Borderland retains apetrologic-geochemical record of the interaction ofa ridge-trench-transform triple junction, the Rivieratriple junction, with the western continental marginof the North American Plate. The implications ofthis interaction were discussed by Atwater (1970) withregard to the tectonic evolution of western NorthAmerica. Later studies focussed on specific facetsof this evolution such as microplate rotation (Kamerling and Luyendyk, 1979), basin formation (Crowell,1976) and the geometry of lithospheric plate subduction (Dickinson and Snyder, 1980). However, fewinvestigations were directed toward an understandingof the associated volcanism and its relation to thetransition from a subduction complex to a mid-oceanridge-subduction interaction. Available interpretations of the volcanic suites suggested the volcanicrocks of the southern California Borderland weresolely related to subduction-related magmatism in anisland arc environment (Higgins, 1976; Crowe andothers, 1976). This interpretation was questioned by

287

Hurst (1978, 1979, in press) based on the similaritiesbetween the Conejo volcanic suite (Santa Monica Mountains and Santa Cruz Island) and mid-ocean ridge basalts. The conejo-Sa§7a Cggz Island volcanic suitesexhibit low initial Sri Sr ratios (0.702530.70372; 20 samples average = 0.70306 + 0.00011), lowK

20, high Ti0

2and FeO/MgO ratios high"E;"r than those

expected in island arc suites. These data, along withthe observed geologic and petrologic data. were interpreted as indicating the direct influence of theEast Pacific Rise on the chemical evolution of thesevolcanic suites. This work presents a summary of thepetrochemical data from the Conejo-Santa Cruz Islandand Catalina Island volcanic sUites as well as thesignificance of these suites to the tectonic evolutionof the Borderland.

THE CONEJO AND SANTA CRUZ ISLAND VOLCANIC SUITES

General Geology

The Conejo and Santa Cruz Island volcanic suitesvary in composition from basalt to rhyolite. Thepredominant compositions range from basalt to dacite,with the occurrence of rhyolite being minor «~ 5%).Both suites contain massive basalt and andesite flows,hyaloclastite, tuff, dacite domes and volcaniclastics(pyroclastics and epiclastics). Pillow lavas occurin the lower to middle members of the Conejo volcanicsuite but are absent in the upper member of this suiteindicating an early subaqueous eruptive stage followedby later subaerial eruptions (Williams, 1977). No

pillow lavas have been observed in the Santa Cruz Island volcanic suite. Some members of this suitecontain intercalated marine sediments indicating amarine influence during portions of the eruptive history of this suite (Crowe and others, 1976; Hurst,unpublished) .

The tectonic evolution of both volcanic suitesis also similar. High angle normal faults are thepredominant type of fault in both areas. Majorfolding events have not affected these suites whichsuggests, along with the predominance of normalfaulting, that an extensional tectonic regime persisted throughout the eruption of these volcanics.Both suites are northward dipping homoclinal sequences.

Petrology

The summaries of the petrology which follow aretaken from Hurst (in press; Conejo volcanic suite),Hurst and Hume (1982, in preparation; Santa Cruz Island volcanic suite) and Crowe and others (1976;Santa Cruz Island volcanic suite).

Conejo Volcanic Suite

The predominant rock types are cumulophyricbasalt to tholeiitic andesite with most texturesbeing intersertal to hyalopilitic. Plagioclase,augite and hypersthene are the most common phenocrystsobserved with plagioclase being the most abundant and

hypersthene the least abundant. Olivine is rare andi8 restricted to the lower portions of the volcanicsection. It is often observed as a corroded remnantin altered hypersthene grains or less frequently asindividual grains. Pigeonite has been observed rimming orthopyroxene but is very rare. Iron-titaniumoxides are never more than 3% of the modal composition of the basaltic rocks. The iron titanium oxidesare restricted to the lower portions of the sections,crystallizing after the major phases.

The crystallization order fluctuates throughout the section as follows (parentheses indicatesimultaneous crystallization):

Lower Section -(olivine + plagioclase + clinopyroxene) orthopyroxene

Middle Section -(clinopyroxene + plagioclase) orthopyroxene - pigeonite

Upper Section -(augite + plagioclase) - orthopyroxene

folloHed byplagioclase - orthopyroxene

The observed crystallization sequence is commonto the fractionation products of tholeiitic basalts.

However the moderate iron-enrichment in this suite(Hurst, in press) does not favor the crystallizationof significant amounts of pigeonite relative to orthopyroxene as observed in classic tholeiitic fractionation trends (e.g. Skaergaard). Another factor whichmust be considered is the probable mixing of moreprimitive, i.e. magnesian, magma with the Conejovolcanic suite differentiation products suggestedby reverse and oscillatory zoning observed inclinopyroxene and orthopyroxene in this suite.Reverse zoning in plagioclase also suggests theintroduction of more calcic magma over the courseof the eruption of this suite. As an example, oneplagioclase phenocryst in the lower lavas has a corecomposition of An 6 which abruptly changes,to An

75in the intermedia~e zones and malntalns thls composition through the rim. The absence of hydrousphases precludes a significant change in liquidustemperatures as a result of variations in PH20 .Minor Amphibole « 1%) does occur in late stagedacite domes where miarolitic cavities are observed.

Santa Cruz Island Volcanic Suite

The volcanic rocks of Santa Cruz Island arehypocrystalline, glomeroporphyritic basalt todacite. Rhyolite occurs but is a minor component of the suite. Plagioclase and augite arethe predominant phases Hith hypersthene and pigeonitegenerally accounting for less than 3% of the modalcomposition of the volcanic rocks studied. Howeverhypersthene and pigeonite do occur throughout the entire Santa Cruz Island volcanic section. Olivine hasnot been observed but its' presence is inferred byiddingsite pseudomorphs in the lower portion of thevolcanic section. Iron titanium oxides are presentthroughout the section but are observed to crystallizeafter the major phases. The modal amount of irontitanium oxides is generally less than 3% withoccasional increases to approsimately 6%.

The crystallization sequence of this volcanicsuite is as folloHs:

LOHer Section -

288

olivine - clinopyroxene - plagioclase orthopyroxene

Middle Section -(clinopyroxene + plagioclase) - orthopyroxene- pigeonite

orplagioclase - clinopyroxene

orplagioclase - (clinopyroxene + orthopyroxene)

Upper Section -plagioclase - (clinopyroxene + orthopyroxene)- pigeonite

or(clinopyroxene + orthopyroxene + plagioclase)- pigeonite

The observed crystallization sequence is complexyet is similar to that observed in the Conejo volcanic

suite and in tholeiitic fractionation products. Thethree pyroxene assemblage reported by Crowe andothers (1976) is a distinguishing characteristic ofthe Santa Cruz Island volcanic suite and the CatalinaIsland volcanic suite to be discussed later in thiswork. Although pigeonite occurs in small quantities(1-2%) it persists throughout the Santa Cruz Islandvolcanic sequence, a trait which is not characteristic of the Conejo volcanic suite Hith which it hasbeen correlated (Nolfand Nolf, 1969: Hurst, in press).Plagioclase compositions in the Santa Cruz suiterange from An45 to An65 and are therefore similarto the Conejo volcanic suite. It should be notedthat the present comparison is based on microprobedata (Conejo volcanic suite) and optical data(Santa Cruz Island volcanic suite). PlagioclaseHas not observed to be extremely zoned in this studywhereas Crowe and others (1976) reported zonedplagioclase ranging in composition from An65 to An35'More extensive sampling may resolve this present dispari ty. Amphibole is observed in some dacite rlOl'sin this suite, a feature similar to that observed inthe Conejo volcanic rocks.

CATALINA ISLAND VOLCANIC SUITE

General Geology

The data reported in the following sections issummarized from Wood (1982) and Hurst (unpublished).

The volcanic rocks of Santa Catalina Islandrange in composition from basalt to rhyolite withthe compositional range basalt to dacite predominant.Basalt to dacite lavas flOl-IS, volcaniclastic breccias,tuff, epiclastic breccias, hyaloclastite and dacitedomes occur in the volcanic section. Evidence fromintercalated sediments indicate a marine influenceduring the eruption of the Catalina Island volcanicsuite as well as a subaerial component as suggestedby the eruption of air-fall tuff.

High angle normal faults occur in the Catalinavolcanic section as in the aforementioned volcanicsuites. The attitudes of lava flows vary considerablyin the area and may be controlled by block-faulting.Extensional tectonism certainly seems to dominate thestructural evolution of the volcanic sections in theBorderland.

Petrology

The Catalina Island volcanic suite is composedprimarily of intersertal to hyalopilitic basalticandesite to rhyolite with hypersthene andesite anddacite predominating. Olivine occurs as phenocrysts

in basalt, andesite and occasionally in dacite.Plagioclase is the dominant phenocryst followed byhypersthene, augite, pigeonite and magnetite.Magnetite is more abundant in the Catalina suite(5-10% of the groundmass mode) than it is in theConejo-Santa Cruz Island suite. Magnetite also

remained a liquidus phase throughout a major portionof the crystallization history of the Catalina suite.A three- pyroxene assemblage is observed in the basaltic andesite with pigeonite forming rims on hypersthene. Magnetite is not observed in the basalticandesite. Hornblende occurs as a phenocryst in thedacitic portions of the suite.

Stratigraphic controls on the volcanic suite onCatalina Island are not clear due to the disruptionof the section by numerous high angle faults. Henceit is difficult to place the crystallization sequencein a relative time frame. The crystallization orderis as follows for the various litho~gic types:

Olivine basalt -(olivine + magnetite) - plagioclase _clinopyroxene

Basaltic andesite -(olivine + plagioclase + orthopyroxene) _clinopyroxene - pigeonite - magnetite

Andesite -(olivine + magnetite) - plagioclase _orthopyroxene - clinopyroxene

Dacite -(quartz + hornblende + plagioclase) _biotite - magnetite

The observed crystallization order is similarto that of calc-alkalic island arc suites althoughthe early crystallization of magnetite and delayedcrystallization of plagioclase are not observed inisland arc suites. Plagioclase compositions rangefrom An43 to An 63 . Normal zoning is most commonalthough some reverse zoning is observed. Abruptcompositional variations are not observed in thissuite which distinguishes the Catalina suite from theConejo-Santa Cruz Island volcanic suite to the north.The lack of iron-enrichment in the Catalina suitecontrasts to the moderate iron-enrichment observed inthe Conejo-Santa Cruz Island volcanic suite. However the appearance of pigeonite as a liquidus phasemust have been associated with some iron-enrichmentduring the crystallization of the basaltic andesitewhere pigeonite is observed.

COMPARATIVE GEOCHEMISTRY OF THE BORDERLANDVOLCANIC SUITES

Table 1 is a compilation of geochemical datafrom the Borderland volcanic suites. The datapresented are those which have been found to be useful in associating a volcanic suite with a particulartectonic regime.

All of the southern California Borderland volcanic suites are similar in thcir low K ° and high

Ti0 2 ,contents over a wide range of sili~a coneentratlons. Santa Cruz Island tends to have very high

Ti0 2 concentrations relative to the other suites whilethe ConeJo volcanic suite contains the lowest values

of K2

0 of all three suites. The Catalina Islandvolcanic suite tends to have higher values of K

20

as the rocks become more siliceous.

FeO/MgO ratios tend to be lower in the Conejoand Catalina Island suites in the basalt to andesiterange when compared to the Santa Cruz Island suite.

289

This particular ratio tends to increase in all suitesduring the eruption of dacite. In general, however,the increase in the FeO/MgO ratio is more pronouncedin the Conejo-Santa Cruz Island volcanic suite.

These results are summarized in Figures 1 and2 which are AFM plots for the Conejo-Santa CruzIsland and Catalina Island volcanic suites respectively. A moderate amount of iron-enrichment isobserved in the Conejo-Santa Cruz Island volcanicsuite whereas no iron-enrichment is observed in theCatalina Island volcanic suite. Two glasses analyzedin the Conejo volcanic suite show a pronounced ironenrichment trend between tholeiitic liquids represented by tholeiitic glass (TG in Fig, 1) and latestage differentiation products represented byrhyodacite glass (RDG in Fig. 1) in this suite. Thedashed line in Fig. 1 represents the approximatedifferentiation trend for the Conejo-Santa CruzIsland volcanic suite. The solid line is the lineof demarcation between tholeiitic and calc-alkalicsuites (Irvine and Baragar, 1971).

The range in Mg# is relatively similar in allthree suites throughout the compositional range ofvolcanic rock types observed. In general, the SantaCruz Island volcanic suite has slightly lower valuesthan the other suites. The range in the Mg # ofeach suite is seen to increase in the andesitic rocksand decrease as the siliceous volcanics erupt. Thismay be related to a similarity amoung the suites withregard to their mode of generation involving the mixing of more primitive magmas with differentiationproduct~ a process which will be discussed in the nextsection.

DISCUSSION

The observed petrogenetic characteristics ofthe Borderland volcanic suites are suggestive ofmixed tholeiitic and calc-alkalic affinities.Certain characteristics of all suites, such as theirlow K20, high Ti0 2 and low initial Sr isotopic compositions (Conejo-Santa Cruz Island suites) suggesta mid-ocean ridge source while the lack of a pronounced iron-enrichment and presence of amphibole arguefor an island arc source.

The differentiation trend of the Conejo-SantaCruz Island suite differs from that of the CatalinaIsland suite in that amphibole and biotite fractionation influenced the course of crystallization inthe Catalina rocks. Pronounced iron-enrichment isnot observed in the Catalina suite (Fig. 2 and Fig.4) while moderate iron-enrichment is observed in the

Conejo-Santa Cruz Island suite (Fig. 1 and Fig. 3).Basalt through andesites of the Conejo-Santa CruzIsland suite clusters about the mid-ocean ridgebasalt trend on a plot of FeO* versus FeO*/MgO(Miyashiro, 1974) while the more siliceous rocksdiverge from this trend (Fig. 3). The entireCatalina volcanic suite is displaced to highervalues of FeO*/MgO relative to the mid-ocean ridgetrend yet these rocks continue to trend sub-parallelto the mid-ocean ridge trend (Fig. 4). An overallsimilarity observed between the suites is the existence of two compositional groups, a high iron-low*FeO/MgO group and a low iron-high *FeO/MgO group.These groups are defined by an abrupt increase inFeO*/MgO from 1 to 2.5-4 when FeO* equals 6%.The value of FeO*/MgO then decreases gradually withFeO*. This trait could be related to the disappearance of pigeonite after the crystallization of thebasaltic andesites.

The Borderland volcanic suites are also similarin that their Feo*/MgO - TiO Z systematics are associated with mid-ocean ridge basalts in the basaltthrough andesite compositional range (Fig. 5 andFig. 6; after Classley, 1974). Dacite and rhyolitetend to cluster in the island arc tholeiite fieldyet do not follow the classic Cascade trend (Fig.6). However the Conejo-Santa Cruz Island suitediffers from the Catalina Island suite in thatearly clinopyroxene fractionation from the ConejoSanta Cruz Island suite followed by plagioclaseaccumulation in the later silicic differentiatescan explain the observed fractionation trend ofthe former suite (Fig. 5). At present, the CatalinaIsland trend requires more information to asses itsdetailed fractionation history (Fig. 6). It is alsoof interest to note the extreme iron enrichment seenin a number of the siliceous Santa Cruz Island volcanic rocks. Also note how the removal of clinopyroxene drives the "liquid" composition toward thecomposition of a tholeiitic glass (TC in Fig. 5)present in the lavas of the middle section of theConejo-Santa Cruz Island volcanic suite. Unfortunately glass is rare and therefore the liquid line ofdescent must be approximated by the whole rockcompositions.

The geochemical and petrologic data presentedsuggest that there are similarities among theBorderland volcanic suites which require similarpetrotectonic evolution yet there are differences.The Conejo-Santa Cruz Island suite is more tholeiitic in character than the Catalina Island suitebased on the former sUite's lower Sr isotopic compositions, moderate iron-enrichment, lower KzO andhigher TiO Z' The spatial association of CatalinaIsland with the Conejo-Santa Cruz Island volcanicsuite cannot be supported based on our work todate. We do feel that Catalina may have been in thesame geologic environment but it should not betectonically associated with the Santa MonicaMountains Microplate (Kamerling and Luyendyk, 1979).

PETROTECTONIC MODELS

Conejo-Santa Cruz Island Volcanic SUites

Numerous models have been proposed to explainthe volcano-tectonic evolution of the CaliforniaBorderland. These include island arc volcanism (Croweand others, 1976: Weigand and Anderson 1979),eduction (Dixon and Farrar, 1980), diapiric upwellingof the asthenosphere (Dickinson and Snyder, 1980) and

Basalt Basaltic Andesite Andesite Dacite Rhyolite

Conejo Volcanic Suite

K2

0

Ti02

FeO/HgO

87 Sr /S6Sr

Hgli

0.27-0.61

1. 40-2 .10

0.86-1.57

0.53-0.67

0.29-0.55

1. 02-1. 60

1.04-1.72

0.51-0.63

0.59-1. 07

0.88-1.27

0.71-2.60

0.70294-0.70372

0.40-0.70

0.59-1. 79

0.53-1. 28

0.37-14.2

0.11-0.49

0.82

0.37

2.27

0.44

Santa Cruz Island Volcanic Suite

Ti02

FeO/HgO

87Sr /86Sr

Hgil

0.42-1. 00

2.33-2.53

1.76-2.20

0.45-0.50

0.42-1.17 0.47-1.79

1.15-2.54 1.11-2.36

1.15-2.26 1.15-4.17

0.70253-0.70313

0.45-0.61 0.30-0.61

1. 09-1. 76

0.93-2.21

2.43-40.5

0.19-0.42

1. 37

0.96

18.3

0.09

Catalina Island Volcanic Suite

K2

0

Ti02

FeO/HgO

87sr /86Sr

Hgil

0.45-0.64

1.20-1.26

1.08-1.23

0.59-0.62

0.57-1.03 0.92-1.16

0.88-1.34 0.93-1.27

0.96-1.54 1.22-2.68

0.70400-0.70420

0.54-0.65 0.40-0.59

0.70-1.83 2.32-6.96

0.86-1.31 0.05-0.31

1.42-4.28 2.01-3.30

0.29-0.56 0.35-0.47

Tab Ie 1. Comparative geochemistry of the southern California Borderland .Volcanic Suites (Data from Hurst 1978, 1979, in press; Wood, 1982:Crowe and others, 1976; Blackerby, 1965)

290

modified ridge volcanism (Hurst, 1978; 1982, in press).These models, with the exception of island arcvolcanism, require a direct magmatic imput from themantle rather than a recycling mechanism and an extensional environment in the Borderland during volcanogenesis.

The results of this work support a model whichrequires the presence of a ridge-related source (low87Sr /86Sr , low K20, high Ti0 2 ). The source chemistryis then modified, perhaps by combined partial meltingand fractional crystallization, to produce a tholeiiti~ suite with suppressed iron-enrichment and volatileenrichment in the silicic differentiates, allowinghornblende to crystallize. These characteristics arenot observed in normal mid-ocean ridge volcanism butare observed in plutonic rocks associated with ridgevolcanics in ophiolites (Hopson, personal communication). We believe the constant influx of primitivemagma observed in normal ridges was suppressed as theEast Pacific Rise interacted with the subduction zone.

•

F

The trenchward flank of the ridge may be partiallysubducted or spread into the trench while the seawardflank spreads episodically as it attempts to avoidsubduction. This results in a temporary capping ofthe magma chamber below allowing differentiation.This process is coupled with the sporadic crystallization of magnetite as f 02 changes as a function ofthe relative influx of primitive magma or 02 fromthe surrounding environment. The effect of theseprocesses is to produce a differentiated suite withvariations in the degree of iron-enrichment. Episodic emplacement of more primitive magma furthermodifies the evolution of the suite by the mixing ofprimitive and differentiated magmas as suggested bythe mineral chemistries.

We suggest that the ridge-trench-transforminteractions in the southern Borderland resulted inlocal mantle-ridge fusion in the extensional environment associated with the Riviera Triple Junction.As the triple junction migrated to the south, so too

• Conejo Vol conicSui te

o Santa Cruz IslandVolcanic Suite

A M

Figure 1. AFM plot of the Conejo-Santa Cruz Island volcanic suite. Thesolid line denotes the tholeiitic (Th) .. calc-alkalic (CA)division of Irvine and Baragar (1971). Data from Hurst (un~

published), Crowe and others (1976) and Blackerby (1965).

291

did the volcanogenic zone resulting in the cessationof volcanism to the north. This process is termedzipper tectonics (Hurst, in press) since magmatismceases as the Riviera Triple Junction zips up themagmatic source in the north as it migrates to thesouth. It is difficult to envision the subductionof the entire ridge as suggested by Dixon and Farrar(1980) due to the problems associated with the sub-

duction of buoyant, hot mantle material. Perhapsthe trenchward flank is preferentially subducted,gradually exposing the overlying continent toasthenospheric upwelling as suggested by Dickinsonand Snyder (1980). Hence in our model, the Borderland volcanism is produced by a form of migratingvolcanic zone while continental volcanism in Western North America is related to a growing slab-windowat depth (Dickinson and Snyder, 1980).

Catalina Island Volcanic Suite

The petrotectonic evolution of the CatalinaIsland volcanic suite is difficult to assess at

present. Wood (1982) suggests that this volcanicsuite may be related to either a mid-ocean ridgesource which fractionated amphibole at depth dueto high PH20 and P02 or an island arc source.Problems arise due to the uncertainty of CatalinaIsland's position relative to the other Borderlandvolcanic suites as a result of complex tectonic movement along NW-trending faults in the Borderland(Howell, 1976).

The chemistry of the Catalina Island volcanicsuite (low K

20, high Ti0 2 ) is similar to that of

other Borderland suites yet its lack of ironenrichment, higher Sr isotopic ratios (circa 0.7040;Wood, 1982) and crystallization sequence suggest arcaffinities. However, Wood (1982) also recognizedsilicic inclusions within this suite which suggeststhe possibility of contamination by continental material. These types of inclusions are not observed inthe Conejo-Santa Cruz Island suite. Perhaps Catalinawas rafted from North America as the migratingmagmatic zone passed below a portion of the North

FeO (total)

oo

Tholeiitic

",- ...

'",

;' "";' \,

'" Calc-alkallna ";'

"'" \,

'" 0 "9'" 0 0 \,

'" 00 °80 o'{,\,

;' \,

0 0 0 000 0

o

MgOFigure 2. AFM plot of the Catalina Island volcanic suite (Wood, 1982).

The dashed lime denotes the tholeiitic-calc-alkalic divisionof Irvine and Baragar (1971).

292

10rI 0

8 • Conejo Volcanic Suite

~ 0 Santo Cruz Island Volcanic9 • Suite

--Mid-Ocean Ridge BasaltTrend

0

8

J7 ('000

I 0

6 f:.*0 • •

ICD 5l1..

4

:3

2

2 :3 4$R

FeO /MgO5

Figure 3. FeO* versus FeO*/MgO (Miyashiro, 1974) for the Conejo- SantaCruz Island volcanic suite. Data sources are the same asthose of Figure 1.

293

8\

\

go \7 \

0 \

\0 \0

6 0 \

0\ 0

0 0 \0 \

Q) 00\5 0

LL \

0 0 \\

cf. 4 \ Tholeiitic0 \

...... \

~\

Calc-alkalic \3 \

\

\

0\

2 \

0\

\

0

2 3 4 5 6

Fe 0" / M g 0

Figure 4. FeO* versus FeO*/MgO (Miyashiro, 1974)for the Catalina Island volcanic suite.Data from Wood, 1982.

294

• Basalt - Basaltic Andesi te0

5 IIIAndesite

018.3 0

f ~ 40.5A~

Dacite

t~ 7.45 E9 Rhyolite4 t 0

~Plog.~

RDG <:t-~

3~

1Il~ ~ ..0- .....~

/ '\ 0/ \/ 0 I §/

2 / I 0 0/

,/ !>g , 0

/ I •/ 0 0)/0,/ 0 • • <:t- T G

/' 0 /

.1ItSJ • • .... .../' ....

/' --..... - •/' --f --\

o 2

Figure 5. FeO*/MgO versus Ti0 2 for the Conejo-Santa Cruz Island volcanicsuite. The solid symbols and (@) represent the Cenejo volcanicsuite while the other symbols represent the Santa Cruz Islandvolcanic suite. The dashed field is the field of mid-ocean ridgebasalt (Glassley, 1974). Data sources are the same as those ofFigure 1.

295

3

Figure 6. f~O* /MgO versus Ti0 2 for the Catalina Island volcanic suite.flelds are those suggested by Glassley (1974). Data from\vood (1982).

American plate. This model warrants further studyas it may explain the origins of other tectonicallyisolated southern Borderland islands such as Sanrlemante.

SUMMARY

Our studies to date suggest a model in which thevolcanics of the southern California Borderlandoriginated as a result of the southerly migration ofa triple junction volcanogenic zone. Volcanismwould cease from north to south after the passageof the active magmatic zone. The chemical characteristics of the volcanogenic zone to the continent, thecharacteristics of the volcanics produced are dependent upon the spatial relationship episodic capping ofthe mid-ocean ridge source due to differential spreading, amount of fractional fusion and crystallization,and the availability of volatiles. In the northern

296

Borderland, volcanics erupted through oceanic crustwith minor volatile transfer during volcanogenesiswhile the volcanics of the southern Borderland mayrecord the interactions of the migrating triplejunction volcanogenic zone with continental crust asthe volcanogenic zone temporarily passed beneath thecontinental margin.

ACKNOVILEDGMENTS

The authors wish to thank T. Davis, P. Ehlig andR. Stull for helpful discussions which have improvedthe manuscript, S. Thompson for continued excellencein drafting and G. Higley for her dependable typingand editing.

REFERENCES CITED

Atwater, T., 1970, Implications of plate tectonicsCenozoic tectonic evolution of western NorthAmerica: Geol. Soc. America BUll., v. 81,p. 3513-3536.

Blackerby, B.A., 1965, The Conejo volcanics in theMalibu Lake area of the western Santa MonicaMountains, Los Angeles County, California,unpublished Ph.D. dissertation, 157 p.,University of California, Los Angeles.

Crowe, B.M., McLean, H., and Howell, D.G., andHiggins,R.E.; 1976, Petrography and majorelement chemistry of the Santa Cruz Islandvolcanics in Howell, D.G., ed., Aspects ofthe geologic history of the California Continental Borderland, Am. Assoc. of Petrol.Geol., Pac. Sect., Misc. Pub. 24, p. 196215.

Crowell, J.C., 1976, Implications of crustal stretching and shortening of coastal Ventura basinCalifornia in Howell, D.G. Ced.) Aspects of'thegeologic history of the California ContinentalBorderland, Am. Assoc. of Petrol. Geol., Pac.Sect., Misc. Pub. 24, 365-382.

Dickinson, W.R. and Snyder, W.S., 1970, Geometry ofTriple Junctions Related to San Andreas Transform: Jour. of Geophy. Research, v. 84, no. 82,p. 561-572.

Dixon, J.W. and Farrar, E., 1980, Ridge sUbduction,eduction and the Neogene tectonics of southwestern North America: Tectonophysics, v. 67,p. 81-99.

Glassley, W., 1974, Geochemistry and tectonics ofthe Crescent Volcanic Rocks, Olympic Penninsula, Washington: Geol. Soc. America Bull.v. 85, p. 785-794.

Higgins, R.E., 1976, Major element geochemistry ofthe Cenozoic volcanic rocks in the Los Angelesbasin and vicinity, in Howell, D.G .. , ed.,Aspects of the Geologic History of the California Continental Borderland, Am. Assoc. ofPetrol. Geol., Pac. Sect., Misc. Publication 24,p. 216-227.

Howell, D.G., 1976, Late Miocene counterclockwiserotation of the south half of Santa Cruz Island in Howell, D.G., ed., Aspects of theGeologic History of the California Continental Borderland, Am. Assoc. of Petrol. Geol.,Pac. Sect., Misc. Pub. 24, p. 449-454.

Hurst, R.W., 1978, The Conejo Volcanics, WesternSanta Monica Mountains, volcanogenesis at anoceanic continental plate interface Cabs.),EOS CAm. Geophys. Un. Trans.), Vol. 59, 12.p. 1213, Am. Geophys. Un., 1978 fall ann.meeting.

_____ , 1979, Petrologic constraints on the differentiation of the Conejo Volcanic SUite, WesternSanta Monica Mountains Cabs.), EOS CAm. Geophys.Un. Trans.), Vol. 60. #46, p. 972. Am. Geophysical Un., 1979 fall ann. meeting.

_____, in press, Petrogenesis of the Conejo volcanicsuite, southern California; Evidence for midocean ridge-continental margin interactions:Geology.

Irvine, T.N., and Baragar, W.R.S., 1971, A guide tothe chemical classification of the common volcanic rocks: Canadian Jour. Earth Sci., v. 8, p.523-548.

297

Kamerling, M.J. and Luyendyk, B.P., 1979, Tectonicrotations of the Santa Monica Mountains region,western Transverse Ranges, California, suggestedby Paleomagnetic vectors, Geol. Soc. Am. Bull.,Part 1, Vol. 90, no. 4, p. 331-337.

Miyashiro, A., 1974, Volcanic Rock Series in IslandArcs and Active Continental Margins, Amer. Jour.of Science, Vol. 274, April, p. 321-355.

Nolf, B. and Nolf, p., 1969, Santa Cruz Islandvolcanics in Weaver, D.W. and others, eds.,Geology of the northern Channel Islands AmAssoc. of Petrol. Geol. and Soc. Econ. Pale~n.and Mineral., Pac. Sect., Spec. PUb., p. 91-94.

Weigand, P.W., and Anderson, T.P., 1979, Geochemistryof the Mid-Miocene Conejo Volcanics from theSanta Monica Mountains, Los Angeles County,California and Tectonic Implications, Cabs.),Geol. Soc. Am. abs. with programs, 1979 Ann.Meeting.

Williams, R.E., 1977, Miocene volcanism in the centralConejo Hills, Ventura County, California: M.S.Thesis, Univ. of California, Santa Barbara,72 p.

Wood, W.R., 1982, Geology, petrography and geochemistry of the Santa Catalina Island volcanicrocks, Black Jack Peak to Whitleys Peak area:M.S. Thesis, Calif, State Univ., Los Angeles146 p. '

MID-TERTIARY DETAClIllENT FAULTING fu'W l1AtlGAllESE MINERALIZATIONIN THE I1IDHAY llOUNTAINS, U1PERIAL COUNTY, CALIFORllIA

Lindee Berg, Gregory Leveille, Pattie GeisDepartment of Geological Sciences


ABSTRACT INTRODUCTION

Detachment faulting of late Oligocene to middleMiocene age has been recognized as a significantfeature of the Cenozoic geology of western Arizona,southeastern California, and Nevada. Field studieshave now shown that a detachment fault is present inthe Midway Hountains of southeasternmost California.The Midway detachment fault separates the rocks ofthe Hid,.my l10untains into an allochthonous upperplate, and autochthonous lm,er plate. The two upperplate rock units in the l1idways are an andesitic member of the Oligocene to Miocene Palo Verde volcanicsequence and the Hiocene Tolbard fanglomerate. Theupper plate has been extended along northwest-striking, northeast-dipping normal faults that transportedupper-plate fault blocks to the northeast. Regionalfolding of the fault surface has warped the Midwaydetachment fault into its present domal form. TheMidway detachment fault can be correlated to similaLfaults exposed to the northeast in the TrigoMountains of western Arizona, and to the west in theChocolate Mountains of California. In the MidwayMountains, deposits of manganese oxides are localizedalong the detachment fault as well as along normalfaults ,;ithin the upper plate. Psilomelane, pyrolusite, and manganite are the most common manganeseminerals present.

The 11idway l10untains are located in the northeastern corner of Imperial County, California,approximately 50 km east of Bra,;Iey and 35 km southof Blythe. Neighboring mountain ranges in Californiainclude the Palo Verde Hountains to the north andChocolate Hountains to the west with the TrigoHountains of Arizona to the east of the l1idways. Theeasiest access to the range is along a jeep trail offCalifornia Higln,ay 78 between mile marker 67 and 68.This jeep tr:1il extends for 10 km back into the range(Fig. 1), and connects with numerous other, moreprimitive roads.

The first geologic studies in the Hich,ayMountains were done by Jarvis B. Hadley (1942), aSurvey geologist interested in the manganese depositsof the Paymaster mining district. Subsequent work';as completed in 1960 by Southern Pacific LandCompany geologists Ilanely, Gamble, and Gardner intheir mineral evaluation of the area (Gardner andothers, 1960). The purpose of their study was todetermine if any potentially economic deposits werepresent on the Southern Pacific land holdings. Theydid not attempt any structural or regional interpretations of the geologic features of the range. Intheir reconnaissance mapping of the area, they considered the detachment fault of this study to be a

Milpitas Wash

-'--l

I

II

ChocolateMtns.

~NI Peter Kane Mtns.

",0,,">,0 5 Mi

~ 5 Km. I'1-'"

'/

Figure 1. Hajor geographic features of the area adjacentto the Mid",ay l·1ountains.

299

normal fault or an unconformity. Most recently, thpPioneer mine of the llid\my Hountains \.,as discussed byHorton (1977) Hho described the history of the miningoperations.

Detailed study of the rock units and structureof the nearby Chocolate, Peter Kane, and Cargolluchacho Mountains has been done by Dillon (1976),Haxel (1977), and Haxel and Dillon (1978). Theydefined the overall structural relationships of thePelona-Orocopia schist and its overlying Precambriancrystalline complex. They have also mapped the contact betHeen the tHO assemblages as the OrocopiaChocolate Hountain thrust of Hesozoic age.

Study of the Tertiary volcanic stratigraphy Hasdone in the adjoining Palo Verde Hountains by Cro",eand others (1979). They described the volcanic rocksof this area as consisting of several major packagesof differing lithologies and varying in age fromOligocene to mid-Hiocene. The exact correlation ofthe Tertiary section in the Palo Verde Hountains tothe Hid",ay Hountains is not as yet kno\m.

Huch ",ork has been done in the past ten years(1971-1981) to define the structure and extent ofdetachment faulting in ",estern Arizona, southeasternCalifornia, and southern Nevada (Anderson, 1971,1977, 1978; Schackelford, 1976, 1980; Davis andothers, 1979, 1980; Rehrig and Reynolds, 1980; Frostand Hartin, 1982). These \vorkers have found that(1) the detachment fault appears to be of late Oligocene to middle Hiocene age, (2) it is a tensionalfeature resulting from northeast-south",est crustalextension, (3) regional folding of the detachmentfault occurred, producing antiformal ranges surrounded by synformal basins, and (4) the detachmentfault separates the geology of this region into t"'oseparate structural units, a lo",er plate and an upperplate ",ith an unknm-m amount of offset bet",een thetlW.

The closest, previously described, exposures ofdetachment faulting to this part of California are inthe Eagle Tail (130 km a",ay, Rehrig and others,1980), Harquahala (140 km a",ay, Rehrig and Reynolds,1980) and Riverside Hountains (90 km a",ay, Carr andDickey, 1980). Recently discovered exposures of theregional detachment fault are described in this volume by Garner and others in the adjacent TrigoHountains, Logan and Hirsch in the Castle Dome110untains, Pridmore and Craig in the Baker Peaks, andHueller and others in the Hoha",k Hountains. The Hid",ay Mountains, thus, appear to be part of theregional continuation of the detachment terrane intosouthl"estern Arizona and southeasternmost California.As such, they also appear to have been affected bythe same manganese mineralization as other parts ofthe detachment terrane, and as the Hhipple Hountains(Ridenour and others, this volume).

LITHOLOGIC UNITS

Lo",er-Plate Rock Units

The rock units in the Hid",ay Hountains can bedivided into lo",er-plate units, upper-plate units,and younger units that postdate detachment faulting.

The oldest lithologic unit exposed in the l1id",ayl10untains is the Chuck1.,alla complex, of Morton(1966), ,,,hich is a Precambrian metamorphic suite, .that is probably about 1.7 billion years old. Thlsmetamorphic-igneous complex consists of quartzdiorite

300

gneiss, foliated hybrid granitic rock, schist, andfine-grained granitic rocks, all of ",hich have beenintensely folded and faulted. Accentuating thefolded appearance of this unit are metamorphic segregations of quartz, feldspars, micas, and hornblendeinto distinct mineral bands. The metamorphic rocksgrade Hithout apparent intrusive contacts into thegranite gneiss and into locally structureless, gray,biotite granite. Variably developed lineations ",ithin this unit trend northeast (Gardner and others,1960), but are not associated ",ith a mylonitic fabric.No mylonitic rocks appear to crop out in the louerplate of the l1id",ay Mountains, although they areexposed in nearby mountain ranges (Crm"ell, 1981).

In the Mid",ay 110untains pegmatite dikes of Cretaceous age (?) cut the lo",er-plate, metamorphicigneous complex. The pegmatite dikes have not beenfoliated and place an upper limit on the deformationpresent in the rocks of the louer plate. Similartypes of pegmatitic dikes pervade nearby ranges suchas the Big Haria Hountains (Hamilton, this volume;l1artin and others, this volume) and Trigo Hountains(Heaver, 1982). In both these ranges the pegmatitesare late Hesozoic in age, and suggest that the pegmatites in the Hidways may be of the same age.

Upper Plate Rock Units

The oldest upper-plate rock unit in the Hidwayl10untains is an andesitic member of the Palo Verdevolcanic sequence (of Horton, 1977). The rocks ofthis unit are red-black in color and are stained bymanganese oxides in places. These volcanic rocksformed -an autobreccia during their deposition. Theautobrecciated nature "'as further accentuated bylater deformation and appears to have played animportant role in controlling the passage of mineralizing fluids. The rocks of this unit exhibit a porphyritic texture ",ith elongated laths of plagioclasein an aphanitic groundmass. The thickness of thePalo Verde volcanic sequence is estimated to be 400to 500 meters in the Mid",ay Hountains. In nearbymountain ranges this unit contains pyroclastic volcanic rocks along with tuff-breccia and mudflm.,-breccia none of Hhich are found in the l1idway Mountains.The'volcanic rocks in the !1id\my Mountains are probably Oligocene to l1iocene in age on the basis of theregional occurrence of similar units (Cro",e andothers, 1979; Dillon, 1976; Keith, 1978). K-Ar agedetermination of the Hidway volcanic rocks is underway.

Overlying the volcanic rocks in the upper plateis a younger sedimentary unit, the Tolbard Fanglomerate, ",hich is l1iocene to Pliocene in age (Danehyand others, 1960). It is ",ell indurated, red-bro",nto tan in color, and is locally stained by manganeseoxide deposits. This unit forms steep irregularslopes ",ith massive, rounded summits. It is composedlargely of subrounded to angular clasts ranging insize up to 60 centimeters. These clasts are set in afine-pebble to coarse-sand matrix. The ratio of sandto clasts is roughly 1.0 to 1.5 ",ith volcanic rocksaccounting for seventy percent of the clast and crystalline rocks making up the remaining thirty percent.Lenses of coarse, crossbedded sandstone, 10 to 50centimeters thick, are common throughout the formation. The Tolbard Fanglomerate is cemented by silicaand hematite, ",ith some secondary calcite cementation. Paleocurrent indicators ",ithin this unit suchas channeling, trough crossbedding, and imbricatedcobbles suggest transport tm.,ard the north",est. Thehigh degree of variation from the mean of N30H may be

a function of the alluvial fan depositional environment and may be the result of flow dispersion acrossthe former fan surface. A detailed study of the sedimentology of this unit is presented in the followingpaper by Jorgensen and others (this volume).

Younger Rock Units

Deposited unconformably upon all older rockunits is the Pliocene-Pleistocene(?) Barren MountainsGroup (Danehy, 1960). The two members of the Barrenl10untains Group present in the Midway Mountains arethe Arroyo Seco and the Vinagre Formation. TheBarren Mountains Group is generally an undeformed,flat-lying fanglomerate containing clasts of volcanicand metamorphic rocks, some of which may be reworkedclasts from the Tolbard Fanglomerate. In places, theBarren llountain Group has been cut by northeaststriking, high-angle faults juxtaposing it againstolder rock units.

The Arroyo Seco Formation is a poorly consolidated fanglomerate, which contains crossbedded sandlenses with gravel to boulder-size clasts in a sandymatrix. This unit contains predominantly volcanicrock clasts in most areas. Locally it contains over50% metamorphic-igneous complex rocks, which give theunit a light gray color (Gardner, 1960).

The Vinagre Formation consists of subangular tosubrounded, gravel to boulder-size clasts. The clastcomposition of the unit is 65% volcanic rocks, 25%gneissic and crystalline rocks, and 10% pegmatiticrocks. Small sand lenses also occur, with faint butfairly common crossbedding. These beds are usuallyflat-lying or dip very gently away from the range.

The youngest deposits are of Quaternary age andhave been subdivided into an older and younger alluvial gravel deposit. These two units lie unconformably on all older rock units and largely obscure thecontac t betl"een the upper and 10l"er plate around muchof the range. No faults are known to cut theseyounger alluvial deposits.

STRUCTURAL GEOLOGY

The Hidway detachment fault separates the rockunits of the Hidway Mountains into upper-plate and10l"er-plate assemblages. The Midway detachment faultis well exposed along the northern flank of the range,but elsewhere is either eroded away or covered byyounger sediments (Fig. 2)'. Its present outcrop pattern and the domal form of the range suggest that thel1idway detachment fault Has once continuous over therange but has since been removed by erosion.

Developed within the 10l"er-plate rocks of thel1idHay 110untains is a chlorite breccia zone. Thiszone has a gradational lower boundary with a sharp,upper surface, Hhich is the detachment fault. RocksHi thin this zone, ,,,hich is up to ten meters thick,have been brecciated on both a microscopic and amacroscopic scale. The brecciation of the crystallinerock appears to have allowed for the transmission oflarge volumes of fluids through this zone, whichleached and altered the rocks. Movement along thedetachment fault seems to be responsible for thebrecciation of these rocks. The combination of theleaching and chloritization of the lower-plate rocksnear the detachment fault gives these rocks their

Figure 2. Oblique aerial view of the northern flank of the HidHayHountains. LOI"er-plate rocks of the Chucbmlla complex form the metamorphic core complex of the range. The upper-plate rocks (foreground)are the andesitic member of the Palo Verde volcanic sequence. lUningoperations have exhumed portions of the detachment fault surface andremoved over 50,000 cubic meters of material from the main ore bodyin the Palo Verde volcanic sequence.

301

Figure 3. A view ofthe lOlifer plate I"here thedetachment fault has beenremoved by erosion, sholifing the ramp surface asdeveloped in the llidwayUountains. The 1 ine drcllmin tlte photograph parallelcthe ramp surface.

Figure 4. (Belo",) Oneof many upper-plate normalfaults in the Tolbard fanglomerate. The upper-platenormal faults generally dipto the northeast.

distinctive coloration with greens and dark redsdominating. Ifhere the upper plate has been removedby erosion, the more resistant lower-plate rocks formwhat is referred to as the detachment ramps. Bytracing the outcrop area of the ramp one can inferthe trace of the detachment fault even thouah thefault surface has been removed by erosion (~ig. 3).The ramp appears to flatten near the top of the rangeand is steeper at the base. The unusually steep dipof the detachment fault (from 30° to 60°) as exposednear the base of the range, may thus not be characteristic of the overall attitude of the faultsurface.

Foliation ",ithin the lower-plate rocks is truncated by the detachment fault as are Cretaceous (?)pegmatite dikes. The dikes do not share any of thefoliation, indicating that the foliation-formingevent ",as pre-pegmatitic and therefore pre-detachmentfaulting. The foliated fabric ",ithin the lo",er plateappears to have no genetic connection to detachmentfaulting and is probably Precambrian in age. LOI"erplate extension in the Midways cannot be equated tothe presence of a mylonitic fabric, since none ispresent.

Although the foliation is not mylonitic, itappears to define a pronounced domal form, much likethat in ranges with mylonitic fabrics. This archedform is best seen in morning light looking ",est intothe range. The development of the domal form of theMidway fault may be related to the development oflarge-scale folds during mid-Tertiary crustal extension (Otton and Dokka, 1981; Cameron and Frost,1981) as described in better known portions of thedetachment terrane. Superposition of two fold sets,one trending northeast and the other north",est, mayhave produced the mid-Tertiary uplif t of the 11idlifay11ountains.

302

Upper Plate

The dominant structural element of the upperplate in the Midway Mountains is a series of northwest-striking, northeast-dipping, high-angle normalfaults, with dips from 54 to 60° (Fig. 4). Striae onthe fault surface indicate many different movementdirections, suggesting complex motion of the faultblocks during transport. On the whole, however,striae and mullion structures indicate a down-dipsense of motion. Tear faults perpendicular to thehigh-angle faults provide another kinematic indicatorfor the direction of transport, and suggest down-dipmotion on the normal faults. Back rotation of theupper-plate into the detachment surface also suggeststhe same northeast (N70E) transport. Some upperplate faults indicate other directions of transportbut do not seem to have major offset across them.

Motion along upper-plate faults was probablycoeval with motion along the detachment fault, as itis in other ranges (Davis and others, 1980) where theupper-plate structure is better exposed. The amountof offset across the detachment fault is unknmmbecause the same units have not yet been identifiedfrom the two plates. If the upper-plate volcanicand sedimentary rocks reflect the full amount ofextension, then the amount of extension does notappear great. Because the rocks of the l1id'vays ,,,eretransported relatively to the northeast, a greatamount of transport does not seem possible becauserocks in the Trigo Mountains moved to the southwest.These rocks in the Trigos lie along the direction oftectonic transport for the l1idways and thus make itdifficult to postulate large offsets during midTertiary extensional tectonics.

sw

Within the study area, faulting in the upperplate has juxtaposed the andesite of the Palo Verdevolcanic sequence with the Tolbard fanglomerate nearthe 11idway detachment fault (Fig. 5). The andesitehas the geometry of a tectonic slice between thefanglomerate and the metasedimentary rocks (Fig. 5).The tectonic slice is present near the hinge of theantiformal bend in the detachment fault. Intenseshattering of the volcanic unit has apparentlyresulted from its motion along the detachment fault.

MANGANESE MINERALIZATION

History of Mining Operations

11ining operations to recover mangnaese in theJ1idway 110untains first began in 1917 and continuedthrough 1918. During this period ore was recoveredfrom the main ore body in the andesitic member ofthe Palo Verde volcanic sequence and had an averagemanganese oxide content of 46% by weight. Productionresumed in 1941, continued throughout most of HorldHar II and ended in 1944. The most recent miningoperations in the Midway Mountains were undertaken aspart of a federal stockpiling program, which wasactive between the years 1952 and 1959. More than22,000 tons of ore were taken from the workings andaveraged 43% manganese oxides by weight. Duringfield work in the Midway Mountains in the fall of1981, several claim markers dating from 1980 wereobserved, suggesting renewed interest in the manganese deposits.

NE

oo~

- 0I- 0UJ 0UJ

t\I

I.L-z:0 0

~:>UJ..JLIJ 0

00t\I

I

00

~I

--- .... ...... ......

Qal

.... .......... ,... DETACHMENT FAULT~,,,,

Figure 5. A generalized cross section of the Midway detachment fault separating the rocks into upper-plateunits (Ttfg and Tpvu) and lower-plate units (Micu). Normal faults cut the upper-plate unit and sole into thedetachment fault surface. Transport of upper-plate fault blocks is to the northeast. The detachment fault formsan antiform over the range.

303

General Description of Manganese Deposits

!Ianganese mineralization in the MidIVay Mountainsoccurs as open-space filling of fractures and porespaces, and as replacement mineralization throughoutthe brecciated rock adjacent to the detachment fault.llineralization is confined to the upper-plate unitsof the detachment terrane and is concentrated in thefault zone directly above the detachment surface.Primary manganese mineralization does not occur inthe Barren 110untains Group or younger alluvialdeposits. The principal manganese minerals presentin the llid\.,ay llountains are psilomelane, pyrolusiteand manganite IVith lesser amounts of braunite andramsdellite (Hadley, 1942).

The main manganese ore body is located adjacentto the detachment fault in the upper-plate andesite(Fig. 6) and contains mostly psilomelane, pyrolusiteand braunite. Nanganese minerals are concentrated inveins that occur as hydrothermal fracture fillings\vithin normal faults and associated stress fractures.Individual veins range in IVidth up to 1.5 meters andextend for lengths of over 100 meters.

T\vo cuts have been extensively mined _vithin theore body, exposing excellent cross sections of theintensely fractured andesite. These cuts are 14 to

18 meters IVide, 100 meters long, and IVere IVorked to adepth of 20 meters so that 32,000 cubic meters ofmaterial IVas removed from each cut (Norton, 1977).Large fragments of unbroken andesite are found IVithinthe more extensive veins. The degree of shatteringin the andesite, thus, appears to be directly proportional to the degree of mineralization.

In the Tolbard fanglomerate, veins containingmanganese mineralization follm., both normal faultsand fracture surfaces and commonly occur in mammillary or botryoidal structures on the fault surface.These faults have IVell-defined footlValls and brecciated hanging IValls, the latter of which are considerably mineralized (Fig. 7). The veins range in IVidthup to 2 meters and are up to 100 meters long. Mining operations IVithin the Tolbard Fanglomerate consisted of working a series of drifts and raises withup to 20 meters of vertical section and 90 metersof horizontal section removed from individual veinsin this manner. Within several of the larger stopes,this mining has exposed IVell-defined mullion structures on the footlVall (Fig. 8). 1,langanese depositsare also present along small fractures that pinch outupward IVithin the fanglomerate. Hydrothermal solutions responsible for the manganese mineralizationthus appear to have radiated upIVard through the unitalong zones of high porosity and permeability. The

Main manganese ore body

JDetachment

Faull

.'" .0 '" 0....

TQbmu

./

Manganesefault

mineralizationsurfaces

along

Figure 6. Geologic map of the northeastern flank of, the NidIVayNountains sholVing areas of manganese concentrations. Also indicatedis the trace of the detachment fault.

304

Figure 7. Northeasttrending veins of manganese ShOH ,,'ell definedfootHalls and gradationalhanging Halls.

Figure 8. (BeloH)Hullion structures apparent along upper-platefault surfaces indicatenormal movement.

brecciated fault zones and associated fracturesapparently localized the passage of the hydrothermalfluids and suggest a through going plumbing systembetHeen most of the vein deposits.

Origin and Chemistry of Hineralization

305

1·1anganese oxide deposits in the l1idHay Hountainsare thought to be hypogene fault deposits formed byhydrothermal processes (Fig. 9). Present-day hotsprings are kno",n to deposit manganese oxides andthus suggest that the Hid\vay Hountain deposits are aproduct of the flOl, of hot springs through structurally controlled vein systems. The hot springs originof these manganese deposits is suggested by bothmineralogy and chemistry. Coarse grained ",hitecalcite, finely divided carbonates and chalcedony arethe principal gangue minerals. This calcite forms athin layer above the manganese and shoHs retrogradesolubility. Hagnetite is found in small veins Hithin the main ore body as Hell formed crystals uhichcommonly Heather to hemitite.

Chemical factors such as pH, Eh and the concentration of dissolved manganese are important factorscontrolling the precipitation of manganese. Hanymeteoric Haters plot in the acidic area of the Ell-pHfield Hhere iron oxides have a 101" solubility andmanganese oxides have a high solubility. Hanganesedeposits are, therefore, common in volcanic associations because the metal is readily dissolved out ofthe rock by dilute acids. \fuere dilute acids arepresent, manganese Hill be dissolved, transported,and after passing the critical Eh-pH conditionsnecessary for the deposition of manganese, redeposited (Fig. 10). Although no fluid inclusion studiesare yet available for the 11idHay Hountains, the pH ofthe transporting solution is inferred to have been inthe acidic range.

The origin of manganese deposits in the l1id",ayMountains \,as influenced by several factors includingthe temperature of the hydrothermal solution thattransported the manganese, the mineral composition ofthe rock unit from Hhich the manganese Has derived

MANGANESE ORES

DEPOSITS INDEPENDENT OFVOLCANIC ACTIVITY

DEPOSITS ASSOCIATED DEPOSITS ASSOCIATEDWITH TUFFS AND WITH IRON FORMATIONSCLASTIC SEDIMENTS OFVOLCANIC AFFILIATION

The age of mineralization is indicated by thestructural control of the are deposits. Hydrothermalfluids were moving along the brecciated zones,requiring that mineralization is either coeval with,or post faulting. The widespread development ofslickensided and striated surfaces that have involvedmineralized rock further require that mineralizationoccurred before at least some of the deformation.Hineralization, thus, appears to have been at leastin part contemporaneous with extensional faulting andrise of the llid,,,ay ilountains.

IRESIDUAL

ACCUMULATIONSLATERITES

IMETAMORPHIC

DEPOSITS

ILOW TEMPERATURESILICATE ANO HAUSMANNITE OEPOSITSASSOCIATED WITHSUBMARINE FLOWS

ISEDIMENTARY

DEPOSITSHYDROTHERMAL

DEPOSITS

REGIONAL CORRELATION

Figure 10. Eh-pH diagram shoHing stabilityfields of common manganese minerals. Assumedconcentrations are: total dissolved carbonate, lH;total dissolved sulfer, 10-6M (after Krauskopf).

"IIII

MnCO JIIIIIIIIIIIIII

4 6 8 10 12 14pH

1.4

1.2

1.0

0.8

0.6

~ 04'0>

~w Mn2 •0.2

0.0H2O

H2-0.2

-0.4

-0.6

0 2

The HidHay detachment fault appears to be an isolated exposure of a much more extensive detachmentfault, ,chich extends over ,,,estern Arizona, southeasternmost California, and Nevada. This wavelikefault surface is exposed on the flanks of severalranges in this region. In the Trigo Hountains ofHestern Arizona, 18 km northwest of the Nidways, aHell developed detachment fault is exposed (Garner andothers, this volume; Kitchen, pers. commun., 1981).The detachment fault is exposed as an antiform in theTrigos with a gentle synform between the Trigo andllidway Mountains (Fig. 11).

Nanganese mineralization and the development ofare bodies in the HidHay Nountains were controlled bythe structural features of the range, including boththe detachment fault and upper-plate faults. Brecciation associated "7ith the detachment fault and upperplate faults appears to have created porous and permeable conduits for the hydrothermal solutions risingfrom below. Intense shattering within the andesiteis probably largely responsible for the concentrationof mineralization within the unit. The preferentialshattering of the andesite is probably a result ofboth the brittle character of the unit and its structural position adjacent to the detachment fault.

Structural Control of }~nganese Nineralization

One possible source rock for the manganese is amafic volcanic member of the Palo Verde volcanicsequence. As a generalization more-mafic, highertemperature igneous rocks are decomposed at a greaterrate by chemical weathering than their more-felsic,lower temperature counterparts. Nanganese is generally more concentrated in mafic igneous rocks thanin felsic igneous rocks (IYedepohl, 1980). Therefore,manganese is leached relatively rapidly from maficrocks, and because it is also more concentrated insuch rocks, these rocks may provide an importantsupply of the element.

Ano.ther, perhaps less obvious, structural control for the mineralization is the anti formal shapeof the detachment fault. hThere the main are bodiesare located in the Hidway Mountains is in proximityto the hinge of the detachment-related antiform. Ifthe mineralizing fluids were rising from below alongthe structurally controlled conduit of the detachmentfault, they would have been concentrated near thecrest of the fold. Flow of hot-spring waters wouldthus be concentrated from over the entire limbs ofthe folds to the hinge area. Precipitation of themineralization is inferred to have taken place inthis general hinge area.

and the behavior of manganese in solution. The depthof the detachment fault in Oligocene to middle Pliocene time could not have been greater than a feHkilometers as determined by reconstruction of upperplate structures. At such depth the temperature ofthe meteoric water, Hhich is the proposed transporting agent of the manganese, would have been only 100to 200°C. \,hile hydrothermal solubility data is notyet available for this element, manganese oxides are

relatively soluble at 25°C and should become evenmore soluble Hith a tendency toward increasingly morecomplex ion formation at higher temperatures (Crerarand others, 1980).

Figure 9. Genetic classification of manganeseare deposits. Deposits in the llid,,,ay llountains arehydrothermal in origin (after Park, 1956).

306

A

VINCENTTHRUST

FAULT

M1DWAYMOUNTAINS

';.)7·W/

TRIGO A'. / MOUNTAI

D': ',,'1/, /- /~- /

"-/'-'.. , "I //, ,

'.' . - /. ,I

/

km,

Figure 11. Schematica1 representation of the antiforma1 and synforma1 nature of the detachment faultbetHeen the UidHav llountains and the adjacent ranges (after Frost),

307

Figure 12. Southwestern view depicting that the detachment fault has been warped into antiforms in theTrigo Mountains (foreground) and the Midway Mountains (background). Between the two ranges is a correspondingsynform in which the Colorado River currently flows.

308

The transport direction of the upper plate inthe Trigos is to the southwest, opposite that of theupper plate in the l1idways. These opposing upperplate movement directions along the direction of tectonic transport may indicate that the upper plate hasnot been transported long distances. It may alsoindicate that individual fault blocks were essentially rotated in place during regional crustal extension.

In the Chocolate Mountains, approximately 10 kmsouthlvest of the Hidways, an exposure of the detachment fault has been reported (Richens and Smith, personal communication, 1981). This contact was wellmapped by Dillon (1976) as a normal fault that truncates tilted Tertiary volcanic rocks from underlyingcrystalline rocks. Correlating this normal fault inthe Chocolates to the regionally developed, wavelikenormal fault system would then indicate that theChocolates have undergone detachment related deformation in mid-Tertiary time. Transport above thisnormal fault has been rather uniformly to the northeast as judged from the northwest strike and southwest dips of large packages of volcanic and crystalline rocks in this area (Morton, 1966; Dillon, 1976).

As defined by the mapping of Dillon (1976) andMorton (1966), the fault that may be the detachmentfault is folded into several doubly plunging, northeast-trending antiforms and synforms. Also folded inthis geometry is the Orocopia-Chocolate !1ountainsThrust, which separates gneissic rocks from the underlying Pelona-Orocopia schist (Dillon, 1976; Morton,1966, 1977). Correlation of these fold structuresand the attendant detachment fault from the Chocolates to the Midways to the Trigos is depicted on adiagrammatic cross section in Figure 11. If thiscorrelation is correct, then the Mesozoic Pelonaschist and the Vincent thrust would also show thisfolding (Fig. 11).

CONCLUSIONS

Field studies in the Midway Mountains havedefined the existence of Oligocene (?) to l1ioceneextensional deformation that appears to correlate withsimilar normal faulting seen on a regional scale.The major fault in the Hidways appears to be a detachment fault, separating upper-plate volcanic and sedimentary rocks from lower-plate metasedimentary units.The domal form of the range exposes the fault, whichis also well developed just across the Colorado Riverin the Trigo !1ountains of Arizona. The folded character of the fault is similar to that seen elsewherein the detachment terrane and suggests that correlative detachment faults may be present in the adjoining Chocolate and Peter Kane Mountains to the westand south.

Motion of the upper-plate volcanic and sedimentary rocks has been to the northeast along northwest-striking, northeast-dipping normal faults. Offset between individual upper-plate faults does notappear great. Offset on the detachment fault isunknOlVll because of the mismatch of upper- and 10lverplate units, the 101, relief, and the res tricted areaof this study. Because upper-plate units in theTrigos moved to the southwest, opposite and on linewith the movement in the Hidways, major offset acrossthe Midway fault is probably not present(Fig. 12).

Manganese mineralization in the Midway !1ountainswas influenced by the presence of the detachment faultand upper-plate normal faults. !1anganese-bearingsolutions used the faults as conduits and precipitatedmanganese oxides along them. Psilomelane, pyrolusite

309

and manganite are the principal ore minerals in themajor manganese deposits within the Midways. Mostmineralization occurs as either open-space filling offractures or as replacement mineralization within thebrecciated fault zones. Mineralizing fluids appearto have been controlled by the geometry of thedetachment surface and reflect the paleohydrology ofthese hot spring type fluids.

ACKNO\iLEDGEMENTS

\ve would like to thank everyone whose contributions helped to bring this report to its final form.He would like to thank Eric Frost whose guidance,help, and tremendous energy made this report possible. His ideas on detachment faulting are reflectedthroughout this paper. Conversations with HaltKitchin, Russ Richens, and Bruce Smith have beenhelpful in preparing this report. We would like tothank Mike Jorgensen, Randall Mathis, Renay Staley,Jerry Lazenby, Phil Trumbly, Bill Shiffrar, and ChrisNatenstedt for their critical reviel' and suggestions.He would like to thank Enos StralVll and Scott Fenbyfor their fine drafting. He would also like toacknOloledge the fine <york of Kathy Jessup andPia Parrish in typing this report.

REFERENCES CITED

Anderson, J. L., Davis, G. A., Frost, E. G., andShackelford, T. J., 1979, Regional Miocenedetachment faulting and early Tertiary(?) mylonitization, Hhipple-Buckskin-Rawhide Mountains,southeastern California and western Arizona, inGeological Excursions in southern California,Geol. Soc. of America guidebook.

Anderson, R. E., 1971, Thin-skinned distension inTertiary rocks of southeastern Nevada: Geol.Soc. America Bull., v. 82, p. 43-58.

Anderson, R. E., 1977, Geologic map of the BoulderCity IS-minute quadrangle, Clark County, Nevada:U.S. Geological Survey, Geol. Quad. Map CG-139s.

Anderson, R. E., 1978a, Geologic map of the BlackCanyon IS-minute quadrangle, Mohave County,Arizona, and Clark County, Nevada: U. S.Geological Survey, Geol. Quad. Map CG-1394.

Anderson, R. E., 1978b, Chemistry of Tertiary volcanic rocks in the Eldorado Mountains, Clark County,Nevada, and comparisons with rocks from somenearby areas: Journal of Research of the U.S.Geol. Survey, vol. 6, no. 3, p. 409-424.

Cameron, T. E., and Frost, E. G., 1981, Regionaldevelopment of major antiforms and synforms coincident with detachment faulting in California,Arizona, Nevada, and Sonora: Geol. Soc. ofAmerica Abstracts with Programs, v. 13, no. 7.

Cameron, T. E., John, B. E., and Frost, E. G., 1981,Development of regional arches and basins andtheir relationship to mid-Tertiary detachmentfaulting in the Chemehuevi Mountains, SanBernardino County, California, and Mohave County,Arizona: Geol. Soc. of America, Abstracts withPrograms, v. 13, no. 2, p. 48.

Carr, H. J., and Dickey, D. D., 1980, Geologic map ofthe Vidal, California, and Parker SW, CaliforniaArizona quadrangles: U. S. Geol. Survey Map1-1125.

Crerar, D. A., et al., 1980, Ceochemistry of manganese: an overvie1d, i.!~ Proceedings of the secondinternational symposium on geology and geochemistry of manganese, Barentsov, I. Ii., Grasselly,G. Y., eds., E. ScilHeizerbart' sche Verlangsbuchandlund, Stutgard, Germany.

err-THE, B. 1'1., CroHell, J. C., and Krummenacher, D.,1979, Regional stratigraphy, Ie-Ar ages, andtectonic implications of Cenozoic volcanicrocks, southeastern California: AmericanJournal of Science, vol. 279, p. 186-216.

Crowell, J. C., 1981, An outline of the tectonichistory of southeastern California, in TheCeotectonic Development of Californi~-;- vol. 1,W. C. Ernst, ed., Prentice-Hall, Inc., EnglewoodCliffs, New Jersey.

Davis, G. A., Anders()ll, J. L., Frost, E. C., andShackel ford, T. J., 1979, Regiona 1 Hiocenedetilchment faulting and early Tertiary (?)mylonl t iza t ion \'1111pple- Bucks kin-Ra\vll ideI-fountains, southeastern CalifornL:l and "h7estern

Arizona, _~.!. Geologic excursions in the southernCalifornia area, Abbott, P. L., ed., San DiegoState University, San Diego, California.

Davis, C. A., Anderson, J. L., Frost, E. G., andShnckelford, T. J., 1979, A regional gravitysl1(le complex of Tertiary age, eastern San13crnardino COllnty, California, and westernArizona: Geol. Soc. America Abstl~acts \lithPrograms, v. 11, no. 7, p. 410.

Davis, C. A., Anderson, .J. L., Frost, E. G., andShackelford, T. J., 1980, Hytonitization anddetachment faulting in thQ Hhipple-BuckskinRel\-7!1 ide Ylounta.ins terrane, sou theas ternC;:lli[ol~nia dlIll weslerll Arizona, in Tectonicsignificance of metamorpl1ic core~Zornpl'2xcs

of tilQ North AmQrican Cordillera, Crittenden,H. D., Jr., Coney, P. J., and Davis, C. I!., eds.,GQo1. Soc. America HQl1loir 153.

Dillon, J. T., 1976, Geology of tile Chocolate andC':l1'go Huchacho r.IotJntains, southc_as ternmos t

Californin: Univ. of California, SantaBarbara, Unpub. Ph.D. thesis, 575 p.

Frost, E. G., 1931, Hid-Tertiary detachment faultingin the HhipplQ Mtns., Cnlif., and BuckskinHtns., Ariz., nnd its relntionship to the development of major antiforrns and syn[orms: Geed.Soc. of America, Abstracts ""ith Programs, v. 13,no. 2, p. 57.

Frost, E. C., 1981, Hid-TQrtiary stratigraphic recordproduced during rQgional detachment fnulting anddevelopment of extensional arclIes and IJClsins,soutl11,est of the Colorado Plateau Hargin: Geol.Soc. of America, Abstracts ,,'ith Progrmns, v. 13.

Cardner, It. C., Camb1e, J., Danehy, E. A., andCoonrad, W. L., 1960, Geology and mineralresources of Township 11 South, Rnnges 19 and20 Enst, San Bernardino bnse and mQridian,Imperial County, California: Report submittedto Southern Pacific Land Company.

Carner, W., Tanges, S., and Germinario, H., 1982,Detachment faulting and mineralization in thesouthern Tl'igo l'Iountains, Yuma County, Arizona:this volume.

lladley, J. B., 1942, ~~nganese deposits in thePaymaster Hining District: U.S. GeologicalSurvey Bull. 93l-S, p. 459-473.

Hamilton, H., 1982, Mesozoic and Cenozoic tectonicsof the Big Hnria and RivQrside Nountains: thisvolume.

Hariya, Y., 1980, On the geochemistry and formationof manganese dioxide deposits, ~n PrDceedingsof the second international symposium ongeology and geocllemistry of manganese, Barentso~

I. H., GrassQlly, G. Y., eds., E.Schweizerbart'sche Verlangsbuchandlund,Stutgard, Cermany.

!laxel, C. B., 1977, The Orocopia Schist and theChocolate Hountain thrust, Picncho-Peter Kanej\lountains <:-lrea southeasternmost California:University of California, Santa Barbara, Ph.D.thQsis, 277 p.

lIaxel, G. ll., and Dillon, J. T., 1978, The PelonaOrocopia Schist and Vincent-Chocolate Hountainthrust system, southern California, in Ho,"ell,D. G., and HcDougall, K. 1\., eds., Hesozoicpaleogeogrnphy of the western United States;Pacific Section, Society Econ. Paleo. and11ineral., p. 453-469.

Jenkins, O. P., 1943, Hnnganese in California: Calif.Div. Mines and Geology, Bull. 125, p. 61-62.

Jenkins, O. P., 1953, Californin Journal of Ilinesand Geology, v. 49, no. 1 and 2, p. 114-121.

Jennings, C. W., 1967, Salton Sea Sheet, CQologicmap of California, California Div. of Hines andCeol., scale 1:250,000.

Keittl, S. n., 1978, Paleosubduction geometries inferred froul Cretaceous and Tertiary magmatic patterns in sout]lwestern Nortll AUlerica, Geoloovv. 7, p. 515-521. 0, ,

Keith, S. B., 1981, Evidence for late Laramide south"lJestern vergent underthrusting in south\\/esternCa 1 lfornia, sou thern Arizona, and sou tlT\·n:s ternI1e\\T Nexico: Geol. Soc. of America, Abstracts\~ith Programs, v. 13.

Keith, S. B., 1981, Lnramide structural, magmatic,and metallogenic evolution of southeastCalifornia, southern Arizona, and southwesternNew HQxico: Ceol. Society of America, Abstracts\~it]l Programs, v. 13.

Frost, E. G., Hartin, D. L., and Krummenacher, D.,1982, Hid-tertiary detachment faulting insoutln'Jestern Arizona and southeastern Californiannd its overprint on the VincQnt-Orocopia thrustsys tern: Ceol. Soc. 0 fArner ica Abs t rac t s 'h7i tilPrograll1s, v. 14.

GamblQ, J., Danehy, E. A., and Gardner, H. C.,compilers, 1959, Areal economic geologic map:Southern Pacific Land Con~nny, Scale 1:62,500.

310

Krauskopf, K.2nd ed.:

p. 698.

ll., 1979, Introduction to Ceochemistry,NeIC York, HcGrm·}-Hill, Inc., -

Logan, B., Hirsch, D., 1982, Geometry of detachmentfaulting and dike emplacement in the centralCas tIe Dome 1louD tains, Yum.a Coun ty, Arizona:this volume.

Martin, D., 1982, K-Ar geochronology of the BigHaria 1,10 un tains Terrane., Riverside Coun ty,California: this volume.

Morton, P. lZ., 1966, GeologIc map of ImperIal County,California. shoHing mines and p.lineral deposits,California Div. of Hines and Gc,ol., scale

1:125,000.

Horton, P. K., 1977, Imperial County report i/7,Calif. DIv. of Nines and Geology.

Mueller, K., Frost, E. G., Haxel, G., 1982, Detachment faulting in the lfuhawk Hountains, YumaCounty, Arizona: tllis volume.

Otton, J. K., and Dokka, R. K., 1981, The role ofstretch folding in crustal extensIon in theHojave and Sonoran Deserts: A low strain ratemode: Geol. Soc. of America Abstracts withPrograms, v. 13, no. 7, p. 524.

Pridmore, C., Craig, C., and Hintzman, K., 1982,Upper-plate structure and sedimentology of theBaker Peales area, Yuma County, Arizona: tl1isvolume.

Rehrig, W. A., and Reynolds, S. J., 1980, Geologicand geochronologic reconnaissance of a north1;vest-trending zone of metamorphic complexes insouthern Arizona, in Tectonic significance ofmetamorphic core complexes of the HorthAmerican Cordillera, Crittenden, H. D., Jr.,Coney, P. J., and Davis, G. H., eds., Geol.Soc. America Hemoir 153.

Rehrig, W. A., Shafiqullah, I!., and Damon, P. E.,1980, Geochronology, geology, and listric normalfaulting of the Vulture llountains, llaricopaCoun ty, Arizona, _~ Studies in Hestern Arizona,Jenney, J. P., Stone, C., eds., Arizona Geol.Soc. Digest, vol. XII.

Ridenour, J., 1982, llineral potential of the proposed Whipple lloun tains wilderness area,San Bernardino County, California: thisvolume.

Roy, S., 1969, Classification of manganese deposits,in Acta Hineral. Petrogr., Szeged., v. 19,no. 1, p. 67-83.

Shackelford, T. J., 1976, Structural geology of theRawhide llountains, Hohave County, Arizona:Unpub. Ph.D. dissertation, University ofSouthern California, Los Angeles, California,175 p.

Shackelford, T. J., 1980, Tertiary tectonic denudation of a Hesozoic-Early Tertiary (?) gneisscomplex, RalJhide Hountains, western Arizona:Geology, v. 8, no. 4, p. 190-194.

Weaver, B., 1982, K/Ar geochronology of the Trigol'[oulltain detachment terrane, Yuma, Arizona:unpub. llasters thesis, San Diego StateUniversity, San Diego, California.

311

Wedepohl, K. H., 1930, Geochemical behavior ofmanganese, in Proceedings of the Second International Sy~~osiurn on Geology and Geochemistryof 1langanese, Darentsov, I. N., Grasselly,G. Y., eds., E. Schweizerbart'scheVerlallgsbnchandlund, Stutgard, Germany.

POSSIBLE RELATIONSHIP BETHEEN HIOCENE CRUSTAL EXTENSION/DETACHHENT FAULTING AND THE DEPOSITION OF THE

TOLBARD FANGLOHERATE IN THE HIDliAY AND HESTERNPALO VERDE 110mlTAINS, U1PERIAL COUNTY, CALIFORNIA

Hichael R. Jorgensen, Christopher J. Natenstedt and Philip N. TrumblyDepartment of Geological Sciences


Abstract

Analysis of paleocurrents and clast compositionsof tHO exposures of the Hiocene Tolbard Fanglomeratehas indicated a Hiocene topography similar to ,,,hatexists today in the 11idHay llountains and Palo Verdel10untains area, southeastern California. The topographic configuration of the area is thought to havebeen produced by folding associated Hith Tertiarycrustal extension/detachment faulting. The depositional facies of the Tolbard Fanglomerate may haverecorded a significant history of the development offolding and crustal extension/detachment faultingduring the 11io.cene.

Purpose and Introduction

The purpose of this paper is to discuss thepossible relationship betHeen Tertiary crustalextension/detachment faulting and Hiocene paleotopography as inferred from depositional facies ofthe Tolbard Fanglomerate in the HidHay l10untainsand the ,,,estern Palo Verde l1ountains. This discussion is an addendum to the preceeding paper, HidTertiary Detachment Faulting and lbnganese l1i~lization in the 11idHay l1ountains, Imperial County,California by Berg, Leveille and Geis. Analysis ofpaleocurrent data and clast composition is used todefine the l1iocene paleotopography. Data Hascollected from exposures of the Tolbard Fanglomeratein the HidHay l10untains (Hadley, 1942) and HesternPalo Verde l10untains (l1orton, 1977) of southeasternCalifornia (Figure 1).

The Tolbard Fanglomerate in the Hid,,,ay HountainsHas first described and named by Hadley (1942) in aninvestigation of the Paymaster l1ining District.Horton (1977) correlated the 'Post-andesite RedBeds' in the l1idHay l10untains Hith fanglomerate inthe Hestern Palo Verde l1ountains. The TolbardFanglomerate is considered to be correlative Hiththe Bear Canyon Fanglomerate described by CroHe,CroHell and Krummenacher (1979) and Dillon (1975).Dillon (1975) considers the Bear Canyon Fanglomerateto be correlative Hith the Coachella FanglomerateHhich is offset by the San Andreas fault system.

Lithologic Units

The principal lithologic units Hi thin thel1idHay Hountains area consist of crystalline,volcanic and sedimentary rocks. As described byBerg and others (1982), the volcanic and sedimentaryrocks are separated from the underlying crystallinerocks by a lOH-angle normal fault, or detachmentfault, Hhich is inferred to be continuous Hithdetachment faults on a regional basis. In thelOHer plate a zone of chloritized metasedimentaryrocks is seen at the detachment fault. The upper

313

plate is comprised of Tertiary volcanic rocks overlain and in fault contact Hith the Miocene (?)Tolbard Fanglomerate. Further discussion of theregional stratigraphy is presented in Berg and others.

The Tolbard Fanglomerate in the HidHay Hountainsconsists of reddish broHn, well indurated alluvialfan ~eposits. The fanglomerate covers approximately3 km Hith close to 300 m (Hadley, 1942) of exposedsection. These deposits are poorly stratified,laterally discontinuous, interbedded amalgamateddebris flolls and sheetflood deposits. The debrisfloHs range in thickness from 15 em to 2 m andcontain clasts averaging 3 to 15 em, Hith rareclasts up to 1 m in diameter. These clasts arepredominantly andesite, floH-banded rhyolite,trachyte, banded gneiss, dacite, chloritized metasedimentary rocks, and pegmatite. Clasts are oftencovered Hith desert varnish making identificationtedious. The deposits have a coarse to very coarsesand matrix composed of quartz, feldspar, lithicfragments and clay minerals. Units are cemented bysilica, iron oxides and locally by secondary calcite.The ungraded to inversely graded debris-floHdeposits are matrix supported.

----,,I

Peter Kane MinI.

.0v',0 !5Mi

~' 5 Km. ','1- ~Pa[,ocurrlnIDlreelion.

Figure I Locallon ond Paleocurrent Map

Conclusions

Figure :3 Imbrication for Ihe Block Hi IIs/ Polo VardeMounlains 29 Doto Points

NI

Is

Imbrication49 0010 Points

Rose Diagrams for Ihe Midway Mounloins

NI

w-

IS

Channel Axes24 Data Points

Figure 2

The characteristics of the Tolbard Fanglomeratemay reflect tectonic events related to Tertiarycrustal extension and proposed detachment faultingas seen in southwest Arizona. A well developeddetachment fault of mid-Tertiary age is exposedjust east of this study area in the Trigo llountainsof Arizona (Garner and others, 1982). A similarfault is present in the study area (Berg and others,1982), indicating that this portion of southwesternArizona and southeastern California was affected bymid-Tertiary crustal extension. On a regionalscale, this mid-Tertiary deformation produced largescale rotation of Tertiary and crystalline rocksabove the detachment fault (Davis and others, 1979;Rehrig and Reynolds, 1980; Reynolds and Rehrig,1980). The presence of such tilted rocks in nearlyall of southwestern Arizona and southeasternCalifornia (Eberly and Stanley, 1978; Dillon, 1975;Cr<me and others, 1979; !lorton, 1977) probablyindicates that much of this region was extended inmid-Tertiary (!.fiocene) time.

Oligocene to early Hiocene volcanic rocks (Crmveand others, 1979). The leucrogranite, biotitegneiss and muscovite schist clasts may have beenderived from the Little Chuck,valla Hountains, whereextensive outcrops of metasedimentary and graniticrocks are exposed.

Conicident with this extension appears to havebeen the growth of major antiformal-synformal foldstructures (Frost, 1981, 1982; Otton and Dokka, 198~

Cameron and Frost, 1981; Otton and Frost, 1981)that are responsible for the present topographicalconfiguration of this terrane. These folds weredeveloping during the same time as detachmentfaulting and associated rotation of upper platerocks over a several million year period. Thestratigraphic record of the upper plate, therefore,records the tectonic history of the fault blocksthrough time. The Tertiary section is thus apreserved record of the strain history of midTertiary crustal extension in this region.

The clast suite consists of rhyolite, vesicularbasalt, scoria, andesite, leucogranite, biotitegneiss and muscovite schist. The volcanic clastsmay have been derived from the Black Hills or theLi ttle Chucbvalla Hountains, both of ''lhich contain

The sheetflood deposits are comprised of mediumto Coarse grains of quartz, feldspar and lithicfragments; cemented by silica, iron oxides andcalcite. Sedimentary structures, other than planarlaminations, are rare; but imbrication, small-scalechanneling, minor tabular cross-bedding andoccasional mud cracks are present. The clastpopulation in these deposits is similar to thatseen in the debris-flow deposits.

Two source areas may be indicated by paleocurrent dispersion, overall fining/thinning andclast population trends seen in the TolbardFanglomerate in the Hidway Hountains. Eastwardfining and thinning of the debris-flow deposits isaccompanied by a decrease in the percentage ofgneissic and chloritized metasedimentary rockclasts, suggesting a source terrane in the area nowoccupied by the Hidway t-fountains. North''lest tonortheast paleocurrent directions, derived fromimbrication and channel axis orientations, infer asource terrane to the south. Rose diagrams wereconstructed from forty-nine imbrication measurementsand twenty-four channel axis orientations (Figure 2).The distal southern source terrane may be in thearea of the Peter Kane Hountains, or may have beeneroded away, then covered by Pliocene alluvium inthe Vinagre \,ash area. Gneissic clasts found in thenorthwest-flowing sheetflood deposits may haveoriginated in the southern source area, or may beincluded due to reworking of the ''les terly deriveddebris flmvs.

Occasionally, trough and tabular cross-stratification can be seen in some of tho sandy sections.The dominant sedimentary structures are parallellaminations of sands and imbrication of conglomerateclasts. Clast imbrications in the sheetflood andbraided stream deposits indicate an east to southeast flow direction (Figure 3) corroborated bysoutheast-northwest orientation of channel axes.The southeast flow direction indicates a sourceterrane to the northwest.

Eighteen miles to the northwes t of the Hid"lay!.fountains, a section of the Tolbard Fanglomerat cropsout in the western Palo Verde lfountains (Horton,1977). This section consists of debris flow,braided stream and sheetflood deposits. Thefanglomerate is exposed over a 4 km2 area, ,vith amaximum relief of G m. lfost contacts are covered byalluvium; however, where contacts can be observeddirectly, a normal fault separates the TolbardFanglomerate from the other units. Sedimentationunits are laterally discontinuous, ranging inthickness from 15 em to 2 m. Ortho and paraconglomerates are dominant volumetrically over sandstoneunits. Generally, the paraconglomerate is ungradedor inversely graded; and is interpreted as debrisflow deposits. Usually, orthoconglomerates fineupward to coarse or medium sandstone, except wherethe beds are a algamated. Sandstones are medium tovery coarse, poorly sorted, subangular arkosiclitharenites, similar to the sheetflood deposits inthe Hidway !fountains. Both sandstones and orthoconglomerates represent braided stream and sheetflood deposits.

314

IHthin this study area, sediments from thepaleohighlands in the Little Chuckwalla/Black Hillsarea were transported to the southeast. Paleohighlands in the Peter Kane/Hid\~ay !1ountains areasupplied sediments to the paleodrainage flowing tothe northwest. The convergence of these paleodrainages may indicate a Hiocene paleobasin in thearea now occuppied by the lower Milpitas Wash.Therefore, Miocene paleotopography of the studyarea probably resembled the existing topography andwas probably produced by the Tertiary extensionaldeformation. As the Tolbard Fanglomerate sedimentswere shed off the Miocene paleohighlands, thedepositional facies may have recorded a significanthistory of the development of Hiocene folding andcrustal extension/detachment faulting.

Acknowledgements

He \~ould like to thank Neil Hack, Lindee Berg,Pattie Geis, and Greg Leveille for sharing theirthoughts, criticisms and company. We wouldespecially like to thank Eric Frost for hisinsights and critical review and Kathy Jessup andPia Parrish for the use of their typing skills. Wewould also like to thank Enos Strawn for draftingthe maps and figures.

REFERENCES CITED

Berg, L., Leveille, G., and Geis, 1'., 1982, l1idTertiary detachment faulting and manganeseminerali zation in the l1id\~ay Hountains,Imperial County, California: this volume.

Cameron, T.E., and Frost, E.G., 1981, Regionaldevelopment of major antiforms and synformscoincident with detachment faulting inCalifornia, Arizona, Nevada, and Sonora: Geol.Soc. of America Abstracts with Programs, v. 13,no. 7.

Crowe, B.E., Crowell, J.C., and Krurnrnenacher, D.,1979, Regional stratigraphy, K-Ar ages, andtectonic implications of Cenozoic volcanicrocks, southeastern California: Am. Jour.Science, v. 279, p. 186-216.

Davis, G.A., Anderson, J.L., Frost, E.G., andShackelford, T.J., 1980, Mylonitization anddetachment faulting in the Ifhipple-BuckskinRawhide Hountains terrane, southeasternCalifornia and western Arizona, in Tectonicsignificance of metamorphic core~omplexes ofthe North American Cordillera, Crittenden, M.D.Jr., Coney, P.J., and Davis, G.H., eds., Geol.Soc. America Memoir 153.

Dillon, J.T., 1976, Geology of the Chocolate andCargo Huchacho Mountains, southeasternmostCalifornia: Univ. of California, SantaBarbara, Unpub. Ph.D. thesis, 575 p.

Eberly, L.D., and Stanley, T.B., 1978, Cenozoicstratigraphic and geologic history of southwestern Arizona: Geological Society ofAmerica Bulletin, v. 89, p. 921-940.

Frost, E.G., 1981a, Mid-Tertiary detachment faultingin the Whipple Htns., Calif., and Buckskin Mtns.Ariz., and its relationship to the developmentof major antiforms and synforms: Geol. Soc. ofAmerica, Abstracts with Programs, v. 13, no. 2,p. 57.

Frost, E.G., 1982, Structural style of detachmentfaulting in the Ifhipple Mountains, California,and Buckskin Mountains, Arizona: ArizonaGeological Society Digest 15.

315

Frost, E.G., and Otton, J.K., 1981, Mid-Tertiarystratigraphic record produced during regionaldetachment faulting and development ofextensional arches and basins, southwest of theColorado Plateau lfargin; Geol. Soc. ofAmerica Abstracts with Programs, v. 6, p. 197.

Garner, W., Tanges, S., and Germinario, N., 1982,Detachment faulting and mineralization in thesouthern Trigo Hountains, Yuma County, Arizona:this volume.

Hadley, J.B., 1942, !1anganese deposits in the Paymaster Mining District, Imperial County,California; United States Geological SurveyBulletin, #93lS, p. 459-473.

!1orton, P. K., 1977, Geology and llineral Resources ofImperial County, California; CaliforniaDivision of Hines and Geology, County Report117, 104 p.

Otton, J.K., and Dokka, R.K., 1981, The role ofstretch folding in crustal extension in theMojave and Souoran Deserts: A low strain ratemode: Geol. Soc. of America Abstracts withPrograms, v. 13, no. 7, p. 524.

Rehrig, W.A., and Reynolds, S.J., 1980, Geologicand geochronologic reconnaissance of anorthwest-trending zone of metamorphiccomplexes in southern Arizona, in Tectonicsignificance of metamorphic core complexes ofthe North American Cordillera, Crittenden,M.D., Jr., Coney, P.J., and Davis, G.H., eds.,Geol. Soc. America Hemoir 153.

Reynolds, S.J., and Rehrig, W.A., 1980, Hid-Tertiaryplutonism and mylonitization, South Mountains,central Arizona; Geological Society of America,Hemoir 153.

GEOLOGIC FRAMEWORK OF THE CHEMEHUEVI MOUNTAINS, SOUTHEASTERN CALIFORNIAli

Barbara E. JohnU.S. Geological Survey

345 Middlefield RoadMenlo Park, California 94025

ABSTRACT

The Chemehuevi Mountains are a part of an Oligocene(?) and Miocene terrane of detachment faul ti ngthat extends along the Colorado River trough fromsouthern Nevada to Sonora, Mexico. The ChemehueviMountains expose at least two allochthonous plates andan underlying autochthon.

In the Chemehuevi Mountains, an upper plate consisting of Precambrian(?) gneissic and granitic rocksis overlain, both nonconformably and in fault contact,by a tilted Tertiary section consisting of volcanicrocks, monolithologic breccia, conglomerate, and sandstone. These rocks are truncated abruptly downward bya regionally developed decollement, the Chemehuevidetachment fault, Which encircles the range. Thisfaul t may be continuous under Tertiary and (or) Quaternary cover with the Whipple Mountains fault, whichlies about 20 km to the south and has large butunknown separation. Structurally below the Chemehuevidetachment faul t lies the lower ,,:ate, separated 1-2km along a deeper decollement, the Mohave Wash fault,from the 1Ll1derlying autochthon. Both the Chemehueviand Mohave Wash faul ts are Tertiary in age. In thecase of each faul t the upper plate moved northeastrelative to the lower plate. The autochthon and lowerplate consist of a Mesozoic(?) calc-alkaline, metaluminous to peraluminous plutonic suite. The Mesozoic(?) plutonic suite exhibits crude concentricalzoning; zones are progressively younger and more felsic toward the center of the intrusion, suggestingthat the entire suite is cogenetic. Homblende- andsphene-bearing phases of the suite are metaluminousand typically form the outer margin of the intrusion,against Precambrian ( ?) gneisses. The youngest andmost leucocratic phases of this suite are muscovitebearing and locally garnetiferous granite and granodiorite that form the central part of the intrusivesui te. The latter chemically resembles the belt ofperaluminous granitoids that is located along the western margin of the Precambrian 'craton in the NorthAmerican Cordillera.

Locally, the rocks of the au toch thon and lowerplate record three episodes of mylonitic deformation.The oldest event is only rarely preserved in orthogneiss inclusions in the plutonic suite and has noconsistent orientation. A later, regionally developedsubhorizontal mylonitic foliation and penetrative subhorizontal northeast-trending mineral lineation wasimposed on early crystallized phases of the plutonicsuite, and on Precambrian(?) gneiss. Thin, discontinuous zones of variably oriented mylonitic gneiss andmicrobreccia related to Tertiary detachment faul tingcut all rock types in the autochthon and lower plate.Precambrian (?) and Tertiary rocks of the upper plateshow only brittle deformation.

317

INT RODUCT ION

The Chemehuevi Mountains and adjacent areasstraddle the Colorado River along Topock gorge and area part of the Oligocene(?) to Miocene detachmentfaulting terrane that extends along the Colorado Rivertrough from Nevada to Mexico. The Chemehuevi Mountains form an inclined horseshoe shape, one that isopen and plunging to the northeast. The northern,western, and southern ridges of the horseshoe riseabruptly as much as 680 meters above the surroundingvalleys, forming steep range fronts. The easternChemehuevi Mountains form a gently inclin8d surfacesloping east to the Colorado River. Although the rangehas a long complex history of deformation, the character of this topography is controlled by wa~ped, multiple Tertiary low-angle faults that both flank and cutthrough the range. Erosion of the extensively brecciated crystalline rocks between these faults has produced the topography in the Chemehuevi Mountains.

A pioneering reconnaissance geologic study by theSouthern Pacific Land Company established the distribution of major rock types and the presence of Tertiary 10'n-angle faul ts in the Chemehuevi Mountains(Coonrad, 1960). Coonrad described the ChemehueviMountains as a "large resistant mass which is vi rtually surrounded by thrusted rocks of various types"(p. 20), and indicated a Tertiary age for the faultmovement. Geologic mapping by Coonrad and Collier(1960) delineated a large pluton that underlies thecore of the Chemehuevi Mountains; they tentativelyassigned a Precambrian age to it.

Drawing on her work in the Whipple Mountains,Terry (1975) suggested that the low-angle faults inthe Chemehuevi Mountains are Mesozoic thrusts. Shenoted that the faults are underlain by related mylonitic rocks having an east-northeast trending lineationdefined chiefly by elongate quartz grains. Both Terryand Coonrad speculated that the Chemehuevi Mountainsthrust may be equivalent to a similar fault in theWhipple Mountains to the south (Fig. 1). Davis andothers (1980) reinterpreted these faults as extensional faults, and considered the Chemehuevi Mountainsto be part of a detachment terrane extending from nearLas Vegas, southward along the Colorado River troughand eastward into southern Arizona. This detachmentterrane was shown by Davis and others (1980) to consist of mountain ranges which are underlain by a subhorizontal Oligocene(?) to middle Miocene decollementthat has been warped and domed. The decollement separates a footwall consisting of Precambrian(?) andMesozoic to early Cenozoic igneous and metamorphicrocks, including mylonitic gneisses, from a hangingwall upper plate consisting of heterogeneous rocks ofall ages.lIThis report is preliminary and has not been reviewedfor conformity with U.S. Geological Survey edi torialstandards and stratigraphic nomenclature.

In the present st udy, three struc tura 1 plates,sepa"ated by two late Tertiary low-angle detachmentfaul ts, have been recognized in the Chemehuevi Mou!1tains. The autochtho:1 of the Chemehuevi Mountains, asused here, refers to the structurally deepest exposedgroup of rocks in the range; it would be considered"lower plate" in the sen.'le of Davis and others (1980).The rocks of the Chemehuevi Mountains are3. '"lave beendivided into two structural assemblages, based onlithology and structural position. The structurallydeepest 3.ssemblage consists of crystalline rock th3.tm3.kes up the Chemehuevi Mountains autochthon and overlyi ng lower plate. The au toch thon and lower plate ar'esep3.rated by the t-lohave \-lash detachment fault, an irregular, locally subhorizont3.1 det3.chment fault with 1to 2 kID separation. The second assemblage occurs inthe upper plate and consists of crystalline rocksoverlain by t-liocene sed imentary and volcanic rocks.The higher Chemehuevi det3.chment faul t of Cameron 3.ndothers (1981) separates the upper and lower plates.Neither the crystalline rocks of the upper plate northe Miocene rocks have sources in the lower plate orautochthon. Lying nonconformably above this "upperplate" assemblage are undeformed post-detachment Pliocene and Quaternary sedimentary rocks, whose ageplaces a mini~um age on detachment faulting.

This paper dis cusses major struc tures and rockunits in the two assemblages, with particular emphasison the assemblage below the Chemehuevi detachmentfaul t. The following descriptions are drawn from ongoing studies by the author. Complementa"y structuraland stratigraphic studies of rocks above the Chemehue vi detachment faul t are being carried out concurrently by Cameron (this volume) and Pike and Hansen(this volume).

Figure 1. Location map of theChemehuevi Mountains and adjoiningregion. Mountain ranges from northto south: H=Homer, D=Dead, Sa=Sacramen to , Pi=Piute, OW=Old Woman,Ch=Chemehuevi, W=Whipple, Mo=Mopah,T=Turtle, I=Iron. Towns: N=Needles, LHC=Lake Havas u City.

318

STRUCTURAL ASSEMBLAGE 1 AUTOCHTHON AND LOWER PLATE ROCKS

Crys ta11 i ne rocks 0 f the au to ch thon and lowe rplate of the Chemehuevi Mountains occupy the core ofthe range and have been subdivided into older crystalline rocks of uncertain age and the Mesozoic( 7) plutonic suite of Chemehuevi t-lountains that intrudedthese older rocks. The older crystalline rocks locally include small plutons, dikes and sills believed tobe derived from the Chemehuevi Mountains plutonicsui te.

Older Crystalline Rocks

The older crystalline rocks of structural assemblage 1 consist of three u~its (not differentiated onthe geologic map of Fig. 2). These are in order ofage a layered gneiss unit, which occurs in the northern, central and souther:1 part of the range in boththe autochthon and lower plate; an injection complex,which occurs in the northwestern part of the autochthon and northeastern part of the 10lVer plate; and aleucocratic gne iss and amphibolite complex that cropsout along the south flank of the Chemehuevi Mountains.These units lVill be discussed in order from oldest toyoungest, insofar as relative ages can be determined.

The oldest unit in the autochthon and lower-platecrystalline complex is a strongly foliated, variablymylonitized group of layered gneisses of Precambrian (7) age that crops out in the central and easternChemehuevi Mountains (Fig. 2). The layered gneissunit has a shallow dipping, north- to northeast-striking foliation, and a locally developed subhorizontalmylonitic lineation. Equivalent rocks in the north

10 20km

Figure 2. Generalizedgeologic map of theChemehuevi Mou~tains,

California and Arizona.

o Quaternary deposits

Fe"l Tertiary sedimentary

o basaltic plug

Chemehuevi Mountains plutonic suite

2

km

4 6

garnet granite, granite and porphyritic granodiorite

~U:..:J granodiorite and quartz monzodiorite (Kgd and Kd)

(Kgg, Kg and Kpg)

~~

lS]

Precambrian igneous and metamorphic rocks (upper plate)

Precambrian gneiss (autochthon and lower plate)

contact

fault

319

~ Chemehuevi detachment fault

,.v Mohave Wash fault

Figure 3. Ternary plot showing modal quartz (Q),alkali feldspar (A), and plagioclase (P) compositionsfor the plutonic suite of Chemehuevi Mountains. Symbols for intrusive phases are Kd=quartz diorite toquartz monzodiorite; Kgd=granodiorite; Kpg=porphyriticgranodiorite to monzogranite; Kg=muscovite bearinggranodiorite to monzogranite; Kgg=garnet monzogranite.Petrographic classification according to Streckeisen(1973) .

western pa~t of the range show an older(?), steep,northeast-striking foliation, with no associated lineation. The layered gneiss u'1it consists of biotitebearing quartzo-feldspathic gneiss and subordinatecoarse pegmatite, amphibolite, and recrystallized(?)gabbro/diorite. The gneiss is fine- to coarse-grainedand contains plagioclase, alkali feldspar, quartz, andmuscovite-rich layers interleaved with biotite, withlesser hornblende and ga~net.

Along the northwestern boundary of the ChemehueviMount3.ins the autochthon consists of an injectiongne iss or in trusive complex (Coonrad, 1960), characterized by subvertical sills and concordant plutons asmuch as 1 km wide separated by narrow screens of orthogneiss and, locally, layered gneiss. The oldest rockin this intrusive complex is stronR;ly foliated, lightcolored, biotite-bearing orthogneiss characterized bybiotite clots containing rare hornblende and tabulargray alkali feldspar. The orthogneiss was intruded bygneissic aplite and pegpl3.tite dikes. Contact relations between the layered gneiss UYJit 3.nd gneiss ofthe injection gneiss units are gradational, with sillscharacteristic of the injection gneiss complex locallyintruding the layered gneisses. These sills are concord3.nt bod ies (up to 4 km by 1 km) of horn blenderich, quartz monzodiorite and quartz diorite that locally intrude both the injection g'1eiss and bandedgne iss complexes, and are considered to be early-crystallizing phases of the Chemehuevi Mountains plutonicsuite. Foliated, northeast-trending subverticalquartz monzodiorite to granodiorite sills of the plutonic suite int~ude the orthogneisses within thenorthern and western area of the injection gneiss complex. Locally these sills compr ise as much as 80% ofthe outcrop.

A 10 35

Q

65 90 p

Lowlands along the south flank of the ChemehueviMountains expose a complex of leucocratic gneiss,amphibolite, and fine-grained, laminated, biotitequartzo-feldspathic metasedimentary(?) gneiss. Layersof well-foliated amphibolite, leucocratic graniticgneiss, and gneissic pegplatite are commonly interleaved with the leucocratic gneiss. Intrusive intothis complex is a distinctive garnet-spotted leucogneiss, characterized by linear trains of garnet in amedium-grained matrix of plagioclase, alkali feldspar,and purple-gray qU3.rtz. Texturally homogeneous, leucocratic, fine- to medium-grained, subequigranular,biotite qua~tz monzonite intrudes the gneissic U'1itsdescribed above and has fine-grained, chilled(?) margins against them. Foliated hornblende-biotite quartzdiorite to quartz monzodiorite, which also locallyintrudes this complex, may be an early mafic phase ofthe Chemehuevi Mountains plutonic suite. The rocksabove the MOhave Wash faul t in the lower plate on thesouth flank of the Chemehuevi Mountains resemblegne issic rocks described by Davis and others (1980)occurring below the Whipple detachment fault but abovethe mylonite front in the western Whipple Mountains.These rocks also resemble gneissic rocks in the MohaveMountains, Arizona, which occur at a structural levelabove the Chemehuevi and Whipple detachment faults.

Plutonic suite of Chemehuevi Mountains

Intruding the Precambrian(?) crystalline complex,and underlying most of the southern and central Chemehuevi Mountains, is the Cretaceous(?) plutonic suiteof Chemehuevi Mou'1tains, first described by Coonrad(1960) as a quartz diorite body. Remapping of thearea demonstrates that the interior of the range isunderlain by a concordant, irregularly zoned plutonicsui te, composed of five intrusive phases. This plUtonic suite spans a wide compositional range from

320

horn blende- and sphene-rich qua~tz diorite, throughquartz monzodiorite and granodiorite, to leucocratic,garnet bearing, bioti te-muscovi te monzogranite (Fig.3). These intrusive phases are crudely concentric,the younger and more felsic rocks inward. This zonation, taken with the overlap in modal compositions,suggests that the entire suite is cogenetic.

Intrusion of the plutonic suite of ChemehueviMountains appears to have been controlled by the steepnortheast-trending Precambrian(?) foliation in theautochthon and lower plate in the northern part of therange, and by a subhorizontal Mesozoic(?) fabric inthe autochthon and lower plate in the southern andeastern Chemehuevi Mountains. The southeastern marginof the plutonic suite is well defined; the' northeastern margin is obscured by Mesozoic(?) mylonitizationand deformation caused by Miocene detachment faulting.

The oldest intrusive phase is variably foliatedhornblende-biotite quartz diorite to quartz monzodiorite with 5 to 10 % modal quartz (unit Kd, Fig. 3).This phase crops out as small concordant bodies up to1 km by 4 km in plan view that are aligned parallel tothe northeast-striking foliation in the lower platenear Whale Mountain. It is characterized by 18-38%medium-grained, blue-green hornblende and blue-greenbiotite with accessory sphene, magnetite, apatite, andzoned allanite with overgrowths of epidote. Similarrock crops out in the lower plate south of ChemehueviPeak, where it intrudes the leucocratic gneiss andamphibolite complex.

A relatively quartz-poor (20 to 27 % modalquartz), variably porphyritic hornblende-biotite granodiorite (unit Kgd, Fig. 3) locally intrudes and isgradational to the more mafic quartz diorite to quartzmonzodiorite phase. The granodiorite includes small,

Kg~~K99

Kpg Q

equant microcline phenocrysts (up to 1 cm), stubbyblue-green hornblende (4 to 20 %), euhedral biotite (5to 5%), coarse sphene (1 to 3%), and accessary magnetite, allanite, and zircon. The granodiorite is commonly strongly foliated and contains wallrock xenoliths and cognate quartz diorite inclusions. Deformation of the granodiorite appears to have been concentrated near the contacts of this intrusive phase withPrecambrian(?) gneiss. Locally, the granodioritebears a subhorizontal northeast-trending myloniticlineation. Aplite and pegmatite dikes intrude thegranodiorite; these can be traced into the more leucocratic porphyritic granodiorite to monzonite phase ofthe Chemehuevi Mountains plutonic suite.

~ 5.0

i 4.0....o~ 3.0

KdII

•Kgd

60 64 68 128102 (wt. 'l6)

16

0.8 L---JL---J_--'_----'-_----'-_--'-_--'-_--'-_--'

Figure 4. Chemical features of the plutonic suiteof Chemehuevi Mountains: Si02 versus K20 (wt. %) andSi02 versus [A1203/(CaO+Na20+K20)] (molar %). Intrusive phase symbols as in Figure 3.

All phases of the Chemehuevi Mountains plutonicsuite have been affected by detachment faulting. Therocks typically contain variable amounts of epidoteand clinozoisite, chlorite, calcite, sericite, albite,and actinolite, formed as alteration products of primd"'"' igneous minerals during faulting.

Limi ted geochronologic data and li tho logic s~m~

larity to dated plutonic suites in the region suggesta Cretaceous age for the Chemehuevi Mountains plutonicsuite (John, 1981). A K-Ar age of 64.1 ± 2.2 m.y. (R.Marvin, personal commun., 1980) on biotite from thesubequigranular, muscovite-biotite monzogranite (Kg)in the southern Chemehuevi Mountains is a minimum datefor the plutonic suite. Detailed K-Ar isotopic studies by Martin and others (1981) suggest that mineralages are commonly reset below major detachment faul ts

1612

Kgg

~p~@_K~a~ __metalumlnous

68

Si02 (wi. 'l6)

64

peraluminous

60

II

Kd

:.::zo:;;: 1.0 f- -

Kgd

1.2

(greater than 69 wt. %), high and subequal amounts ofNa 20 and K20 (2.8 to 5.0 wt.%), and primary(?) muscovite (Fig. 4). These chemical and mineralogic characteristics resemble those of the inner Cordilleran beltof peraluminous granitoids defined by Miller and Bradfish (1980). However, in the Chemehuevi Mountains theapparent association of peraluminous granitoids witholder, cogenetic hornblende-sphene-bearing metaluminous intrusive rocks is atypical of this belt. Sillsin the Whipple Mountains to the south (Anderson andRowley, 1981) span a compositional range very similarto the Chemehuevi Mountains plutonic suite. Bothsuites have between 60 and 74 wt.% Si02 , 14 to 18 wt.%A1 20

3, 2 to 5 wt.% CaO and 3 to 4.5 wt.% Na20 (Ander

son and Rowley, 1981; John, unpubl. data). Peraluminous granitic rocks west of the Chemehuevi Mountainsin the Old Woman-Piute Mountains (Miller and Stoddard,1980) and in the Iron Mountains (Miller and others,1981) have a similar range in major elements, .butA120

3and CaO are lower for these ranges.

The most voluminous phase of the Chemehuevi Mountains plutonic suite is a variably porphyritic biotitegranodiorite to monzogranite (unit Kpg, Fig. 3). Zonedmicrocline phenocrysts, measuring up to 5 cm in greatest dimension, make up as much as 40% of the volume ofthe rock; they are set in a medium-grained groundmassconsisting of quartz, plagioclase, microcline, andbiotite (5 to 12%) as well as accessory sphene, magnetite, apatite, zircon, allanite, and rare primary(?)muscovite. The porphyritic granodiorite has intrudedthe layered gneiss mit of the older crystalline complex as fine- to medium-grained sills in a lit-par-litarrangement. Borders of this phase against the layered gneiss are fine- to medium-grained, and containrare phenocrysts up to 4 cm long. Inwa>"d, and awayf rom the layered gne iss, these phenocrysts increase inabundance and primary igneous flow struc tures appea>".Contacts with the hornblende-biotite granodioritephase are typically gradational but locally, the porphyritic unit is younger. Medium- to coarse-grained,garnet-bearing, muscovite aplite and pegmatite dikescommonly intrude the porphyritic granodiorite in thecentral Chemehuevi MOQntains.

The most felsic rocks of the Chemehuevi Mountainsplutonic suite occur as dikes and small bodies lessthan 2 km across. Leucocratic, subequigranular, garnet-bearing, muscovite monzogranite (u'1it Kgg, Fig. 3)occurs as small bodies in the central part of theChemehuevi Mountains plutonic suite. This phase hasgradational contact relations with the muscovite-biotite monzogranite, but clearly intrudes the porphyritic granodiorite. Locally, biotite-rich schistoseinclusions with large (1.5 to 3 cm across) red-brownanhedral garnet define a weak igneous foliation. Accessary minerals include euhedral apatite, muscovite0-5%), biotite (less than 1%), garnet (up to 5 mm ingreatest dimension), rare allanite, and zircon; nosphene was seen in this phase.

Medium-grained, subequigranular to porphyritic,muscovite-biotite granodiorite to monzogranite (unitKg, Fig. 3) is the youngest phase of the ChemehueviMounta ins pluton ic suite. The rock weathers to alight tan color and contains microcline phenocrysts upto 1.5 cm in greatest dimension in a gromdmass ofquartz, plagioclase, orthoclase (2-4%), biotite (24%), muscovite (1-3%), magnetite, apatite, zircon,allanite, and rare sphene. Locally, irregular blocksof porphyritic granodiorite occur in muscovite-biotitemonzogranite. Elsewhere, however, contact relationsbetween the two phases are ambiguous.

Preliminary chemical data on samples indicatethat the Chemehuevi Mountains plutonic suite is calcalkaline and metaluminous to weakly peraluminous (Fig.4), and spans a wide range in Si02 content of 59.4 to73.8 weight percent (Fig. 4). The three youngestunits of the Chemehuevi Mountains plutonic suite, Kpg,Kg and Kgg, are all peralQ~inous, with high Si02

321

Dike rocks

Light-colored quartz porphyry dikes form a westnorthwest trending swarm that cuts the Precambrian(?)

--,

@Whale Mtn.

km

Quaternaryundivided

Tertiary 1/

dikes in Kgu

Precambrian 1/

o

contact

~low angle fault

/\....

In the we stern Chemehuevi Mountains, upper-platecrystalline rocks are composed of augen gneiss, amphibolite, orthogneiss and paragneiss, and the graniteporphyry (described above), all of Precambrian(?) age.

monzodiorite inclusions, and minor fine-grained leucocratic orthogneiss. The granite porphyry and associated rocks resemble rocks in the Whipple 110untainsthat Davis and others (1980) pointed out have textural, mineralogic and compositional affinities to rocksof 1.4 to 1.5. b.y. age. The entire suite is cut bycoarse-grained pegmatite dikes and quartz veins.

Intrusive into the Precambrian(?) granite porphyry and gneisses in the western Chemehuevi Mountains isa suite of northwest-trending diabase dikes, which arecharacterized by hornblende intergrown with laths ofplagioclase.

Figure 5. Generalized map shOWing the distributionof dikes in the autochthon and lower plate, westernChemehuevi Mountains. Dikes intrude the ChemehueviMountains plutonic suite (Kgu).

Older crystalline rocks

Rare altered, xenolith-bearing, garnet-bearinghornblende diorite dikes that intrude the older !1ornblende- bioti te granod ior ite ph"lse of the ChemehueviMountains plutonic suite are considered the oldestdikes. One steeply dipping east-west trending dike ofthis type crops out in the southeastern ChemehueviMountains and a second crops out in the northeasternpart of the range. Mafic and ul tr"lmafic xenoliths inthese dikes that range in size from 1 cm to 6 cmacross are commonly associated with coarse hornblendecrystals (as large as 4 cm), in a mediull-grained matrix consisting of hornblende, clinopyroxene, biotite,plagioclase, and garnet.

STRUCTURAL ASSEMBLAGE 2 - UPPER PLATE ROCKS

Crystalline rocks crop out above the Chemehuevidetachment fault in the western Chemehuevi Mountains,and along both sides of the Colorado River (Fig. 2).Grani te porphyry and associated quartz monzonite andquartz monzodiorite are the major rock types in theupper-plate assemblage along both sides of the Colorado River. The granite porphyry is characterized byabundant, coarse grained (1 to 5 cm across) alkalifeldspar phenocrysts set in a medium-grained matrixconsisting of plagioclase, quartz, alkali feldspar,biotite, and hornblende. The phenocrysts are palegray to violet in color and are typically aligned in amoderately- to well-developed primary flow foliation.Included in the granite porphyry are older hornblenderich, variably porphyritic, quartz monzonite to quartz

Dike rocks

Nor theast- and northwest-trending olivine gabbrodikes containing fine-grained plagioclase laths andinterstitial Olivine, magnetite, and secondary greenhornblende are considered to be of intermediate age.Light-gray to tan weathering hornblende-biotite dacite(?) and biotite dacite porphyry comprise the youngest common dikes; they trend north to northwest.Dark colored, porphyritic andesite dikes with plagioclase phenocrysts form a minor intrusive phase; thesedikes have no apparent systematic orientation. In thenorthern Chemehuevi Mountains, west of Whale Mountain,fine-grained hornblende-bearing biotite granodioritedikes intrude both the Chemehuevi Mountains plutonicsuite and Precambrian(?) older crystalline rocks.

in the region. Therefore, the Late Cretaceous minimumage cited above may be substantially younger than theintrusive age of the rock.

Swarms of dikes forming two sets intrude the autochthon and lower plate rocks in the western and central region of the Chemehuevi Mountains (Fig. 5). Thedike-swarms are centered in the Cretaceous(?) Chemehuevi Mountains pluton ic suite; few dikes intrude thePrecambrian(?) older crystalline wallrocks. Whereintrusive relations have been established, northeasttrending dikes appear to be older than those trendingnorthwest. The dikes range in composition from coarsehornblende diorite and olivine gabbro to biotite-bearing dacite porphyry. Both composition and relativeage of the dikes appear to be only casually related toorientation. Dikes vary in thickness from a few centimeters to several meters. Locally, the swarms account for as much as 10% of the volume of rocks withinthe plutonic suite. An orthogonal pattern produced bynorthwest- and northeast-trending dikes prevails inthe southwestern Chemehuevi Mountains, whereas in thecentral and northern parts of the range this patternchanges to predominantly east-west in trend (Fig. 5).

322

rocks and the diabase dikes. These dikes are characterized by resorbed quartz, plagioclase, and alkalifeldspar phenocrysts in a fine-grained groundmass.Dikes of this :,ock type crop out along the westernflank of the Chemehuevi Mou~tains; they may be relatedto 'In epizonal, hornblende-bearing, quartz-rich biotite-monzogranite pluton that is exposed in the southeastern Sacramento Mountains (Fig. 1).

Cenozoic rocks

Sedimentary and volcanic rocks of Tertiary agelie in the upper plate along the flanv.3 of the Chemehuevi Mountains; these rocks have nonconformable andfaul t contacts with the older crystalline rocks anddikes.

Upper plate Tertiary rocks in the southern Chemehuevi Mountains crop out nonconformably above a thinsheet of older crystalline rocks of the upper plate.Volcanic rocks in this area include andesite and dacite flows overlain by rhyolite and olivine-bearingbasal ts. Locally, welded tuff and tuffaceous conglomerate are interstratified within the volcanic section.

The Tertiary section in the eastern ChemehueviMou'1tains is composed of flows of basalt, andesite,and dacite; welded tuff and monolithologic breccia;all overlain by a sequence of we Ided tuff, basal t andthick interstratified conglomerate and sandstone.Contacts of this section with crystalline rocks of theupper plate are low-angle faults. Locally, Tertiaryrocks above the Chemehuevi detachment faul t are overlain in low-angle fault contact by a higher plate ofTertiary volcanic and sedimentary rocks.

POST-DETACHMENT ROCKS

Flat lying marl, clay, silt, and sand of the Pliocene Bouse Formation unconformably overlie structuralassemblages 1 and 2. Younger coarse-grained, locallyderived fanglomerates and terrace deposits of ColoradoRiver gravel consisting of well-rounded quartzite andlimestone pebbles and cobbles dip 2 to 50 into theColorado River trough. The lack of deformation ofthis sequence of sediments suggests a Pliocene upperlimi t on the age of detachment faul ting.

Four small pluglike bodies of vesicular augiteolivine basalt intrude and partially fuse brecciated,chloritized, porphyritic granodiorite of the lowerplate in the west central Chemehuevi Mountains. Thebasalt has a distinctive red-b:'own color that contrasts with the dUll green to brown-weathering dikeswarms of the lower plate and autochthon. The vesicularity of these plugs suggests shallow emplacement,and thus these plugs appear to post-date detachmentfaUlting and its attendant brecciation, chloritization, and tectonic denudation. They may be correlative with the post-detachment olivine-bearing basaltflows intercalated with the upper Miocene fanglomerateof Osborn Wash, a unit that is exposed to the south inthe Whipple and Buckskin Mountains (Davis and others,1980; Carr, 1981).

STRUCTURE

The structural history of the Chemehuevi Mountains is complex and is not yet completely understood.Crystalline rocks in the autochthon and lower platerecord a series of intrusive and ductile deformationalevents that range in age from Precambrian (7) to Miocene. The apparent synkinematic nature of plutonismand mylonitization that has been noted by many workersin the southern Cordillera (Davis and others, 1980;

323

Keith and others, 1980; Miller and others, 1981) isexpressed in the Chemehuevi Mountains as well. Superimposed on the resulting ductile features are brittleand ductile structures related to late Tertiary detachment faul ti ng.

Mylonitic rocks

Locally developed mylonitic fabrics in the Chemehue vi Mountains apparently formed during three deformational events. The oldest mylonitic gneiss in therange is found in the injection gneiss complex in thenorthHestern Chemehuevi Mou'1tains, where it occurs assparse zones in the biotite orthogneiss unit. Theorientation of this mylonitic fabric is va~iable because blocks of the biotite orthogneiss occur as inclusions in Cretaceous granodiorite in this area.These blocks preserve evidence of a Precambrian(7)mylonit izat ion.

A you'1ger mylonitic fabric was imposed on earlycrystallized phases of the Chemehuevi Mountains plutonic suite, in particular on the quartz diorite andgranodiorite intrusive phases. This fabric parallelsthe trend in mylonitic gneiss that is developed regionally throughout southeastern California and western Arizona. Precambrian( 7) gneiss in the easternpart of the range typically has a shallowly-dippingfoliation and a penetrative subhorizontal lineationtrending N400E to N600E. Locally intrusive into thisgneiss are the quartz diOrite to quartz monzodioriteand granodiorite phases of the Chemehuevi Mountainsplutonic suite, both of w\1ich bear a primary igneousfoliation. Near contacts with the Precambrian(7)gneiss, these intrusive phases acquired a weak metamorphic foliation and a similar subhorizontal northeast-trending mylonitic lineation, the latter definedby elongate quartz grains and broken hornblende crystals. Younger plutonic phases lack this fabric andlocally intrude it. The mylonitic fabric therefcreappears to have formed in part during Cretaceous (7)intrusion, as has also been suggested for the WhippleMountains (Davis and others, 1980) and Iron Mountains(Miller and others, 1981), both in California; and forthe Santa Catalina-Rincon-Tortolita Mountains, Arizona(Keith and others, 1980).

Despite the vast region underlain by myloniticgneiss having a consistent subhorizontal northeasttrending lineation, and the large number of currentstudies of these rocks, no satisfactory consistentexplanation for their origin has been proposed. As inthe case of the Whipple Mountains, and in other rangesalong the Colorado River trough, mylonitic gneiss inthe Chemehuevi Mountains having this consistent northeast-trending mylonitic lineation is truncated by theTertiary detachment faults.

The youngest deformational event which producedmylonitic gneiss in the Chemehuevi Mountains was Tertiary detachment faulting. All rock types of the autochthon and 10Her plate of the Chemehuevi Mountainsare cut by variably oriented microfaults and thin ductile shear zones which increase in number toward eachof the two detachment faul ts, the Chemehuevi and Mohave Wash detachment faul ts. At the deepest structural level exposed below the Chemehuevi detachmentfaul t, thin (up to 1 m thick) zones of myloniti cgneiss with well-developed lineation and/or foliationcrop out. Individually, these zones of myloniticgneiss cannot be traced for more than ten meters, butas a group they appear to be transitional upward tomore brittle microfaults. These microfaults aremarked by quartz±chlorite±epidote±calcite±hematitegouge or microbreccia veinlets 5 mm to 1.5 em thick,

the latter consisting of and bordered by siliceousselvages. Where dikes or compositional layering inthe crystalline rocks a~e cut by these microfaul ts andductile shear zones, separations up to tens of centimeters can be measured. No systematic orientation ofthese microfaul ts has yet been recognized. The relationship of these rnicrofaul ts and ductile shear zonesto detac~ent faulting has not yet been determined butthey are clearly concentrated subjacent to the twodetac~ent faults.

Major Faul ts

The most prominent structural feature in the Chemehuevi Mountains is the Tertiary low-angle Chemehuevidetachment fault that flanks the range and separatesupper plate Precambrian(?) to Miocene rocks from lowerplate and autochthonous crystalline r'ocks. Separationalong this faul t is not known because no source ofupper plate rocks has been recognized in the lowerplate or autochthon. The similar> (equivalent?) Hhipple Mountains fault in ranges to the south is inferredto represent tens of kilometers of northeastward r>elative movement of the hanging wall (Davis and others,1980; Carr, 1981).

The Chemehuevi detachment faul t consists of awell-defined planar surface, underlain by a ledge-forming layer of micro breccia and green chloritized breccia measuring several meters to tens of meters thick.Locally, the fault may be marked by a zone of chloritized gouge and dark red clay and silt, rock typesthat are inter layered with relatively U'1a1 tered gneissand granite fragments derived from the lower plate.The remarkable layering may owe its or igi n to gra insorting during fault movement. Cameroon and others(1981) showed that the faul t surface undulated with awavelength of several kilometers and an amplitude ofseveral hundred meters. The Mohave Wash faul t liesstructur-ally below the Chemehuevi detachment fault, inthe interior of the range, and is cut by steep DOr'thwest-trending normal faults. Unlike the str'ucturallyhigher Chemehuevi detachment faul t, in general theMohave Wash fault cuts across the interior' of therange, and only locally is paralleled by the presenttopography. This low-angle faul t has offset plutoniccontacts in the lower plate between 1 and 2 km northeastward relative to the autochthon. Horizontal striae that trend S600 W on the top of the autochthon areexpos ed in a wi ndow along Trampas Wash. In c r'oss section, the Mohave Wash fault is spoon-shaped and opensnortheastward. It dips between 20 and 250

, dependingon underlying rock type. Where the faul t cuts massiveporphyritic granodiorite in the western ChemehueviMountains, it is defined by a moderately north-dippingzone as much as 70 m thick of chloritized, finely comminuted rock. Where it cuts subhorizontally foliatedgneiss of the eastern part of the range, the fault ischaracterized by a thin, subhorizontal ledge-formingmicrobreccia that is sandwiched between two zones ofchloritic breccia. Locally, the lower plate betweent he Mohave Wash and Chemehuevi detachment faul tspinches out, and the upper plate lies in tectonic contact on the autochthon.

Chlorite breccia, used as a general term to describe variably faul ted to locally comminuted rock derived from the autochthon and lower plate, is foundbeneath the Chemehuevi detachment fault, and bothabove and below the Mohave Hash faul t to a thicknessof hundreds of meters. Chlorite and epidote with lesser clinozoisite, sericite and calcite have imparted adistinctive pale green color to rocks in the zone ofchlor ite breccia. Count less small faul ts offset dikesand their country rocks centimeters to tens of meters

324

in the chlor>ite breccia zone.The timing of detach;nent faul ting is poorly con

stra ined in the Chemehuevi Mou11tains. The oldest undeformed sedimentary rocks are those of the PlioceneBouse Format ion whicl-t overl ies the faul ts unconformably, thus placing an upper limit on the age of detachment faulting. Steeply southwar>d dipping Tertiaryvolcanic rocks above the Chemehuevi detachment faultinclude a sphene-bear'ing, blue-sanidine tuff; sanidinefrom the tuff yielded a K-Ar age of 18.1 ± 0.6 m.y.(Howa~d and others, 1982). This age indicates tiltingof the upper plate after that time. The timing ofmovement along the Mohave Wash fault is even less wellconstrained. Rocks above the Chemehuevi detachmentfaul tare noHhere cut by the Mohave \-lash faul t, andlocally upper plate rocks sit in faul t contact Hi ththose of the autochthon. These relations suggest thatthe Mohave Hash fault is either slightly older or contemporaneous Hith the Chemehuevi detachment fault.

SUMMARY

The Chemehuevi Mountains of southeastern California are underlain by a large Cretaceous(?) plutonicsuite that is situated along the eastern limit of voluminous Mesozoic plutonism. Intrusive phases of thissuite form a zoned complex and span a wide compositional range fr'Om metal~~inous quartz diorite to peraluminous garnetiferous granite. Locally, the ea~ly

crystallized phases of the plutonic suite and theirwallrocks bear a regionally-developed subhorizontalmylonitic foliation and penetrative northeast-trendingmineral lineation. The close spatial and temporalassociation of mylonitization and plutonism in theChemehuevi Mountains implies a genetic relationship.Younger, thin ductile shear zones and brittle microfaul ts cut across these mylonites, and are concentrated below the two Tertiary detachment faults that areexposed in and around the flanks of the range.

ACKNOWLEDGEMENTS

The author Hould like to acknowledge field tripsand discussions with Keith Howard, David Miller,LaHford Anderson, Eric Frost, Greg Davis, and VickiHansen, all of whom have contributed tremendously tothe work presented here. Reviews of this paper atvarious stages by Keith Howard, David Miller, andVictoria Todd are gratefully acknowledged.

REFERENCES CITED

Anderson, J. L., and ROWley, M. C., 1981, Synkinematicintrusion of peraluminous and associated metal uminous granitoids, Whipple Mountains, California:Canadian Mineral., v. 19, p. 83-101.

Cameron, T. E., Frost, E. G., and John, B. E., 1981,Development of regional arches and basins andtheir relationship to mid-Tertiary detachmentfaulting in the Chemehuevi Mountains, San Bernardino County, California, and Mohave County, Arizona: Geol. Soc. America Abstr. Programs, v. 13,no. 2, p. 48.

Carr, w. J., 1981, Tectonic history of the VidalParker region, California and Arizona, in Howard,K. A., and others, eds., Tectonic framework ofthe Mojave and Sonoran deserts, California andArizona: U.S. Geol. Survey Open-File Rpt. 81503, p. 18-20.

Coonrad, w. L., 1960, Geology and mineral resources ofTownship 6 North, Range 23 and 24 East, San Bernardino base and meridian, San Bernardino County:Report submitted to Southern Pacific Land Company,. 23 p.

Coonrad, W. L., and Collier, J. P., 1960, Areal economic geology, T5N, R23-24E; T5N, R23-24E; T7N,R23-24E, San Bernardino base and meridian, SanBernardino County: unpublished geologic mappingfor Southern Pacific Mineral Survey, scale1: 24,000.

Davis, G. A., Anderson, J. L., Frost, E. G., and Shakelford, T. J., 1980, Mylonitization and detachment in the Whipple-Buckskin-Rawhide Mountainsterrane, southeastern California and western Arizona: in Crittenden, M. D., Jr., and others,eds ., Cord illeran Metamorphic Core Complexes:Geol. Soc. America Memoir 153, p. 79-129.

Howard, K. A., Stone, P., Pernokas, M. A., and Marvin,R. F., 1982, Geologic and geochronologic \"econnaissance of the Turtle Mountains area, California: west margin of the Whipple Mountains detachment terrane: Geol. Soc. America, this volume.

John, B. E., 1981, Reconnaissance st udy of Mesozoicplutonic rocks in the Mojave Desert Region: inHoward, K. A., and others, eds., Tectonic framework of the Mojave and Sonoran deserts, California and Arizona: U.S. Geol. Survey Open-FileRpt. 81-503, p. 48-50.

Keith, S. B., Reynolds, S. J., Damon, P. E., Shafiquallah, M., Livingston, D. F., and Pushkar, P.D. , 1980, Evidence for mul tiple intrusion 'inddeformation within the Santa Catalina-RincanTortolita crystalline complex, southeastern Arizona: in Crittenden, M. D., Jr., and others,eds, Cordilleran Metamorphic Co\"e Complexes:Geol. Soc. America Memoir 153, p. 217-267.

325

\Martin, D. L., Krummen'icher, D., and Frost, E. G.,

1981, Regional resetting of the K-Ar isotopicsystem by mid-Tertiary detachment faulting in theColorado River region, California, Arizona andNevada: Geol. Soc. America Abstr. Programs, v.13, no. 7, p. 504.

Miller, C. F., and Bradfish, L. J., 1980, An in'1erCordillera.n belt of muscovite-bearing plutons:Geology, v. 8, p. 412-416.

Miller, C. F., and Stoddard, E. F., 1981, The role ofmanganese in the paragnesis of magmatic garnet:an example for the Old Homan-Piute Range, California: J. Geol., v. 89, p. 233-246.

Miller, D. M., Howard, K. A., and Anderson, J. L.,1981, Mylonitic gneiss related to emplacement ofa Cretaceous batholith, Iron Mountains, southernCalifornia: in Howard K. A., and others, eds.,Tectonic framework of the Mojave and Sonoran deserts, California 'ind Arizona: U.S. Geol. SurveyOpen-File Rpt. 81-503, p. 73-75.

Streckeisen, A. L., 1973, Plutonic rocks classifica-tion and nomenclature: Geoti1les, v. 18, p. 26-30.

Terry, A. M., 1975, Thrust faulting with associatedmylonites in the Chemehuevi Mountains, southeastern California: Geol. Soc. America Abstr. Programs, v. 7, no. 3, p. 380.

Co>

~

View of the Hewberry Detachment Fault and its ramp near Big Bend as seen looking southwest toward the Dead Mountains and its ramp.

~lID-TERTIARY DETACilllENT FAULTING IN THE SOUTHEASTERNNEHBERRY MOUNTAINS, CLARK COUNTY, NEVADA

Randall S. MathisDepartment of Geological Sciences

San Diego State UniversitySan Diego, California 92182

ABSTRACT

Field studies in the Newberry Mountains ofsoutheastern Nevada have defined the existence of anOligocene (?) to middle ~liocene lm,,-angle normalfault, referred to as a detachment fault, whichencircles the range. This fault was partiallymapped by Volborth (1973) who interpreted it as aTertiary thrust fault. This detachment fault isconsidered to be part of a regional wave-like faultsurface related to mid-Tertiary extension within thewestern United States. The detachment fault is asubhorizontal surface Hhich has been warped anddomed, producing fault surfaces that dip away fromthe core of the range. This fault is inferred to becontinuous "lith detachment faults to the south inthe Dead l1ountains, to the I"est at Homer Mountain,to the north in the Eldorado Mountains, and to theeast in the Black l1ountains. The upper-plateallochthonous units consist of Precambrian rapakivigranite, containing large potassium-feldsparphenocrysts, Hhich is depositionally overlain byTertiary volcanic rocks. These volcanic rocksappear to be equivalent to the Patsy Mine Volcanicsof Anderson (1978) in the Eldorado l10untains andAlcyone Volcanics of Thorson (1971) in the Blackl1ountains. These volcanic rocks have beeninterpreted as a caldera sequence of late Oligoceneto early ~1iocene age. The lmver-plate complexconsists largely of fine-grained rapakivi granitethat has been assigned a Tertiary age by Volborth onthe basis of fission-track age dating. These rocksare almost certainly Precambrian or Mesozoic,hOHever, and record a Tertiary isotopic resettingevent related to detachment faulting. Striae on thedetachment surface and the southHestward blockrotations of the upper-plate Tertiary volcanic rocksindicate northeastward relative movement of the upperplate. A chlorite-breccia zone (la's to laO's ofmeters deep) exists Hithin the lOHer-platecrystalline rocks, just below the detachment surface.Hithin this brecciated zone, numerous anastomosingfaults exist that indicate varied movement, butcollectively depict a southwest-northeast movement.The direction of motion along these faults indicatesa genetic relationship betHeen them and the detachment fault. The existance of these faults, alongwith their relative motion, suggests that themovement of the upper plate may have been acollective process of slip along the lower-platefaults together "lith that of the detachment fault.

INTRODUCTION

Location and Accessibility

The NeHberry 110untains are a north-trendinggroup of ridges located at the southern tip ofClark County, Nevada. The Newberry Mountains are

327

bound by the Piu te Valley on the Ives t, the EldoradoHountains to the north, the Colorado River to theeast, and the northern Dead Mountains to the south(Figure 1).

A geologic map was made of an area located inthe southeastern Newberry Mountains consisting ofT.32S./R.66. and the lower half of T.3lS./R.66E.~IDBM, Hhich covers approximately 115 km2 (Figure 2).This covers the area from Big Bend, on the ColoradoRiver, to roughly 6 km north of Davis Dam; an arealdistance of 15 km.

The main access roads into the area consist oftHO paved roads: Nevada State Highway 163 Hhich runseast-Hest, and River Front Road Hhich parallels theColorado River on the Nevada side. The access tomore remote areas is via unpaved utility roads,unimproved dirt roads, and jeep trails. The utilityroads have been very Hell maintained, but some ofthe unimproved dirt roads and jeep trails require

BLACKMTNS.

CA

AZ

Figure 1. Generalized diagram shoHing the detachmen t terrane surrounding the NeHberry 11ountains.Ruled pattern designates the study area. Dashedregions are the lOHer-plate cores of the ranges.

N

t~~fN

V

mc:oN

IOOOH.500oBEND

____lIII!i~1==_~2_~~~1lIi3~_c==~4~i 5 mi.

o 2:3 4 5 km.

Figure 2. Geologic map of southeastern Newberry Mountains, Clark County,southeastern Nevada. Abbreviations used are as follolvs: peg = Precambrianrapakivi granite; Tpv = Tertiary Patsy Mine Volcanics; Qs = Quaternary alluvialsediments; NDF = llewberry Detachment Fault.

328

Figure 3. Upper-plate Precambrian rapakivi granite.with their thin plagioclase rims.

the use of a four-wheel drive vehicle. Fortunately,there is some type of road within 450 meters of thedetachment fault outcrops within the entire studyarea.

Purpose and Nethod of Study

Published maps and reports dealing with thegeology of the Newberry Hountains are all of areconnaissance nature. This area, therefore, offersprospects of new information that may be critical fordeciphering the history of detachment faulting in theregion. The present study was undertaken to extendand supplement current knowledge of detachmentfaulting in the northern Colorado River Trough, andmore specifically the southeastern Newberry Mountainsof southern Nevada.

2 The area studied consists of approximately 115km , much of it with rugged terrain, making itimpossible to cover in detail during the availablefield time. Therefore, the resulting map isnecessarily of a reconnaissance nature. Thedetachment fault, however, was mapped in as muchdetail as possible (Figure 2).

Volborth's (1973) geologic map of the areaproved most helpful in locating the detachment fault,eVen though the author disagrees with many ofVolborth's structural interpretations. He mappedthe detachment fault as a thrust fault of Tertiaryage, partly on the basis of the involvement ofTertiary volcanic rocks and partly on the basis ofTertiary ages of the crystalline rocks. Volborth'smap was generated at 1:125,000 scale, but the author

329

Note the large potassium-feldspar phenocrysts

used 7.5 minute U.S.G.S. topographic sheets forfield mapping. This scale was later reduced to1:62,500 scale to facilitate its inclusion in thisreport (Figure 2).

Previous Geologic Studies in the Area

One of the first geologic reports of the area<vas done by J. S. Newberry (1861), for whom themountains were named. Newberry was a geologistwith the Ives expedition of 1857-58 which ascendedthe Colorado River to the mouth of the Virgin Riverand made numerous traverses into the adjacentcountry. The next person to study the range wasG. K. Gilbert (1875), who served as a geologist withthe I-Iheeler Survey in 1871-73 and follmved theColorado River from the southern tip of Nevada intothe Grand Canyon. Gilbert made notes on theprincipal kinds of rocks along the route and onlocations of major faults. in 1909, F. C. Schraderproduced a 1:250,000 scale geologic map of westernArizona covering a belt along the Colorado Riverextending south of latitude 36° and westward intoNevada to longitude 114°45'. F. L. Ransome (1923)mapped and described the Tertiary volcanic sequencesand the granite prophyry masses exposed in theOatman mining district, located in the southern partof the Black Mountains, Arizona. Carl Lausen (1931)added little to Ransome's geologic findings and dealtprimarily with the gold-silver mineralization of thedistrict. Recent studies by Thorson (1971) haveproved very helpful in providing petrologic descriptions of the Tertiary volcanic units, along withK-Ar age determinations, within the Oatman district.

Figure 4. Volcanic ash-flmv tuffs; basal Patsy Hine Volcanics (late Oligocene (?) to early tilocene).These strata are dipping roughly 50° tmvard the southIVest.

Recent Studies

C. R. LonglVell (1963) performed reconnaissancemapping of the geology betIVeen Lake Head and DavisDam, and although he mapped the Tertiary volcanicrocks of the Eldorado and NeIVberry Hountains at1:125,000 scale, he did not differentiate the Precambrian crystalline rocks. Also, on the basis ofstratigraphic position, thickness, and composition,LonglVell suggested correlation of the Oatman districtvolcanic rocks IVith the volcanic rocks that he mappedto the north betIVeen Lake Head and Davis Dam (including the NeIVberry Hountains). R. E. Anderson (197la)later redefined the Patsy Hine Volcanics of Longwell(1963), which the author believes to be correlativeto those volcanic rocks found in the eastern Newberry11ountains. Longwell (1963) also described a low-anglethrust-like fault zone in the Precambrian granitesnear Davis Dam. Recent work by Anderson (197la) onthe tectonic structure of the Nelson block agreeswell IVith this interpretation if extended southwardto the present area.

Anderson's study (1971a) of low-angle faultingin the Eldorado Hountains, just north of the NewberryHountains, was the first to attribute the Imv-angle

330

faults of the Colorado River Trough to extensionalrather than compressional tectonics. He describesthe Eldorado llountains as an area of maj or imbricatenormal faulting accompanied by pronounced eastwardrotation of Tertiary strata. Hiocene listric normalfaults were interpreted as merging with a subhorizontal basal fault surface below which extension bynormal faulting had not occurred. Displacement ofTertiary rocks above the inferred basal surface was,as a consequence of fault geometries, westIVard(S 70° W) relative to lower-plate autochthonousunits.

Volborth (1973) described the geology of theEldor.odo, Nelvberry, 'ind northern Dead 110untains asa granite complex. Volborth described the rangesas anticlinal zones IVhose axial parts are occupiedby Tertiary granitic plutons enveloped by Precambriangneisses, schists, and granites which were metamorphosed during the Laramide Orogeny. Volborthmapped these Precambrian rocks in thrust faultcontact with the "Tertiary" crystalline rockscomposing the cores of the ranges. Although theauthor disagrees Ivith Volborth' s interpretation ofthe structural and geochronological data, his petrologic descriptions have proved invaluable.

Figure 5. Detachment fault exposed at klippe, Hest of Big Bend. Sample location of 10l,er-platecrystalline rocks dated at 12 m.y.B.P. by K-Ar method.

LITHOLOGIC UNITS

Upper-Plate Units (Allochthonous)

Crystalline Basement Rocks

Crystalline rocks of the upper plate primarilyflank the sides of the NeHberry Mountains, but afeH klippen are found on the roof portion of therange, east of Spirit Mountain. Petrologicdescriptions of these rocks have been reported byRansome (1923), Lausen (1931), LongHell (1963), andVolborth (1973).

The upper-plate crystalline rocks consist ofPrecambrian coarse-grained "rapakivi" granite. Thisdark, brOHnish granite is distinctly porphyriticand contains gray to pink, perthitic microclinephenocrysts, Hhich are mantled by fine-grainedallotriomorphic quartz, microcline, and albiticplagioclase. These phenocrysts average 1 to 2 em,but may be as large as 3 by 6 em, and account for20 to 80 percent of the total rock (Figure 3).The matrix of the rock in \vhich the large phenocrysts are set consists of quartz, plagioclase, andbiotite. The quartz is of an opalescent, bluishcolor and is translucent rather than transparent.Plagioclase phenocrysts rarely exceed 2 by 5 mm,and can be recognized in hand specimen, especiallyon \veathered surfaces, by their yellowish-graycolor and brighter appearance as compared to thepotassium feldspar. Due to the dark color of thebluish quartz and its integral association withbiotite, of Hhich the rock contains 3 to 8 percent,the whole rock appears to be much darker than oneHould assume for a rock containing 30 to 45 percentmodal quartz. Even in fresh cuts the granite isdark gray, and resembles less acidic rocks such as

331

diorite (Lausen, 1931; Volborth, 1973). h'here thisrapakivi granite is juxtoposed against the detachment fault it displays a reddish-broHn hue. Theseporphyritic granitic rocks are probably equivalentto the 1.4 - 1.5 b.y.B.P. rapakivi granite suitethat extends from h'isconsin to California (Silverand others, 1977).

Tertiary Volcanic Rocks

The upper-plate rapakivi granite is nonconformably overlain by Tertiary volcanic rocks.These volcanic rocks Here first mapped by LongHell(1963) as Patsy Hine Volcanics. Anderson C1971a)later redefined the Patsy Mine Volcanics in theEldorado Uountains and described this formation in1977 and 1978. In Hhat appears to be an equivalentvolcanic unit in the Black Mountains, Ransome (1923)and Lausen (1931) described the Alcyone Trachyte.Thorson (1971) later redefined this unit and calledit the A1cyone Formation consisting of five members.Thorson interprets these rocks as a caldera sequenceof late Oligocene to early Hiocene age.

LongHe11 (1963) suggested that the lOHesttrachyte unit in the Oatman district (A1cyoneTrachyte) corresponds in a general Hay to the lowerpart of the Patsy Jfine Volcanics. He concluded thatlargely on the basis of megascopic examination,brmvn andesite seems predominate in the latter, butdetailed petrographic study may show that much ofthe rock is trachyte. Also in the favor of thesuggested correlation are: (1) similar averagecomposition of the assemblage, near or identicalwith that of andesite; (2) location in the lowerpart of each section; and (3) similar maximumthicknesses.

Figure 6. Exposure of detachment fault which separates reddish upper-plate rapakivi granite fromlight-green; sh lmcer-plate crystalline rocks. Located at detachment fault north of Nevada StateHiglllJay 163.

In general, the colors of this unit, whenviewed from a distance, are characteristicallybrown to greenish gray. Some flows are reddish,perhaps due to the oxidation of the ferromagnesiansilicates, while others, more basic in composition,are of a rather dark gray hue (Figure 4). Flowsvary in thickness from a few meters to many tensof meters, but are commonly 5 to 10 meters thick.Hany are vesicular or amygdaloidal in their upperparts. Interbedded with the flows are ash-flowtuffs which exhibit welded glass shards and fiammestructures of collapsed pumice. A very Icell beddedlithic tuff provides an excellent marker horizon.This light colored unit resembles a sedimentary tuffbreccia, composed of green or maroon, angular tosub-rounded fragments of volcanic rocks in a finegrained light green matrix. Locally, explosionbreccia of rhyolitic composition is found. Suchbreccia units are made up largely of angularfragments of volcanic rocks, which vary in size fromsmall to relatively large dimensions (several metersacross) (Lausen, 1931; Longlcell, 1963; Thorson,1971) .

Lm.,er-Plate (Autochthonous) Units

General Statement

Because of the domical form of the NewberryMountains, autochthonous crystalline rock units

332

belm., the Nel.,berry detachment fault occupy the coreof the range and underlie its highest peaks(Figures 2).

Older Metamorphic Rocks

Schist and banded gneiss, which contain large(1 to 2 em) potassium-feldspar porphyroblasts, areexposed in the northern Newberry Mountains, locallyalong its eastern flank, and in the northern DeadHountains. An isotopic age of 1. 7 b. y. has beensuggested by Volborth (1973) as an age for theserocks. This age determination is compatible withregional packages of Precambrian rock units presentacross Arizona (Reynolds, 1980).

Precambrian Intrusive Rocks

The most. widely distributed granite of thearea is a light colored, orthoclase-rich rapakivigranite that occupies the core of the NewberryMountains. These granitic rocks display plagioclasemantles on the potassium-feldspar phenocrysts.Mafic minerals comprise less than 10 percent ofthese rocks.

The grain size of this unit is gradational fromcoarse to aphanitic (1.5 em - 0.5 em to less than0.1 em). Coarse, granular textures are mostpronounced in the central, more deeply exposed parts

Figure 7. Areal photograph of upper-plate klippe, located along River FrontRoad, near Big Bend. Note the planar fabric of the lmver-plate rocks. Use Hidthof tHo-lane road for scale.

Figure 8. Resistant ledge of microbreccia exposed along the detachmentfault. Close-up of detachment surface seen Figure 6.

333

Figure 9. 11icrobreccia (5 - 10 em thick) exhibiting flow-banding, collected[rom exhumed detachment surface in the northern part of the study area.

of the plutons. Border zones of the large plutonsare mostly medium to fine-grained, with micrograniticand aphanitic varieties present along the contacts.These- granitic rocks are often found intruding intothe older metamorphic rocks. In the roof portion ofthe mountains (e.g. southeast of Spirit Hountain),this granite is conspicuously miarolitic (Volborth,

1973) •

Volborth (1973) has assigned a Tertiary age tothese rocks on the basis of fission-track isotopicage dating. His ages range roughly from 10 to 20m.y.B.P. (the period of detachment faulting, approximately) for these lower-plate granitic rocks.Donna Hartin, of San Diego State University, hasdated a sample of these same lower-plate graniticrocks using the K-Ar method. Her age determinationfor a sample collected near the klippe across fromBig Bend was 12 m.y. (Figures 2 and 5). Sheinterprets this relatively young age as a product ofthe mid-Tertiary isotopic resetting due to detachment faulting Q1artin and others, 1980, 1981). Theauthor believes that this age may record the lastmovement along the detachment fault, since thesample was coll~cted within one meter of the erodedfault surface. This date is very close to thatsuggested by R. E. Anderson for the last movementalong the low-angle fault in the Eldorado Hountains,at 11 m.y.B.P. (Shackelford, pers. corum., 1981).

The original age of these "Tertiary" lOI"erplate rocks is probably 1.4 - 1.5 b.y.B.P. basedlargely on their characteristic rapakivi texture.Such rocks are seen widely across Arizona and southeastern California and have been dated in theHualapai l10untains (Kessler, 1977), Hhipplel10untains (Anderson and others, 1979), and HarbleHountains (Hasserburg and Lanphere, 1965).

334

l1esozoic Intrusive Rocks

Intrusive granitic rocks which contain phenocrysts of muscovite, biotite, and garnet are foundlocally intruding into the lower-plate Precambrianmetamorphic and granitic rocks. For the sake ofsimplification these rocks were not mappedseparately, but as part of the lower-platecrystalline rocks. This two-mica garnetiferousgranitoid is typical of Hesozoic rocks in the area.Intrusive rocks with the same characteristicmineral assemblage seen elsewhere in the region havebeen assigned a l1esozoic age by isotopic age dating(Hiller and Bradfish, 1980).

STRUCTURAL GEOLOGY

Newberry Detachment Fault

The most impressive single structural elementof the Newberry l10untains is a low-angle (10°-30°)normal fault, previously mapped by Volborth (1973)as a thrust fault, which separates the terrane intoupper- and lower-plate assemblages. The surface isspectacularly exposed at numerous localities withinthe southeastern Newberry Hountains (Figures 2, 5,and 6). It is clearly visible in the Bureau of Landl1anagement 1: 24,000 colored areal photographs as anirregular contact betl"een light-colored, lower-platerocks and darker, upper-plate rocks which aretypically reddish-brm"ll (Figure 7). It typicallyresembles the low-angle faults present in otherareas (e.g. the Eldorado Hountains to the north,Anderson, 1971a; and the Dead Hountains nearNeedles, California, Frost, pers. comm., 1981).

Figure 10. Vie,,, looking south at the ramp of the detachment surface. Notice the dark-coloredklippe at the base of the ramp. Location: west of Big Bend.

The fault is most obvious in the field where thereddish-brown upper-plate granite is in contact withthe light greenish lower-plate granite, usuallyflanking the base of the mountain. Aphanitic, sometimes flinty microbreccia underlies the surfacealong much of its trace. These finely pulverizedrocks form a layer that is typically 2 to 10 cmthick and resistant to weathering and erosion(Figures 8). As a consequence, the fault surface iscommonly left exhumed when sheared and brecciatedupper-plate rocks are eroded. The resistant cap ofmicrobreccia produces a distinctive ledge immediatelybelow the fault. Recently exhumed exposures of thedetachment surface are commonly planar and typicallydisplay a reddish-brown patina. This microbrecciaexhibits flow-banding (Figure 9), and small-scalenormal faulting. This microbreccia is believed tobe a reflection of superplasticity, suggesting thatthe layer may act as a fluid-like lubricant alongwhich the upper plate moves (Phillips and Sammis,1982; Phillips, this volume).

The resistant brown microbreccia tends toprotect the outline of the fault surface andtypically the ramp of the detachment fault ispreserved (Figure 10). The outline and relief of themountain range, at least in the area studied, seemsto be controlled by the resistance of the detachmentfault microbreccia to erosion.

Slickenside striae on the detachment surfacedisplay a slightly varied movement, but generallytrend northeast-southwest -- usually N70° + 10oE.

335

This transport direction is further supported by theminute fractures, or normal faults, which formwithin the microbreccia. These fractures trendroughly N200W and are formed perpendicular to themovement of the upper plate (Frost, pers. comm.,1981).

Although the actual contact between upper andlower plates is razor-sharp, movement along thedetachment surface has had a profound effect on therocks below most segments of the fault. Enigmatically, shearing and brecciation of allochthonousrocks are found confined to \dthin several metersabove the fault. Below the fault, however, theeffects of faulting carl extend downwar~ as far as250 to 300 meters (Davis and others, 1979). Theseeffects include the pulverization of lower-platecrystalline rocks to fine-grained, structurlessbreccia. An intense and pervasive alteration occursthroughout the entire structurally disturbed zone.Chlorite and epidote are major secondary mineralsthat give a pale green color to the rocks of thealtered zone which is termed the chlorite-brecciazone.

Hithin this highly deformed chlorite-brecciazone, numerous faults exist that are subparallel tothe detachment surface. Hany of these faultsurfaces are exposed at a rock quarry near Big Bend.These faults form an imbricate, curvi-planar fabricwithin the lm"er-plate rocks, which is easily seenin areal view (Figure 7). On a smaller scale,these faults appear to have an anastomosing behavior

Figure 11. Lower-plate fault "hich exhibits striae, and the beginning of separation into twosubparallel faults. Location: rock quarry 0.5 km north of klippe, near Big Bend.

in which they weave in and out of each other(Figures 11 and 12).

Striations and mullion structures on thesefault surfaces indicate varied movement, butcollectively depict a northeast-southwest movement.The direction of motion along these faults indicatesa genetic relationship between them and the detach

ment fault. Movement of the upper plate may havebeen a collective process of slip, involving boththe lower-plate faults and the detachment fault.This conclusion is suggested both by the veryexistence of these anastomosing faults, and theirrelative motion.

Geometry of the Upper-Plate Structures

The eastern Newberry Hountains, near Davis Dam,exhibit imbricate listric (?) normal faultingaccompanied by pronounced southwestward rotation ofthe Tertiary volcanic strata. These strata diproughly 50° ± 6° to the southwest where they aretruncated by northeast-dipping normal faults.Displacement along some of these faults is believedto have been enough to juxtopose the Tertiary rocksof the hanging wall against crystalline rocks of thefotwall (Figure 13). These normal faults arebelieved to flatten downward, merging with thedetachment surface, as suggested by Anderson (197la)in the Eldorado Mountains to the north.

336

The northwest strike of the normal faults,southwest sense of rotation of the Tertiary volcanicblocks, orientation of the fault striae (on allfault surfaces), and the minute fractures found onthe microbreccia are kinematic indicators thatcollectively indicate that displacements along theNewberry detachment fault and synchronous extensionin the upper-plate allochthonous units werenortheast-directed with trends of N70° + 100E.

Geometry of the Detachment Surface

The general form of the detachment surfaceexposed in the Newberry Mountains is a north-trendingantiform. This domal shape has previously beenmentioned by Lausen (1931), Lon~vell (1963), andVolborth (1973). The Newberry antiformal arch ispart of a regional wavelike detachment fault surfacewhich is known to extend from Nevada to Sonora,Nexico.

Hore than one hundred major antiforms andsynforms of the detachment surface have been notedin the region. The development of these elongateantiforms and synforms appears to have beencontemporaneous with detachment faulting, and agenetic relationship between the two has beensuggested by Cameron and others (1981). Cameron andothers (1981) also suggested mid-Tertiary northeast-

Figure 12. Lower-plate fault surfaces which display an anastomosing behavior. The secondaryfault joins the main fault surface at its ends, forming a lenticular shape. Location: same as Figure 17.

southwest crustal extension as a principal cause ofthe detachment faulting and large-scale folding.

The sinusoidal shape of the fault is displayedon all levels. It is present not only on theregional scale, as mentioned above, but also on amore localized scale. This wave-like form isexhibited by the undulating shape of the southeasternNewberry 110untains. This pehnomenon can be seen onthe geologic map (Figure 2), where the outline ofthe detachment fault tends to control the shape ofthe lo"er-plate crystalline rocks making up the coreof the range. Also, this phenomenon exists at theoutcrop level, where the undulation of the faultsurface can be seen by the variations of strikesand dips taken along that surface.

337

REGIONAL CORRELATION

Relationship of Detachment Faultingto Neighboring Ranges

The Newberry detachment fault form an anticlinaldome which is part of a wave-like surface evidentthroughout the detachment terrane. Due to regionaluplift, this domal feature was exposed to erosion,"hich has consequently laid bare the autochthonouslower-plate rocks in the central part of the range,leaving remnants of the allochthonous upper-platerocks encircling its flanks.

swNE

o 1/2 1 mi

Figure 13. Diagrammatic cross section across the southeastern Newberry tlountains, southeastern Nevada, illustrating mid-tliocene rotational, normal fault displacement along the detachment surface. Note the anastomosing faults within the lo"er-plate complex.

The neighboring ranges display this same typeof relationship, lm,er-plate rocks occupying thecores of the ranges, and upper-plate rocks encompassing the basal margins of the ranges. This relationship between upper- and lower-plate rocks can be seenin Figure 1.

The detachment fault is exposed in the NewberryMountains, the Eldorado Hountains to the north, thenorthern Black Hountains to the northeast, theHomer and Sacramento Hountains to the soutlnvesL, andthe Dead and the Chemehuevi Mountains to the south(Frost, pers. comm., 1981). In these mountains itforms a regional wave-like surface, with the 1mverplate rocks pervading the antiforms (cores of theranges), and the upper-plate rocks occupying thesynforms (low-lying areas between ranges).

Relationship to Regional Extension

Detachment faulting in the south'vestern UnitedStates has been interpreted as having been formed byregional, mid-Tertiary northeast-southwest crustalextension (Figure 14). This may be related to theclosing stages of subduction and arc-relatedvolcanism along with the inception of Pacific-NorthAmerican plate interactions as expressed in theregion (Cameron and Frost, 1981).

Anderson (197la; and Anderson and others (1972)suggests a period of extensional tectonism (11 - 15m.y.B.P.) of colossal magnitude in the EldoradoMountains, 40 km north of the Newberry Mountains.Anderson (197lb) describes the style of extensionaltectonism during this period as featuring closelyspaced shingling normal faults that flatten withdepth, followed by Basin-and-Range type high-anglenormal faulting that may have penetrated deep intothe crust.

Hithin the Ne'vberry Mountains, northwest-trending "listric" normal faults dip to the northeast androtate mid-Tertiary volcanic rocks to the southwest.This, along with the possibly correlative nortlnvesttrending dike swarms within the lower-plate rocks,suggests northeast-soutlnvest extension. The age of

338

this extensional event can be placed in limits bythe ages of the volcanic beds that the normalfaults offset. Also, kinematic indicators such asfault striae, mullion structures, and detachmentrelated fracture orientations suggest a northeastward movement of allochthonous upper-plate unitsrelative to the autochthonous lower-plate rocks.The northeastward direction of motion adds supportto the regional northeast-southwest extensionalpicture.

CONCLUSIONS

Volborth (1973) attributes the contact betweenupper- and lower-plate units to gravity-sliding.He concludes that the klippen of the upper plateare the vestiges of Laramide thrust plates whichare now clearly in the process of gravitationallysliding down the present slopes. He also attributes

the rotation of Tertiary volcanic blocks to Basinand-Range type normal faulting unrelated to thepreviously mentioned structural event.

The author is in disagreement with Vo1borth'sstructural interpretation, and would like to suggestnortheast-southwest extensional tectonism (Anderson,1971a, b) as coevally forming the detachment fault(mapped by Vo1borth as a thrust fault) and upperplate structures (rotation of Tertiary blocks alongnormal faults).

The relationship of the Colorado River troughstructures with Basin-and-Range type faulting isunclear. Anderson (197lb) suggests that extensionalfaulting in the Colorado River trough predates themajor period of crustal extension by normal faultingin the Great Basin area to the north. The mountainranges of the area seen in Figure 1 are definitelynot outlined by range-front faults of the Basin-andRange type.

Evidence suggesting that the detachment surfacein the Newberry Hountains formed at very shallowlevels is the stratigraphic thickness of rotatedfault blocks (Tertiary volcanic strata plus Precambrian basement), "hich is approximately 3 to 5 km(Thorson, 1971; Anderson, 1978). The cumulative

/" /~/'/'"./". Y

/'"""-/ /

"

0>--_~.......-:_2_0~0---r ~4-,90 mi200 600km

Figure 14. Distribution of extensional tectonics in the southwesternUnited states. Areas affected by normal faulting of basin-range type are indicated by NE-SH ruled pattern. Areas of 10l,,-angle detachment faulting are stippled. Hodified after Davis and others, 1979.

thickness of these upper-plate rocks provides amaximum limit on the depth at which detachment faulting took place.

Consistent northeastward direction of transportof allochthonous units above the detachment surfacein the Ne\"berry llountains is exemplified by characteristic northwest strike and southwest dip ofupper-plate Tertiary strata, which have been rotatedalong northeast-dipping listric (?) normal faults(Figure 13). Also, the northeast orientation of thefault striae and northwest-trending fractures withinthe microbreccia provide support to the northeastwarddirection of transport. This northeastward directionof transport is consistent with the transportdirection of the allochthonous units in the IVhippleBuckskin-Rawhide Mountains to the south (Davis andothers, 1979), but is opposite to the directionfound in the Eldorado Mountains to the north(Anderson, 1971a). The opposite directions oftransport can be explained by the regional northeastsouth\"est crustal extension in the Colorado Rivertrough, enabling different domainal regions to moveopposite directions at the same, or similar, times.

ACKNOliLEDGEHENTS

I am particularly grateful to Eric G. Frost,who heartily encouraged this investigation. Thispaper greatly benefited from his critical reviewand suggestions. For Mr. and Mrs. Joe Masopust, whovery generously allowed me to stay with them whilein the field area, I am most appreciative. Iwould also like to thank Murray Miles for hiseditorial advise and suggestions; his contributionswere extremely beneficial to the completion of thispaper.

REFERENCES CITED

Anderson, J.L., Davis, G.A., and Frost, E.G., 1979,Field guide to regional lliocene detachmentfaulting and early Tertiary (?) myloniticterranes in the Colorado River Trough, southeastern California and western Arizona: Abbott,P.L., ed., Excursion in Southern California,

339

Guidebook for 1979 National Geol. Soc.America lfeeting, San Diego State Univ.,p. 109-133.

Anderson, R.E., 1971a, Thin skin distension inTertiary rocks of southeastern Nevada: Geol.Soc. America Bull., v. 82, no. 1, p. 43-58.

________, 1971b, Thin skin distension in Tertiaryrocks of southeastern Nevada--Reply: Geol.Soc. America Bull., v. 82, no. 12, p. 35333536.

________, 1977, Composite stratigraphic section ofTertiary rocks in the Eldorado Hountains,Nevada: u.S. Geol. Survey Open-File Rept.77-483, 5 p.

________, 1978, Chemistry of Tertiary volcanic rocksin the Eldorado Hountains, Clark County,Nevada, and comparisons with rocks from somenearby areas: U.S. Geol. Survey Jour.Research, v. 6, no. 3, p. 409-424.

Anderson, R.E., Long,,,ell, C.R., Armstrong, R.L., andHarvin, R.F., 1972, Significance of K-Ar agesof Tertiary rocks from the Lake Mead region,Nevada-Arixona: Geol. Soc. America Bull.,v. 83, no. 2, p. 273-288.

Cameron, T.E., and Frost, E.G., 1981, Regionaldevelopment of major antiforms and synformscoincident with detachment faulting inCalifornia, Arizona, Nevada, and Sonora:Geol. Soc. America Abstr. with Programs,v. 13, no. 7, p. 421-422.

Cameron, T.E., Frost, E.G., and John, B., 1981,Development of regional arches and basinsand their relationship to mid-Tertiarydetachment faulting in the Chemehuevillountains, San Bernardino County, California,and llohava County, Arizona: Geol. Soc.America Abstr. with Programs, v. 13, no. 2,p. 48.

Davis, G.A., Anderson, J.L., Frost, E.G., andShackelford, 'LJ., 1979, Regionailliocenedetachment faulting and early Tertiary (?)mylonitization, Hhipple-Buckskin-Rawhidellountains, southeastern California andwestern Arizona: Abbott, P.L., ed.,Excursions in Southern California, Guidebookfor 1979 National Geol. Soc. America Heeting,San Diego State Univ., p. 74-108.

Gilbert, G.K., 1875, Report on the geology of portions of Nevada, Utah, California, andArizona: U. S. Geog. and Geol. Surveys H.100th Meridian Rept. (Hheeler), v. 3, p. 17187.

Kessler, E.J., 1976, Rubidium-strontium geochronology and trace element geochemistry ofPrecambrian rocks in the northern HualapaiHountains, llohave County, Arizona: TheUniversity of Arizona, unpublished Haster'sthesis, 73 p.

Lausen, C., 1931, Geology and are deposits of theOatman and Katherine districts, Arizona:Arizona Bureau of lfines Bull. 131, Geol.ser. 6 (Univ. Bull., v. 2, no. 2), 126 p.

Lon~vell, C.R., 1963, Reconnaissance geology betweenLake Mead and Davis Dam, Arizona-Nevada:U.S. Geol. Survey Prof. Paper 374-E,p. El-E51.

Martin, D.L., Barry, H.L., Krummenacher, D., andFrost, E.G., 1980, K-Ar dating of mylonitization and detachment faulting in the\Vhipple Mountains, San Bernardino County,California, and the Buckskin Hountains,Yuma County, Arizona: Geol. Soc. AmericaAbstr. with Programs, v. 12, no. 3, p. 118.

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Hartin, D.L., Krummenacher, D., and Frost, E.G.,1981, Regional resetting of the K~Ar isotopicsystem by mid-Tertiary detachment faulting inthe Colorado River region, California,Arizona, and Nevada: Geol. Soc. AmericaAbstr. with Programs, v. 13, no. 7, p. 504.

Miller, C.F., and Bradfish, L.J., 1980, An innerCordilleran belt of muscovite-bearingplutons: Geology, v. 8, no. 3, p. 412-416.

Newberry, J.S., 1861, Geological report, in Ives,J.C., Report upon the Colorado River of theWest: U.S. 36th Cong., 1st sess., HouseExecutive Doc. 90, pt. 3, 154 p.

Phillips, J.C., 1982, Cataclasite formation alongthe \Vhipple Fault, lVhipple Mountains, SanBernardino County, California: this volume.

Phillips, J., and Sammis, C.G., 1982, Cataclasiteformation along the lVhipple Fault, lVhipplellountains, San Bernardino County, California:A case for low temperature structural superplastic flow: Geol. Soc. America Abstr. withPrograms, v. 14, no. 3.

Ransome, F.L., 1923, Geology of the Oatman golddistrict, Arizona, a preliminary report: U.S.Geol. Survey Bull. 743, 58 p.

Reynolds, S.J., 1980, Geological framework of westcentral Arizona: Arizo~a Geol. Soc. Digest,v. 12, p. 1-16.

Schrader, F.C., 1909, Mineral deposits of the CerbatRange, Black l1ountains, and Grand HashCliffs, Mohave County, Arizona: U.S. Geol.

~ Survey Bull. 397, 226 p.Silver, L.T., Bickford, M.E., Van Schmus, H.R.,

Anderson, J.L., Anderson, T.H., and Medaris,Jr., L.G., 1977, The 1.4-1.4 b.y. transcontinental anorogenic plutonic perforation ofNorth America: Geol. Soc. America Abstr.with Programs, v. 9, no. 7, p. 1176-1177.

Thorson, J.P., 1971, Igneous petrology of the Oatmandistrict, Mohave County, Arizona: Univ.California, Santa Barbara, unpublished Ph.D.thesis, 173 p.

Volborth, A., 1973, Geology of the granite complexof the Eldorado, Newberry, and northern DeadHountains, Clark County, Nevada: NevadaBur. Hines and Geol., Bull. 80, 39 p.

Hasserburg, G.J., and Lanphere, H.A., 1965, Agedeterminations in the Precambrian ofArizona and Nevada: Geol. Soc. America Bull.v. 76, p. 735-758.

GEOLOGIC AND GEOCHRONOLOGIC RECONNAISSANCE OF THETURTLE MOUNTAINS AREA, CALIFORNIA: WEST BORDER OF THE

WHIPPLE MOUNTAINS DETACHMENT TERRANE

Keith A. HowardPaul Stone

Marth~ A. PernokasU.S. Geological Survey

345 Middlefield RoadMenlo Park, California 94025

ABSTRACT

The Turtle Mountains area comprises three ranges,the Stepl~dder, Turtle and Aric~ Mountains, that liealong the .rest border of a terl'ane dominated by theWhipple Mountains detachment fault of Tertiary age.The Turtle Mountains area may represent a segmentedterrane of headwall faults, from which rocks slid downand to the northeast along faul ts that merge or feeddownward into the Whipple Mountains detachment fault.

Pre-Tertiary rocks in the three ranges arePrecambrian crystalli ne basement rocks and discordantMesozoic plutons. The Precambrian rocks record acomplex history of sedimentation, multiple plutonism,metamorphism and deformation, and renewed plutonism.K-Ar and fission-track ages for these rocks are partlyreset but are older than in nea"'by ranges; the oldestapparent age determined is 1350 m.y. Mesozoic plutonsof granodiorite, granite and diorite give Ea"'ly andLate Cretaceous K-Ar ages; one diorite possibly is,Jul'assic. The Precambrian and Mesozoic rocks lie nomore than 1 !<In structurally below Miocene volcanicrocks and they represent a middle Tertiary high levelcrust. Equivalent rock units are found in allochthonsin ranges to the east and northeast.

Oligocene(?) and Miocene volcanic rocks are 1 to2 kID thick; p~rt or all them were erupted from theeastem Turtle Mount~ins. Samples of volcanic rockshave yielded ages clustering a"'ound 18 to 20 m.y.

Normal faults that cut these rocks dip steeplyeastward in the southern and central Turtle Mountains,and faults farther to the northeast are progressivelymore gently dipping. Bedding in volcanic strata istypically perpendicular to underlying normal faults.Progressi ve til ting suggestive of growth faulti ngresul ted in an angular unconformity within theTertiary sequence 18-19 m.y. ago. Together theserelations support a model of progressively greaterrotation and displacement of detached faul t blocksnortheastward from the Turtle Mountains. To thenortheast the headwall terrane merges, throughincreasing displacement, with highly tiltedallochthons of the detachment terrane. The isolatedArica Mountains may be within the terrane ofa lloch thonous rocks.

INTRODUCTION

Detachment faults or decollements of largehorizontal and perhaps large vertical displacementcharacterize many areas of Tertiary crustal extensionin the Cordillera (Crittenden and others, 1980). Theterrane of middle Tertiary extensional faulting along

341

Richard F. MarvinU.S. Geological SurveyDenver Federal CenterDenver, Colorado 80225

the Cal iforn ia-Arizona border is one such area (fig.1). The Whipple Mountains detachment fault dominatesmuch of this area; it separates numerous tilted blocksof Tertiary and pre-Tertiary rocks from a footwall ofgenerally dissimilar pre-Tertiary rocks (Carr andDickey, 1976; Carr, 1981; Davis and others, 1980). Inthis paper we refer to the terrane exposing this faultand its allochthons as the Whipple Mountain detac~~ent

terrane.

An outstandin;>; quest ion is how this and otherdetachment faults terminate. Davis and others (1980)suggested th~t the Whipple Mountains faul t may erld bysurfacing west of the Whipple Mountains at a headwallin the Turtle Mountaim, from which upper-plate rocksslid northeastward and rotated southwestward. Therocks and structures exposed in the Turtle Mountainsand in ranges to the north and south may thereforeprovide clues to the geometry and processes of lowangle normal faul ting.

In this report we discuss the geology andgeochronology of the Turtle, Stepladder and AricaMountains in order to help define the western bounda.ryof the Whipple Mountains detach~ent terrane (fig.2) • We also suggest how the rocks exposed in theTurtle Mountains area relate to rock units juxtaposedby the Whipple Mou:1tains faul t to the east. Table 1describes the rock units. Radiometric ages arepresented in Tables 2 and 3 and are su~marized also onthe map, Figure 2.

REGIONAL SETTING AND PREVIOUS WORK

The Turtle Mountains area and the WhippleMountians detachment terrane lie along thenortheastern fringe of the Mesozoic orogen (Burchfieland Davis, 1981). Bel ts affected by intense Mesozoicplutonism and orogenic deformation lie to thesouthwest, and relatively undeformed craton in theColorado Plateau lies to the northeast. MiddleTertiary rocks in this marginal region rest on abasement of Precambrian crystalline rocks that areintruded by scattered Mesozoic plutons. The cover ofPaleozoic and Mesozoic strata had been largelystripped by Oligocene and Miocene time. Mesozoicmetamorphism and deformation of the Precambrianbasement and vestiges of the Paleozoic cover have beendemonstrated in ranges immediately to the west, south,and east of the Turtle Mountains (Hamilton 1960; Stoneand others, 1981; Miller and others, 1982; Davis andothers, 1980; Anderson and Rawley, 1981).

Earlier reconnaissance studies showed that theTurtle and Stepladder Mountains are formed ofPrecambrian gneiss and granite and Mesozoic plutons,

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overlain nonconformably by gently west-dippingTertiary volcanic and sedimentary rocks (Coonrad,1960; Cooksley, 1960a, b; Bishop, 1963; Embree, 1967;Armstrong and Suppe, 1973; Woodward McNiell andAssociates, 1974; Hathaway and Kuniyoshi, 1975; Carrand others, 1980). Other studies reported that theArica Mou'ltains, a small isolated range to the south,expose metamorphosed Paleozoic strata faul ted againstPrecambrian gneiss (Bishop, 1963; Davis, 1974; Carrand Dickey, 1977). Geologic maps are available forthe southern Mopah Range (Carr and others, 1980), thesouthern Turtle Mounta.ins (Woodward McN.iell andAssoc.iates, 1973), and pa"'ts of the Ar.ica (Davis,1974), northern Turtle (Embree, 1967) and StepladderMounta.ins (Coonrad, 1960; Cooksley, 1960a, b).

These ranges lie west of known exposures of theWhipple Mountains detachment fault (fig. 1); but thicksections of Tertiary rocks, downfaul ted along theeastern part of the Turtle Mountains and in theadjoining Mopah Range and Stepladder Mountains, mergeeastward w.ith jostled blocks in the western WhippleMounta.ins above the fault (Woodward McNiell and

Assoc.iates, 1974, Carr and others, 1980; Davis andothers, 1980).

The footwall of the Whipple Mountains faul t tothe east exposes Precambr.ian rocks that were subjectedto intrusion, metamorph.ism and mylonH.ic deformationin the Mesozoic (Anderson and others, 1979; Davis andothers, 1980; Anderson and Rawley, 1981). Across analluviated valley west of the Turtle Mountains, theOld Woman, Iron and Granite Mountains likewise exposerocks of the middle crust including Cretaceousbatholithic and migrnatitic rocks and mylon.iticgne.isses (D. Miller and others, 1981; C. Miller andothers, 1982). In contrast, the cl'ystalline rocks ofthe Turtle and Stepladder Mountains as discussed belowrepresent a higher crustal level, subjected to lessMesozoic heating and partly preserving a Tertiarycover. The crystalline rocks resemble rock.s exposedabove the Whipple Mountains faul t. Th is resemblancesupports the possibility that the Turtle andStepladder Mountains lie near the source of theWhipple Mountains allochthons.

342

Sedimentary rocks (Tertiary)

Granodiorite (C retaceous)

Alluvium (Quaternary)

Volcanic rocks (Tertiary)

Gronite (Cretaceous)

Diorite (Cretaceous)

Augen gneiss of Johnsons Well (Precambrian)

Metasedimentary rocks (Mesozoic and Paleozoic)

Upper basalt (Tertiary)

Porphyritic granitic (Precambrian)

Mylonitic gneiss (Cretoceous 7)

Diorite (Jurassic 7)

Granite gneiss of Virginia May Mine (Precambrian)

Undifferentiated metamorphic rocks, mainly gneiss(Precambrian)

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Contact

EXPLANATION

The small, isolated Arica Mountains, in thesouthern Turtle MoU'1ta in'3 area, contain elementsresembling both the deep and shallow crustal levels.Their place in the regional framework is not yetclear. Metamorphosed Paleozoic rocks here are faul tedover Precambrian gneiss. Two features resemblefeatures of the allochthons in the Whipple Mountainsdetachment terrance: (1) a low-angle normal faul tthat cuts the Paleozoic-against-Precambrian f3ultcontact (Davis, 1974), and (2) nearby outcrops ofsteeply dipping Tertiary strata. An isolated exposureof mylon.itic gneiss contrasts with these and suggeststhe possibility that different crustal levels may beJuxtaposed here as they are in the Whipple Mountains.

The framework established in the WhippleMountaim by the excellent studies by W. J. Carr andby G. A. Davis and J. L. Anderson and their colleaguesprovides much of the basis for our study, as will beapparent from the numerous references to them in thetext.

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343

CRYSTALLINE ROCKS

Pre-Tertiary crystalli ne rocks crop out in fourareas in the study area (fig. 2). The main Turtle

Mountains block exposes a Precambrian terraneconsisting largely of orthogneisses, intruded in thesouth and east by Mesozoic plutons. The area to thenortheast, here called the Mopah-Stepladder block,exposes two areas of crystalline rock, one in theStepladder Mountains containing a Mesozoic pluton andits w'111 of Precambrian gneiss, and a second in thenorthe"n Turtle Mountains consisting of undatedPrecambrian(?) granite. The compact Arica Mountainsarea exposes Precambrian gneiss, a Mesozoicgranodiorite stock, metamorphosed Paleozoic andMesozoic st"ata, and mylonitic gneiss. We use theIUGS classification for plutonic rocks (Streickeisen,1973) .

Precambrian Rocks

The Precambrian basement consists mostly ofmetaplutonic rocks, along with some undeformedplutonic rocks and deformed supracrustal rocks. Theoldest rocks in the Turtle Mountains include rare darkquartzi te and metaconglome"a te, and more commonamphibolite, banded gneiss and sc~ist. Together theserocks account for less than five percent of the TurtleMountains. They a"e thoroughly injected andmigmatized by younger Pre,cambrian granite gneisses.

The migmatizing granitic gneisses includeleucogranite, biotite-garnet granite, and the mostwidespread variety, the fine-grained granite gneiss ofVil'gina May Mine (Table 1). This last is a regionallyimportant lithology that also is recognized in theSacramento and Mohave Mountains to the northeast (fig.1). The common occu"rence of leucocratic s'neat veins(fig. 3) and of retrograded garnet pOl'phyroblasts bothin the veins and the host gneiss of Virginia May Minesuggest a polyphase metamorphic history.

Intruding this gneiss is an extensivegranodiorite augen gneiss, the augen gneiss ofJohnsons Well (Table 1), which crops out over much ofthe Turtle Mountains. Similar augen gneiss is presentin allochthonous rocks in the eastern WhippleMountains (Anderson and others, 1979, and Davis andothers, 1980) and in the Sacramento Mountains. Incontrast to abundant leucocratic veining in the gneissof Virginia May Mine, the augen gneiss of Joh:1sonsWell is with one exception free of cross-cuttinggneissic leucogranite. It may therefore postdatemetamorphism and anatexis of the gneiss of VirginiaMay Mine. The augen gneiss itself is foliated, inplaces mylonitized, and the foliation in turn deformedinto wavy folds. The gneissic deformation predates anundeformed diorite stock which has yielded a'1ornblende K-Ar age of 1350 m.y. (Table 3, no. 1)suggesting a minimum middle Proterozoic age. Theorientation of foliation in Precambrian "ocks variesconsiderably but commonly strikes east or northeastand is steep, in co~cert with dominantly northeaststrikes for ~Jcks of early Proterozoic age in Arizona.

A va"iety of porphyritic granites (also calledgranite porphyries here) of probable Precambrian agecut older gneisses. They postdate most metamorphicdeforma tion ~ut have conspicious flow(? )-a lignedtabula" feldspar phenocrysts. Two of these U'1its inthe southern and !,estern Turtle Mountains (Table 1)are significantly radioactive, like porphyriticgranite of the Marble Mountains, 75 km to thenorthwest, that lfiS dated at 1.4 to 1.5 b.y. by Silverand McKinney (1963) and Lanphere (1964). Two unitsres'C'"o1e rocks described as gpanite porphyry by Davisa~d others (1980), Anderso~ and others (1979) andAnderson and Frost (1981) in the Whipple Mountainsallochthon. These authors favor an age of 1.4 to 1.5b.y. for the granite porphyry in the Whipple Mountainsbased on its compositional affinity to other rocks oft'1at age, and on preliminal'y Rb-Sr isotopic data.Coarse-grained granite in the northern Turtle

Figure 3. Gneiss of Virginia May Mine containing ptygmatically folded garnet-bearingleucocratic veins. The veins are bordered by mafic-rich rock and are i~terpreted asproducts sweated out of the host gneiss. In nearby exposres, the gneiss containsdisoriented inclusions of amphibolite that demonstrate an igneous protolith for thegneiss.

344

Mountains resembles rock in the Sacramento Mountainsin which border phases are flow- foliated and resemblethe granite porphyry of the Whipple Mountains.

Mafic rocks that cut the gneisses of the Turtleand Stepladder Mountains include the small hornblendediorite stock discussed above. The hornblende age of1350 m.y. is the oldest K-Ar age obtained for rocks inthis region. Other mafic rocks are dikes ofhornblende diabase and gabbro that trend mostly eastin the Turtle Mountains and north in the StepladderMountains. Similar dikes are present in allochthonousrocks to the east and northeast in the Whipple (Davisand others, 1979; Anderson and others, 1979), Mohave,Buck, and Bill Williams Mountains (Nakata, 1982;Howard and others, 1982a). These dikes form a swarm ofregional importance. Anderson and others (1979) pointout textural and compositional resemblances of thesedikes to Middle Proterozoic diabase associated withthe Pahrump Group of the southern Death Valleyregion. The K-Ar hornblende age of 439 m.y. (Table 3,no. 2) for a diabase intrusion in the western TurtleMountains is a minimum age -- probably a much reducedage. Because no diabase is known to intrude Cambrianor younger strata in the region, these diabases aremost probably Precambrian. If the amphibole in therocks is secondary after pyroxene, as suggested by theophitic texture, this old age suggests that theuralitization may have been Precambrian also. Theplatform character of the Paleozoic section in theregion (Stone and others, 1981) precludes Paleozoicmetamorphism.

This dated sample (sample 2) and samples 1 and 3(Table 3) and the fission-track sample (Table 2) allwere obtained within 3 kID of each other, in an area ofthe western Turtle Mountains distant from largeMesozoic intrusive bodies (fig. 2). The four apparentages range widely and suggest a complex thermalhistory since formation of Precambrian rocks. The 439m.y. hornblende K-Ar age on the diabase unit appearsto be more reset than the hornblende K-Ar age of 1350m.y. on the diorite stock. A 230 m.y. biotite K-Arage from the older augen gneiss of Johnsons Wellsuggests even greater argon loss in the biotite. Agranite porphyry dike in this area, suspected onlithologic grounds to be middle Proterozoic, yielded afission-track age on zircon of 88 m.y. This agesuggests that Late Cretaceous or younger heating,sufficient to anneal tracks in zircon, contributed tothe partial argon loss in nearby Precambrian rocks.Nearby quartz porphyry dikes of Cretaceous or Tertiaryage may be partly responsible.

Pre-Mesozoic K-Ar ages for Precambrian rocks ofthe Turtle Mountains are unusual in this region.Similar rocks in the Whipple Mountains to the eastyield Jurassic or younger K-Ar ages, indicative ofsubstantial resetting (Anderson and Frost, 1981). Theclosest ranges where Precambrian K-Ar ages are knownare the Marble Mountains, 75 kID northwest (Lanphere,1964), the southern Mohave Mountains, 70 kID northeast(Conoco, unpublished data) and the Hualapai Mountains,100 kID northeast (R. F. Marvin, unpublished data).The retention of old argon in Precambrian rocks of theTurtle Mountains is surprising in view of regionalmetamorphism recorded in Paleozoic strata in theregion.

Paleozoic and Mesozoic Strata

In the Arica Mountains we recognize themetamorphosed equivalents of the Redwall, Supai,Hermit, Coconino and Kaibab Formations, ranging in agefrom Mississippian to Permian (Stone and others,

345

1981), and a conformably ( ?) overlying Mesozoicsandstone. The strata form southeast-vergentisoclinal folds hundreds of meters in amplitude thatplunge 550 south. The strata are thrust over veinedgneiss and are repeated by a low-angle normal fault(Davis, 1974). The Paleozoic strata are part of awidespread platform sequence ranging from Cambrian toPermian in age that once blanketed the region with athickness of 1 to 2 kID (Stone and others, 1981). Theabsence of this sequence above the Precambrianbasement in the Turtle and Stepladder Mountains and alarge region to the north and east reflects extensivepre-Miocene erosion.

Mesozoic Plutonic Rocks

Plutons of granodiorite, granite, and dioriteintrude Precambrian rocks discordantly in the TurtleMountains; granodiorite plutons intrude Precambrianrocks discordantly in both the Stepladder and AricaMountains. Previous reconnaissance suggested that thegranodiorite plutons are Cretaceous (Ar~~trong andSuppe, 1973; John, 1981). Diorite in the southwesternTurtle Mountains is Jurassic or older--perhapsPrecambrian--as indicated by discordant K-Ar ages onhornblende (167 m.y.) and biotite (100.7 m.y.; Table3, no. 4). The biotite age matches that of a lateEarly Cretaceous(?) granite 0.6 kID away (Table 3, no.5), which intrudes the diorite and is so closelyassociated as to suggest they are related. Allpreviously identified Jurassic intrusive bodies inthis pa!"t of the Cordillera lie southwest of theTurtle Mountains (Burchfi el and Davis, 1980; John,1981). The possibility that there are outlierJurassi9 plutons in the Turtle Mountains area mighthelp explain the Jurassic K-Ar ages reported byAnderson and Frost (1981) on Precambrian rocks to theeast in the Whipple Mountains allochthon.

A hornblende-biotite diorite stock that intrudesPrecambrian gneiss in the western Turtle Mountains maybe one of the younger Mesozoic intrusives. It yieldeda hornblende K-Ar age of 86.8 m.y. (Table 3, no. 8) inan area where other K-Ar ages retain pre-Mesozoicsignatures (fig. 2). K-Ar dating of threelithologically different diorites in the TurtleMountains thus indicates that some are Precambian,some possibly Jurassic, and some probably LateCretaceous.

Two granodiorite plutons in the southeasternTurtle Mountains yielded late Early and early LateCretaceous K-Ar ages (Armstrong and Suppe (1973; thispaper). For the northern, or Castle Rock pluton, weobtained an age on hornblende of 101 m.y. and an ageon biotite of 93.0 m.y. (Table 3, no. 7). . Thesouthern, or Turtle pluton yielded an age onhornblende of 105.9 and an age on biotite of 97.3 m.y.(Table 3, no. 6). The southern pluton borde,rsporphyritic granite on the west. An outlying stock ofthe porphyritic granite yielded a similar biotite K-Arage, 101.3 m.y.; however, as noted above, this stockis closely associated with diorite for which aJurassic or older age is indicated. Granite dikesfrom the Turtle granodiorite pluton resembleallochthonous Cretaceous granite in the WhippleMountains (adamellite of Davis and others, 1980, andAnderson and others, 1979). The late Early Cretaceoushornblende ages for plutons in the Turtle Mountainsmay approach the age of intrusion, as indicated by (1)the closeness of biotite and hornblende ages, and (2)the retention, in that range, of older ages inPrecambrian rocks thus sugges ting a relati vely coolhost rock and rapid cooling of the plutons.

Plutons in the Stepladder and Arica Mountains areporphyritic granodiorites closely similar to eachother in lithology and apparent K-Ar age. Biotitefrom the large Stepladder pluton gave a K-Ar coolingage of 72 m. y. (Armstrong and Suppe, 1973, correctedfor new decay constants by Calzia and Morton, 1980).Biotite from the small Arica pluton yielded a 72.4m.y. K-Ar age (Table 3, no. 9). Owing to theirisolation, these intriguingly identical ages must beconsidered simply as minimum ages. Both of theseplutons contain discrete quartz and phenocrysts ofzoned alkali feldspar with ragged margins indictive oflate interstitial overgrowth. Similar texture is seenin porphyritic parts of the Turtle pluton. Hypabyssaldikes emanate south from the Stepladder pluton.Similar dikes intrude Paleozoic rocks in the AricaMountains. These rocks occur in the upper plate of alow-angle fault of unknown displacement; the Aricapluton is in the lower plate.

Long dikes of quartz porphyry rhyoli te radiatesouth and southwest across much of the maincrystalline block of the Turtle Mountains. Several ofthe dikes follow faults. The dikes cut the CastleRock and Turtle plutons, but are not known to cutnearby Miocene and Oligocene(?) volcanic rocks, and soappear to be between late Early Cretaceous and Miocenein age. The dikes converge toward a dike-rich area inthe northern Turtle Mountains, suggesting a sourcepluton of Late Cretaceous or early Tertiary age maylie in that direction. Similar dikes occur inallochthonous crystalline rocks to the northeas t inthe western Chemehuevi and Sacramento Mountains, wherethey may be associated with an epizonal monzogranitepluton (John, 1982).

Mylonitic granodiorite gneiss crops out in asmall isolated exposure 0.8 km southwest of the AricaMountains (fig. 2). It resembles syntectonicallydeformed Late Cretaceous or early Tertiary myloniticgneiss in the footwall of the Whipple Mountains fault(Davis and others, 1980; Anderson and Rawley, 1981)and in the Iron Mountains (Miller and others, 1981),although its lineation is askew the regional pattern(Howard and others, 1982b) By analogy with theWhipple Mountains, the gneiss may represent deepseated (ca. 8-10 km) rock juxtaposed by an unseendetachment fault against shallower rocks in the AricaMountains area. If this area is similar to theWhipple Mountains, then steeply dipping Tertiary rocks3 km wes t of the Arica Mountains may be allochthonous.

TERTIARY ROCKS

Tertiary rocks include (1) small isolatedexposures near the Arica Mountains, (2) rocks cappingcrystalline rocks of the Turtle Mountains block, and(3) extensi ve expos ures in the Mopah-S tepladderblock. The rocks near the Arica Mountains are poorlyexposed sandstone and monolithic breccia that aresteeply west-dipping and possibly allochthonous.

Rocks that cap the Turtle Mountains block areerosional remnants of basal t, locally underlain bycrystalline-clast conglomerate (Hoodward McNiel andAssociates, 1974). Together these rocks are as thickas 200-300 m. K-Ar ages for the basal t in thesouthern Turtle Mountains (fig. 2) cluster around 20m. y. (Hoodward McNiell and Associates, 1974; Calziaand Morton, 1980).

In contrast, the Mopah Range and Stepladder andnorthern Turtle Mountains have sections 1 to 2 kmthick of andesite, rhyodacite, basalt, sedimentarybreccia and rhyolite. Rocks younger and some that may

346

be older than 20 m.y. are present. Volcanic rocks andfeeder dikes demonstrate that the Mopah Range andnorthern Turtle Mountains were volcanic centers(Chesterman, 1949; Bishop, 1963; Embree, 1967; Carrand others, 1980). Silicic and intermediate volcanicrocks dominate the lower part of the section in boththe southern Mopah Range (Carr and others, 1980) andthe northern Turtle Mountains and StepladderMountains. A basal basalt is locally present in theStepladder Mountains.

Hoodward McNiell and Associates (1974) reported a27.9 m. y. apparent K-Ar age on biotite from volcanicrock in the Mopah Range; this are would make the rockolder than most or all the other Tertiary rocks in theregion. However, the mapping of Carr and others(1980) suggests that this dated rock overlies arhyolite for which Carr and others report K-Ar ages of19.4 m.y. (biotite) and 18.6 m.y. (hornblende). Twoashflow tuffs in the northern Turtle Mountains weredated at 20.2 m.y. for a tuff about 1 km above thebase of the Tertiary section, and 20.0 m.y. foranother welded tuff of uncertain stratigraphicposition (Table 2, nos. 10, 11).

Embree (1967) reported an angular unconformitywithin the section in the northern Turtle Mountains,below which rocks dip 10-150 more steeply westwardthan overlying basalt flows. This upper basalt, whichforms mesas along the west sides of the Stepladder andnorthern Turtle Mountains, is increasingly less tiltedhigher in the section (Embree, 1967). A sample from anorthern inselberg along the trend of the mesasyielded a whole-rock age of 18.1 m.y. (Table 2, no.12) •

Welded tuff that closely resembles in age andlithology the widespread Peach Springs Tuff of Young(1966) crops out both east of the Stepladder Mountainsin the Sawtooth Range (Chemehuevi Mountains) and west,in the Little Piute Mountains. Dates of 18.1 m.y. and18.3 m.y. for the tuff at these respective localities(fig. 1; Table 3, nos. 13, 14) suggest that the tuffis younger than the lower unit of the Mopah-Stepladderblock. This relation seems confirmed by the presencebelow the tuff in the Sawtooth Range of plagioclasephyric silicic volcanic rock identical to rock in thelower unit in the northern Turtle Mountains.

Three stratigraphic relations suggest growthfaul ting and successive westward rotation duringdeposition of the Tertiary units: (1) The unconformitywithin the Tertiary section in the Mopah-Stepladderblock, (2) increasingly gentler dips and apparentofflap of higher basalt flows above the unconformity(Em::,ree, 1967), and (3) abrupt eastward thickening ofunits below the basalt across a fault in the MopahRange (Carr and others, 1980, cross-sec tion A-A'). Itcan be concluded that the Mopah-Stepladder block wassegmenting and dropping down relative to the TurtleMountains block during the deposition of volcanicrocks 18-20 m.y. ago.

TERTIARY FAULTING

Tertiary rocks are progressively down-dropped tothe east or northeast across numerous nOT'fllal faul ts inthe Turtle and Stepladder Mountains. Such faul ts inthe Turtle Mountains block dip steeply to the east andhave displacements of as much as 150 m. The faultzone between the Turtle and Mopah-Stepladder blocks isincompletely understood (Woodward McNiel1 andAssociates, 1974) and remains a high-priority targetfor further study. Displacement appears to be a fewhundred meters and one measured fault dips 600

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Ini tial position of b3.sal detachment,

northwestwa~d. Normal faul ts are numerous in theMopah Range, where Carr and othe~s (1980) measuredfaul t dips of 30-55 degrees no~theast, and in thenorther'n Turtle Mountains, where measured northeastfault dips a~e 40-60 degrees (Embree, 1967). Nu~erous

normal faul ts, dipping 30-500, downdrop volcanic rocks

progressively no:'theastwa~d in the StepladderMountains. A pattern emer'ges from these data: Faultsencou1tered farther no:,theast dip at increasinglygentler angles compared to faul ts to the southwest.The Stepladder pluton, farthest no~theast towa"dexposures of detachment faults, appear's to be til tedbut relatively w1broken. Otherwise, rocks in theTurtle Clnd Stepladder Mountains are generally morehighly faul ted and til ted towa~d the northeast.

The Tertiary rocks tend to dip roughlyper'pendicular" to the faul ts that cut them. ~:.:-':3twa:"d

dips of 20 degrees are reco:,ded in the TurtleMount 3. ins block, mos t ly about 5 to 40 degreessouthwestward in the southern Mopah Range (C3.rr andothers, 1980), 25-35 degrees southwestw3.~d in thenorthe~n Turtle Mountains (Embree, 1967) and 25-50degrees southwestwaY'd in the lower un it in theStepladder Mountains. Toward the east, westward dipsbecome progressively steeper as seen from the airabove the northern Mopah Range.

DISCUSSION

The faul ting and tilting of Tertiat'y rocks areconsistent with a concept of progressive breakqwaytoward the Whipple Mountains detach:nent terrane, inwh ich "ocks fa"ther from the headwall a:'ea areincreasingly out of place (Davis and others, 1980).In this concept (fig. 4), rocks are progressively:'otated along listric faults that merge downward andeastward, feeding into one (or more) decollement(s) atdepth. Isostatic upwarping has brought up and exposedthe decollement in the Whipple Mountains from anoriginal depth of pe"haps 5-12 km. The resultingflexure contributes to the observed increasingly lowerdips of faul ts and steeper dips of beds eastwardacross the Mopah-Stepladder block. Crude estimates oforiginal depth of the fault suggests that east of theMopah Range the flexure may be as p;reat as 30 to 40degrees. Regardless of the flexure, gently dippingfaul ts and steep til ts that persist eastwa~d acrossdomal culminations of the detachment fault in theWhipple Mountains (Davis and others, 1980) stand insharp contrast to the steep faults and shallow til tsin the headwall terrane.

347

The ultim3.te surfa~ing of the headwall may lie inthe valley west of the Turtle Mountains block,3.ccounting fOr the lowering of that block relative tothe Old Woman Mountains to the west. Headwall faultscontinue nOrthw'lrd into the southern Piute Mountains(fig. 1). Southward, Tertiary beds dipping 550

southwestwa"d near the Arica Mountains suggest that aheadwall lies to the west.

This listric headwall model implies that theTurtle Mountains block, with its steep faults andnea"ly flat-lying Tertiary rocks, is only slightlydisplaced. Rocks become increasingly displacedlaterally, a·s well as vertically, east-northeastwardacross the Mopah-Stepladde" block and into thedet3.chment terrane (fig. 4).

The Precambrian and Mesozoic rocKs in the Turtleand Stepladder Mou'1tains Y'esemble allochthonous rocksin the uppe:, plate of the Whipple Mountains detachmentfault. Allochthonous crystalline rocks that areequivalent to :'ock units in the Turtle Clnd StepladderMountains include gneiss in the Sacramento, Mohave andBuck Mountains, augen gneiss in the Whipple andSacramento Mountains, granite porphyry in the Whipple,Chemehuevi, Sacramento, Moh3.ve and Bill WilliamsMountains, diabase dikes in the Whipple, Sacramento,Chemehuevi, Mohave and Buck Mountains, quartz porphyrydikes in the Sacramento and Chemehuevi Mountains, andpe~haps granite in the Whipple Mountains. Thisdistribution of rock types is consistent with apalinspastic reconstruction reducing and compressingthe upper plate rocks back toward the Turtle Mountainsarea, along the S400 to 600 W azimuth determined byDavis and others (1980) for fault displacement in theWhipple Mountains. Most of the allochthonouscrystalline rocks noted above ar'e less than 1-3 kmstructurally beneath the base of the Tertiary section,in the til ted blocks in which they now lie. Theserocks were the upper' part of the crust beforedetachment faulting.

In the Turtle Mountains, caps of Miocene rockssuggest that 20 m.y. ago none of the rocks presentlyexposed were buried deeper than 1 km, although part ofthe Stepladder pluton, now tilted and relativelyunbroken, lay deeper. Exposed rocks in the TurtleMountains are unlike most rocks exposed below theWhipple Mount3.ins fault. The rocks below that faul tprobably resided at much deeper levels until exposedby tectonic denudation and isostatic rebound.

We think it reasonable that deep beneath the

MariaU.S. f

B, p.

Turtle Mountains are flat-lying syntectonicallydeformed Cretaceous granites like those of the WhippleMountains footwall (Davis and others, 1980), the IronMountains (Miller and others, 1981) or Old WomanMountains (Miller and others, 1982). The discordantplutons now exposed in the Turtle Mountains may betheir shallower equivalents, that are exposed betweenstructurally uplifted rocks on either side (fig. 4).

ACKNOWLEDGEMENTS

Dave Miller, Lawford Anderson, Ed DeWitt and JohnGoodge all joined us in the field and contributedobservations. Anderson pointed out severalsimilarities to plutonic rocks he has studied in theWhipple Mountains. Charles Meyer performed thefission-track analysis. We thank Jane Pike and JonSpencer for very helpful reviews of the manuscript.

REFERENCES CITED

Anderson, J. L., Davis, G. A., Frost, E. G., 1979,Field guide to regional Miocene detachmentfaulting and Early Tertiary(?) mylonization,Whipple-Buckskin-Rawhide Moutains, southeasternCalifornia and western Arizona, in Abbot, P. L.,ed., Geologic excu~sions in the southernCalifornia area: Geological Society of AmericaGuidebook, p. 109-133.

Anderson, J. L., and Frost, E. G., 1981, Petrologic,geochronologic and structural evaluation of theallochthonous crystalline terrane in the CopperBasin area, eastern Whipple Mountains,California: Final technical report(unpublished), February, 1981, 63 p.

Anderson, J. L., and RaWley, M. C., 1981, Synkinematicintrusion of peraluminous and associatedmetaluminous granitic magmas, Whipple Mountains,California: Canadian Mineralogist, v. 19, p. 83101-

Armstrong, R. L., and Suppe, John, 1973, Potassiumargon geochronometry of Mesozoic igneous rocks inNevada, Utah, and southern California:Geological Society of America Bulletin, v. 80, p.1375-1392.

Bishop, C. C., 1963, Geologic map of California,Needles sheet (1:250,000): Calif. Division ofMines and Geology.

Burchfiel, B. C., and Davis, G. A., 1981, MojaveDesert and environs, in Ernst, W. G., ed., Thegeotectonic developmentof California: PrenticeHall, Englewood Cliffs, N. J., p. 217-252.

Calzia, J. P., and Morton, J. L., 1980, Compilation of

isotopic ages within the Needles 1 by 20

quadrangle, California and Arizona: U.S.Geolgical Survey Open-File Report 80-1303.

Carr, W. J., 1981, Tectonic history of the VidalParker region, California and Arizona, inTectonic framework of the Mojave and SonoranDeserts, California and Arizona: U.S. GeolgicalSurvey Open-File Report 81-503.

Carr, W. J., and Dickey, D. D., 1976, Cenozoictectonics of eastern Mojave Desert (abs.): U.S.Geolgical Survey Professional Paper 1000, p. 75.

1977, Major tectonic zone in the eastern Mojave---Desert (abs.): U.S. Geolgical Survey

Professional Paper 1050, p. 70.Carr, W. J., Dickey, D. D., and Quinlivan, W. D.,

1980, Geologic map of the Vial NW, VidalJunction, and parts of the Savahia Peak SW andSavahia Peak quadrangles, San Bernardino County,California (1:24,000): U.S. Geolgical Survey Map1-1126.

348

Chesterman, C. W., 1949, Dike complex in the TurtleMountains, eastern San Bernardino County,California (abs.): Geological Society of AmericaBulletin, v. 60, no. 12, pt. 2, p. 1937.

Cooksley, J. W., 1960"1, Geology and mineral resourcesof Township 6 of North, Ranges 19 and 20 east,San Bernardino Base and Merician, San Berna~dino

County, California: Southern Pacific andCompany, unpublished report.

1960b, Geology and mineral resources of Township7 of North, Ranges 19 and 20 east ,San BernardinoBase and Merician, San Bernardino County,California: Southern Pacific and Company,unpublished report.

Coonrad, W. L., 1960, Geology and mineral resources ofTownship 7 of North, Ranges 19 and 20 east, SanBernardino Base and Merician, San BernardinoCounty, California: Southern Pacific LandCompany, unpublished report.

Crittenden, M. D., Jr., Coney, P. J., and Davis, G.H., eds., 1980, Penrose Cordilleran metamorphiccore complexes: Geologic Society of AmericaMemoir 153, 490 p.

Davis, G. A., Anderson, J. L., Frost, E. G., andShackelford, T. J., 1980, Mylonitization anddetachment faulting in the Whipple-Buck,kinRawhide Mountains terrane, southeasternCalifornia and western Arizona, in Crittenden, M.'D., Coney, P. J., and Davis, G. H., eds.,Cordilleran metamorphic core complexes:Geological Society of America Memoir 153, p. 79130.

Davis, G. A., 1974, Geology of Arica Mountains, inSouthern California Edison Company, informationconcerning site characteristics, Vidal Nuclear,,'Generating Station: v. V, Appendix 2.5-E.

Embree, Glenn, 1967, Geology of a portion of theTurtle Mountains quadrangle, San BernardinoCounty, California: San Diego State College,unpublished report, 40 p.

Hamilton, W. B., 1960, Structure in the BigMountains of southeastern California:Geolgical Survey Professional Paper 400277-278.

Hathaway, A. W. and Kuniyoshi, S., 1975, SouthernTurtle Mountains: A possible area of early basinand range structure in the southeastern MojaveDesert: Geological Society of America, Abstractswith Programs, v. 7,. no. 3, p. 322-323.

Howard, K. A., Goodge, J., and John, B. E., 1982"1,Detached crystalline rocks in the Mohave, Buckand Bill Williams Mountains, western Arizona:this volume.

Howard, K. A., Miller, D. M., and John, B. E., 1982b,Regional character of mylonitic gneiss in theCadiz Valley area, southeastern California: thisvolll1le.

John, B. E., 1982, Geologic Framework of theChemchuevi Mountains, southeastern California :this volume.

John, B. E., 1981, Reconnaissance study of Mesozoicplutonic rocks in the Mojave Desert region, inTectonic framework of the Mojave and SonoranDeserts, California and Arizona: U.S. GeolgicalSurvey Open-File Report 81-503, p. 48-50.

Lanphere, M. A., 1964, Geochronologic studies in theeastern Mojave Desert, California: Journal ofGeology, v. 72, p. 381-399.

Miller, C. F., Howard, K. A., and Hoisch, T. D., 1982,Mesozoic thrusting, plutonism and metamorphism inthe Old Woman-Piute Range, souteasternCalifornia: this volume.

Miller, D. M., Howard, K. A., and Anderson, J. L.,1981, Mylonitic gneiss related to emplacement of

a Cretaceous batholith, Iron Mountains, southernCalifornia, in Tectonic framework of the Mojaveand Sonoran--neserts, California and Arizona:U.S. Geolgical Survey Open-File Report 81-503, p.13-75.

Nakata, J. K., 1982, Preliminary report on dikingevents in the Mohave Mountains, Arizona: thisvolume.

Silver, L. T., and McKinney, C. R., 1963, U-Pbisotopic age studies of a Precambrian granite,Marble Mountains, San Bernardino County,California:

Stone, Paul, and Howard, K. A., 1979, Compilation ofgeologic mapping, Needles l ox2° sheet, California

and Arizona: U.S. Geological Survey Open-FileReport 79-388.

·Stone, Paul, Howard, K. A., and Hamilton, W. B., 1981,Paleozoic metasedimentary rocks of thesoutheastern Mojave Desert region, California andwestern Arizona, in Tectonic framework of theMojave and Sonoran Deserts, California andArizona: U.S. Geological Survey Open-File Report81-503, p. 104-106.

Streickeisen, A. L., 1973, Plutonic rocks:Classification and nomenclature recoIIlJlended bythe IUGS Subcommission on the systematics ofigneous rocks: Geotimes, v. 18, no. 10, p. 2630.

Woodward McNiell and Associates, 1974, in SouthernCalifornia Edison Company, Information--concerningsite characteristics, Vidal Nuclear GeneratingStation: Apendices 2.5-E and 2.5-1.

Young, R. A., 1966, Cenozoic geology along the edge ofthe Colorado plateau in northwestern Arizona:Dissertation Abstract, Section B., v. 27, n. 6,p. 1994.

349

Table 1. Rock Units in the Turtle Mountains Area 1

Des criptionPlutonic rocks classified after lUGS

(Streckeisen, 1973)

Upper basalt: Basalt and andesite flows.West-dipping mesas along west flank ofStepladder and northern TurtleMountains and Mopah Range.Unconformably over unit Tv.

Peach Springs Tuff of Young (1966)(?):Welded crystal-rich ashflow tuff containing blue sanidine. Present eastand west of Stepladder Mountains inSawtooth Range (Chemehuevi Mountains)and in Little Piute Mountains.

Volcanic rocks: Flows and flow breccias ofrhyodacite, rhyolite, dacite, andesiteand basalt, thin ashflow tuffs,sedimentary volcaniclastic breccias.Sequence probably thousands of feet thick.In Mopah Range, Turtle Mountains, andStepladder Mountains.

Sedimentary rocks: Crystalline-clastboulder conglomerate in southwesternTurtle Mountains. Arkose in northernTurtle Mountains. Near AricaMountains: sandstone and monomictbreccia derived from mylonitic gneiss.

Intrusive rocks: Dikes and necks ofandesite, basalt, and rhyodacite,cutting preTertiary and Teritiaryrocks in Turtle Mountains.

Quartz porphyry: Silicic dikes up to20 m thick radiati nil. Sand SW acrossPrecambrian rocks from N-centralTurtle Mountains. Five to fiftypercent phenocrysts of feldspar,rounded quartz, biotite al tered tomuscovite, rare hornblende. CutsCastle Rock and Turtle plutons.Resembles dikes in Sacramento andChemehuevi Mountains.

Diorite near Martins Well: Hornblendediorite stock in western Turtle Mountains.

MapSymbol

(Fig. 2)

Tb

Tv

Ts

Kd

Plutonic RocksColor ExposedIndex Area (km2 )

25-40

ApparentAge (m.y.)

18.1

20.2,19.9(19.0,20.3,20.4, 27.4)

86.8 H

Geologic Age

Miocene

Mioceneand

Oligocene (?)

Tertiaryor

Cretaceous

Stepladder pluton: Light-colored porphyriticgranodiorite with pinkish coarse-grainedK-feldspar, quartz and plagioclasephenocrysts.

Kgd 7 75 (72.0 B) Cretaceous

1Apparent ages listed are K-Ar and one fission-track age. Ages compiled byCalzia and Morton (1980) in parens. B, biotite, H, :1omblende, Z, zircon.

350

Table 1. Rock Units in the Turtle Mountains Area (cont.)

Des cription Map Plutonic Rocks Apparent Geologic AgePlutonic rocks classi fied after lUGS Symbol Color Exposed Age (m. y. )

(S treckeisen, 1973) (Fig. 2) Index Area (km2 )

Arica pluton: Light-colored porphyritic 9 >1 72.4 Bbiotite with pinkish coarse-grainedK-feldspar phenocrysts.

Castle Rock pluton: Medium-grained biotite Kgd 15 > 7.5 101 Hhornblende granodiorite to monzogranite. 93.0 BNorth of Turtle pluton between Mopah (98.0 B)Range and Turtle Mountains block. Cretaceous

Turtle pluton: Biotite-hornblende medium-grained granodiorite, locally porphyritic. 9-15 > 50 105.9 HForms southeastern Turtle Mountains. 97.3 BMapped as quartz diorite by WoodwardMcNiell and Associ ates (1974) •Peripheral biotite-muscovite monzogranitedikes resemble granite(adamellite of Davis and others, 1980)in Whipple Mountains.

Granite of southern Turtle Mountains: Kg 5-10 > 5 101.3 BPorphyritic biotite monzogranite to grano-diori te. Locally minor hornblende ormuscovite. Coarse-grained tabular K-feld-spar phenocrysts are commonly aligned.May be gradational to Turtle pluton.Mapped as quartz monzonite byWoodward McNiell and Associates (1974).

Mylonitic gneiss: Granodiorite gneiss withSE-trending mylonitic lineation. my 10 (t: 1 Cretaceous(?)Isolated exposure, conceivably a giantmegabreccia block. Resemblesmylonitic gneiss in Iron and in WhippleMountains.

Diorite of southwestern Turtle Mountains: Jd 25-55 ') 167 H Jurassic(?) ;Medium-grained hornblende-biotite diorite 100.7 B may beto quartz monzodiorite. Precambrian

Metasandstone: Purplish sandstone, gritstoneand sandy schist and green feldspathic pz Earlyschist in Arica Mountains. Mesozoic

Metamorphosed Kaibab Limestone: Calcitic Permianand dolomitic marble, in part stripedby brown-weathering metachert, inArica Mountains.

Metamorphosed Coconino Sandstone: Platymetaquartzite in Arica Mountains.

Metamorphosed Hermit Shale: Phyllite havingcalcitic laminations in Arica Mountains.

Metamorphosed Supai Formation: Dark- brown- Permian andweathering calc-silicate rock interlayered Pennsylvanianwith marble in Arica Mountains.

351


DescriptionPlutonic rocks classified after lUGS

(Streckeisen, 1973)

Metamorphosed Redwall Limestone: Whitecalcitic marble in Arica Mountains.

MapSymbol

(Fig. 2)

pz


ApparentAge (m.y.)

Geologic Age

Mississippian

Diabase: Hornblende diabase and gabbro dikesup to tens of meters wide in Turtle andStepladder Mountains. Blue-green hornblende may be secondary. Commonly ophitic.Most dikes in Turtle Mountains trend N-S.

Diod te near J ohnsons Well: Horn bl endediori te in undeforrned stock, cuts augengneiss of Johnsons Well in western TurtleMountains. Texture variable.

Granite of northern Turtle Mountains:Coarse-grained dark biotite-hornblendemonzogranite, local medium-grainedquartz monzodiorite.

Granite porphyry of southern Turtle Mountainsnear Rice: Biotite syenogranite tomonzogranite with aligned coarse-grainedtabular gray K-feldspar phenocrysts inmedium-grained matrix. High gamma radia-.tion. Intrudes metasedimentary gneisses.

Granite porphyry dike in western TurtleMountains: Dike 1 m wide cuts gneiss.Quartz-rich syenogranite having alinedcoarse-grained K-feldspar phenocrysts.High gamma radiation.

Granite porphyry in northwestern TurtleMountains. Irregular body few metersacross of quartz-riCh syenogranite.Aligned tabular coarse-grained K-feldsparphenocrysts. Resembles granite porphyry ofWhipple Mountains (Davis and others, 1980).

Granite porphyry in Stepladder Mountains.Several small irregular bodies of monzogranite with purplish quartz and alignedtabular feldspar phenocrysts. May resemblemafic granite porphyry of Andersonand others (1980) in Whipple Mountains.

Augen gneiss of Johnsons Well: Biotitehornblende granodiorite and lesser granitegneiss. Medium-grained. Gray, purplish orpink K-feldspar phenocryst augen0.4-1 em across. Cuts most other gneissesbut is cut by one folitated leucogranitebody. Forms large massif and many smallerbodies through much of Turtle Mountainsblock. Resembles allochthonous augengneiss in Whioole Mountains (Davis andothers, 1980) and in Sacramento Mountains.

pCg

pCj

50-60

40-60

10-25

5-10

2

2

15

8-30

352

< 1

1/20

>15

> 1/2

«< 1

«< 1

> 100

439 H

1350 H

88.2 Z

230 B

Precambrian(Proterozoic

Y ?)


X?)


DescriptionPlutonic rocks classified after lUGS

(Streckeisen, 1973)

MapSymbol

(Fig. 2)


ApparentAge (m.y.)

Geologic Age

Granite gneiss of Virginia May Mine:Throughout much of Turtle Mountain block.Medium-light gray fi ne-grained biotitemonzogranite to syenogranite gneiss.Commonly contains leucocratic sweat(?)veins with dark borders and bluish quartz.Both these veins and the gneiss containgarnet porphyroblasts, largely nowretrograded to phogopite. Intrudesamphibolite. Similar gneiss is foundin the Sacramento and Mohave Mountains.

Gneissic rocks: Undivided leucogranite,veined and banded granitic gneisses inArica, Turtle and Stepladder Mountains.Garnet-spotted leucocratic gneiss inStepladder Mountains similar to gneiss inMohave Mountains. Some (unknown) fraction of gneisses may be metasedimentary.

~pbibolite: Small layers and pods,commonly migmatized, throughout much ofTurtle Mountains block and in StepladderMountains. Amphibolite is found also inmost other ranges in the region.

Metasedimentary rocks: Dark quartzite,metaconglomerate, gneiss, schist.

pCv

pC

5-10 >75

25

1-5


X?)

Table 2. Fission-track age of zircon

MapNo. Field No. Rock

NorthLatitude

WestLongitude

Spontaneous tracksdensity no.

(cm- 2 )

Induced tracksdensity no.

( cm- 2 )

Neutron Dose Agel!

(m.y. )

13 H79TM-196 Graniteporphyry

4.452 x 106 316 1.474 x 107 523 88+6

Combined data of two zircons.C. E. Meyer, Analyst

AF = 7.03 x 10 -17yr-1, 235 u/258 u = 7.252 x 10-3 , ~ = 580 x 10-24 cm2•I!~One standard deviation.

353

Table 3. K-Ar ages

MapNo. Field No. Rock and

Analyzed Mineral

NorthLatitude

WestLongitude

PercentK20

(avg. )40Ar*X)0-1O

mol gm

!;arcent40Ar*1

Ar total

Calculated WeightedAge Avg.Agem.y. m.y.

bio

quartz hbl 34°11'25" 114°50'03"monzodiorite

230:t.7

439:t.32

167.±..5

100.7.±..3.0

231.4229.1

439.0

96.5103.5

1326 1350:t.4013341371

165.6167.5

989863

89

9479

7358

3744

32.4332.09

4.105

1.7691.797

12.4613.38

0.52

9.125

1.528

8.725

0.7085

gabbro

Diorite hbl

granodiorite bio 34°16'02" 1"14°54'17"gne iss

H79TM-99z

4 H79TM-486

2 H79TM-97b

3 H79TM-103

5 H79TM-486a monzogranite bio 34°11'20" 114°49'55"

104+6104.1

104.6107.3

99.195.7

101.0101.5

72

8089

6357

8187

11.7511.33

1. 371

1.2901. 324

12.2012.26

0.832

0.92

8.155

8.007bio

granodiorite hbl 34°10'49" 114°45'00"6 H79TM-121

7 H79TM-137

8 H79TM-112

9 H79Ar-466

10 H79Mo-149

11 H79Mo-157

12 KH78-111

13 H79CH-299

14 H79LP-56

bio

diorite

welded tuff bio 34°25'25" 114°48'35"

welded tuff bio 34°27'17" 114°49'39"

welded tuff san 34°35 '46" 114°39' 02"

welded tuff san 34°37' 24" 115°04' 22"

8.56

0.4225

8.835

8.585

8.253

2.745

8.74

9.41

11.17

0.5084

9.396

2.509

2.388

0.7182

2.285

2.498

95

51

84

33

65

63

73

67

93.0

86.8

72.4

20.2

20.0

18.1

18.1

18.3

93. 0.±..3. 2

86.8+2.6

20.2+1.0

20.0.±..0.7

18.1+0.6

18.1+0.6

Ages are calculated using the following constants

40K decay constants:

Abundance ratio :

1\+ A{=0.581 x 10- 10 yr- 1

"' 4 962 10- 10 yr- 1/\{3=' x

40K/Ktotal =1.167 x 10-4 atom percent.

Numbers 1, 3, 4, 5, 6, 8, 10 and 11 analysed by M. A. Pernokas, D. V. Vivit, P. R. Klock, and S. T. Neil.

Numbers 2, 7, 9, 12, 13 and 14 analysed by R. F. Marvin, H. H. Mehnert, V. M. Merrit and E. L. Brandt.

Samples prepared by D. H. Sorg and M. A. Pernokas.

hbl, hornblende; bio, biotite; WR, whole-rock; san, sanidine.

354

c.>O'iCI>

Figure 1. Exhumed Baker Peaks-Copper Mountains detachment fault, southern Baker Peaks. Fault surface dips 10 to 15 degrees to the west.

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