Tehuama Colusa Serpentine

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66 HOPSON AND PESSAGNO

progressed behind the ridge on the east. The Stony CreekBeehive Flat composite fault sytembetween the TCSM and CRO was a dextral transform faultduring stage 2 (TithonianValanginian)and a W-vergent reverse faultduring stage 3 (Valanginian), during underthrusting and tilting of TCSM/CRO basement. The extensional Coast Range fault, bounding TCSM on the west, brought updeeply subducted Franciscan Cretaceous high-P/T metamorphic rocks in the late Cretaceous (Jaykoet al., 1987).

A Question of Identity

THE GEOLOGIC MAP of California (California Divi-sion of Mines and Geology, 1966, scale 1:2,500,000;Jennings, 1977, scale 1:750,000) and the UkiahSheet of the state geologic map (Jennings andStrand, 1960, scale 1:250,000) show a prominentN-Strending purple map unit along the easternflank of the Coast Range, west of Sacramento Valley.This is shown as Ultramafic Rocks in the map leg-ends and described as mostly serpentinite. But italso encompasses mafic rocks including remnants of a Jurassic ophiolite in the Digger CreekElder Creek area near Paskenta (Bailey et al., 1970;Hopson et al., 1981; Robertson, 1990; Blake et al.,1992; Shervais, 2001; Shervais et al., 2004), and athick pile of mostly basaltic submarine lavas near Stonyford (Brown, 1964a, 1964b; Bailey et al, 1970;

Shervais and Kimbrough, 1985b; Shervais andHanan, 1989; Shervais et al., 2002 and in press, b)plus similar basalt with red chert near Wilbur Springs (Hopson et al., 1981; McLaughlin et al.,1989). Other submarine basalt-diabase-gabbro-ultramafic successions farther south in the CoastRanges were also recognized as Jurassic ophiolites,were inferred to be remnants of former oceanic crustand upper mantle, and were collectively named theCoast Range ophiolite(Bailey et al., 1970). Theremaining, more extensive serpentinite, and also theStonyford and Wilbur Springs lavas, were then com-monly portrayed as part of the Coast Range ophiolite(CRO). The serpentinite is commonly assumed to bethe lower, relict mantle portion of the ophiolitebecause of its peridotite tectonite protolith andbecause it was situated at the apparent base of thetilted east-facing Digger CreekElder Creek CROremnant and its Great Valley Group (Ingersoll,1990) sedimentary cover.

Moreover, the serpentinitic beltlike the ophio-

liteis unmetamorphosed except for low-P/Thydrous alteration; apparently it was never sub-ducted. In contrast, it is faulted on the west againsthigh-P/T metamorphic rocks of the South ForkMountain Schist (SFMS), part of the widespreadFranciscan Complexthat comprises much of the

northern Coast Ranges (e.g., Blake et al., 1985,1988, 1992). The SFMS protolithUpper Jurassic/Lower Cretaceous (i.e., upper Tithonian to Valangin-ian) graywacke and mudstone, plus slices of basalticlava and chertwere subducted to depths of >25km, deformed, and metamorphosed at blueschist-facies P/T conditions (Jayko et al., 1986; Blake etal., 1988) during the Early Cretaceous (Suppe andArmstrong, 1972; Lanphere et al., 1978; McDowellet al., 1984), then uplifted tectonically and exhumedby extensional unroofing (Jayko et al., 1987; Harmset al., 1992) toward the rear of the seaward-growingFranciscan accretionary wedge (Platt, 1986). Thiscontrast of metamorphic grades seemed further reason to link the serpentinite belt with the CRO,rather than with the local Franciscan (i.e., the SouthFork Mountain schist).

Thus, the regional geologic sketch maps within

many CRO papers since 1970 show the entire widthof the purple belt, including the Stonyford subma-rine lavas, as part of the California Coast Rangeophiolite (or Jurassic ophiolites). This practicebegan with Bailey et al. (1970), Bailey and Blake(1974), Hopson et al. (1981), and Crerar et al.(1982), all of whom lumped the ultramafic purplebelt with the Coast Range ophiolite. However, acontrary note was sounded by Shervais and Kim-brough (1985a, 1985b, 1987; also Hanan et al.,1991):

This serpentinite-matrix mlange, whichforms a belt that extends from Stonyford inthe south to Paskenta on the north, has beeninterpreted as CRO by most recent investiga-tors (e.g., Bailey et al., 1970; Bailey andBlake, 1974; Hopson et al., 1981). We sug-gest that correlation with the Franciscan com-plex may be more likely, based on thegeochemical similarity between volcanicrocks of the mlange and Franciscan volcanicrocks. (Shervais and Kimbrough, 1985b, p.828829)

A prescient suggestion indeed! But most subse-quent workers remained content to lump this hugeultramafic belt (Fig. 1) as part of the Coast Range


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TEHAMA-COLUSA SERPENTINITE MLANGE 67

FIG. 1. Generalized geologic map of the northern Coast Range bordering the Sacramento Valley, showing the Tehama-Colusa serpentinite mlange and the Mesozoic terranes that border it on each side. Geology modified from Jennings andStrand (1960) with improvements from MacPherson (1983), McLaughlin et al. (1989), Blake et al. (1992), and Shervaiset al. (in press, a).


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ophiolite on small regional maps that depict CROdistribution. These include MacPherson and Phipps(1985); Lagabrielle et al. (1986); Blake et al.,(1987); McLaughlin et al. (1988); Seiders, (1988);Robertson (1989, 1990); Shervais (1990, 2001);Stern and Bloomer (1992); Cowan and Bruhn, 1992;Dickinson et al. (1996); Giaramita et al. (1998);Coleman (2000); Godfrey and Dilek (2000); Inger-soll (2000); Pessagno et al. (2000), Constenius et al.(2000), and Huot and Maury (2002). Even the NapaCounty serpentinite massifs south of Figure 1 aredescribed as part of the Coast Range ophiolite(Phipps, 1984).

Problems with this interpretation arose, however.First, the serpentinitic belt was not originally the

base of the Coast Range ophiolite (Paskenta andElder Creek remnants) but is separated from it bythe Beehive Flat fault, which diagonally truncatesthe CRO members (Fig. 2). Second and more telling,much of the serpentinitic belt is a serpentinite-matrix tectonic mlange(e.g., Hopson et al., 1981;Shervais and Kimbrough, 1985a, 1987; Jayko andBlake, 1986; Jayko et al., 1987; Robertson, 1990;Huot and Maury, 2002; Shervais et al., in press, b),not simply a serpentinized peridotite-tectonite basal

member of the ophiolite succession. Up to 3040%of the serpentinitic belt is comprised of tectonicmlange blocks enclosed in a sheared, pulverizedserpentinite matrix. Most of the blocks are alteredsubmarine basaltic lava, including pillow lavas andlava capped by radiolarian ribbon chert. Nativeblocks of serpentinized harzburgite and dunitetectonitegrading into the enclosing shearedserpentinite matrixand sparse blocks of ribbonchert are widespread but minor. Diabase and plu-tonic blocks are rare.

Jayko and coworkers mapped the serpentinitemlange separately from the ophiolite. They inferredthat it lies structurally below the CRO and origi-nated differently (Jayko and Blake, 1986; Jayko etal., 1987). But the two units are now faulted together and occasionally treated as a cohesive tectonic unit:

The Coast Range ophiolite consists of a ser-pentinite-matrix mlange structurally over-lain by a disrupted ophiolitic fragment.(Jayko et al., 1987, p. 1057)

The present study takes another look at theserpentinite belt to assess its lithology, structure,age, and origin, and also the question: Is it partof the Coast Range ophiolite (CRO)? Or theFranciscan Complex? Or some different, perhaps

unrelated, terrane? How did it originate, and howwas it emplaced in its present position? Our findingshave important implications for the Mesozoictectonic evolution of the region.

The ultramafic belt is here called the Tehama-Colusa serpentinite mlange(TCSM) to better denoteits geologic character and its geographic locationspanning Tehama, Glenn, and Colusa counties N-Salong the Coast Range front west of the SacramentoValley.

Tehama-Colusa Serpentinite Mlange

The ultramafic belt (Fig. 1)the Tehama-Colusaserpentinite mlange (TCSM)is convenientlydescribed in two segments: (1) a northern segment,informally called the Tehama serpentinite mlange(known also as the Round Mountain serpentinitemlange); and (2) a broad southern segment, theColusa serpentinite mlange. They are describedbelow, following a brief description of the serpen-tinite that is common to both segments.

The serpentiniteThe TCSM belt, whose protolith was peridotite, is

pervasively serpentinized throughout its >90 kmlength. Massive serpentinite with relict peridotitetextures is widely overprinted by rock with spacedshear surfaces (slickentite), schistose serpen-tinite, and locally serpentinite gouge, recording var-ious degrees of internal deformation. Hard blocks of massive, serpentinized peridotite weather out withtopographic relief, forming native mlange blocksthat grade into surrounding, low-relief, sheared ser-pentinite or gouge. The serpentinite shear surfacesand schistosity are variable locally and warp aroundmlange blocks, but in general are subvertical andstrike roughly N-S, subparallel to the outcrop belt.

The serpentinite terrane has a characteristic veg-etation marked by sparse although ubiquitous dig-ger pines, leather oak, and abundant manzanitabrush on the mountainsides, and also by cypresstrees along some stream canyons. The vegetationcover varies from relatively open, with local barerocky ground, to heavy brush cover that is difficultto penetrate on foot. In most cases, however, the

large proportion of manzanita to other brush types isindicative of serpentinite bedrock.

The serpentinite mineral assemblage is chieflychrysotile-lizardite-brucite-magnetite chlorite.Antigorite has been reported (Huot and Maury,2002) but seems not to be a major phase. Submicro-


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several percent of some serpentinites and remainlargely fresh. Cr-Al spinel (~23 %) is largelyaltered to opaque oxides. Carbonate mineralsincluding magnesite locally fill fractures. Maficdikes that cut the original peridotite are altered towhitish rodingite composed chiefly of hydrous Ca-Al silicate minerals. A detailed discussion of theserpentinite mineralogy, chemistry, and the serpen-tinization process in the Stonyford area is given byShervais et al. (in press, a).

The chrysotile-lizardite-brucite serpentinite isinferred to be the product of low-temperaturehydrous alteration (Coleman, 1971). Antigoriteserpentinite, the product of higher-temperaturemetamorphism, has not been reported from theTehama-Colusa ultramafic belt.

Tehama (Round Mountain) serpentinite mlangeThe mlange character of the serpentinitic belt

(Fig. 3A) and its cross-cutting fault contact againstthe Digger CreekElder Creek CRO remnant wererecognized by Maxwell (1974), Fritz (1975), Hopsonet al. (1981), Shervais and Kimbrough (1985a,1987), Jayko and Blake (1986), and Jayko et al.(1987). Jayko and Blake (1986) provided an infor-

mative small-scale map of the serpentinite mlangeshowing distribution of the larger mlange blocks.Hopson et al. (1981) named this and similar rocks tothe south the Tehama County Ophiolite mlangebelt, from which the present name is modified. Butthey left it as part of the CRO for want of definitiveevidence to the contrary. However, Jayko et al.(1987, p. 480) noted that In the Paskenta area asmall but complete remnant of the Coast Rangeophiolite is structurally underlain by a serpentinitemlange, which they depicted separately fromCRO on a geologic map (their Fig. 6; see also Jaykoand Blake, 1986; Blake et al., 1992). Theydescribed and interpreted this unit, which theycalled the Round Mountain serpentinite-matrixmlange, (see also Huot and Maury, 2002). Robert-son (1990) also showed this belt as tectonic mlangefaulted against the CRO units. Shervais and Kim-brough (1985b, 1987) and Shervais and Hanan(1989) recognized that serpentinite-matrix mlangecomprised the ultramafic belt in the Stonyford area,

since mapped by Shervais and Zoglman-Schuman(Shervais et al., in press, a, b).The Tehama (Round Mountain) serpentinite

mlange (Fig. 2) is well exposed (less now thanformerly, due to roadcut deterioration) along roadtraverses through the ultramafic belt west and south-

west of Paskenta, and in a stream traverse throughthe spectacular gorge of Thomes Creek (i.e., thePaskenta, Bennett Creek, and Thomes Creek rem-nants described by Hopson et al., 1981). Themlange is quite similar in all three traverses, butthe description of the Bennett Creek road sectioncaptures its essence (Hopson et al., 1981, p. 459):

The ophiolite [i.e., serpentinite mlange] beltis well exposed along the Covalo road atBennett Creek, 11 miles southwest of Paskenta. The belt is 1.7 km wide at thispoint and composed entirely of ophioliticmlange. Steeply dipping faults bound themlange on both sides, separating it fromTithonian strata of the Great Valley Sequenceon the east and from metagraywacke andphyllite of the South Fork Mountain Schist(Franciscan) on the west.

The ophiolite [serpentinite] mlange consistsof tectonic blocks of pillow lava, massive lava(greenstone), and rare blocks of radiolarianchert, and remnants (native blocks) of serpen-tinized peridotite enclosed in a matrix of ser-pentinite. The blocks of volcanic rock rangefrom less than 1 m up to 500 m in diameter,with blocks 100 to 200 m being common-place. The larger blocks form resistant knobsand hillocks amid hummocky mlangeterrrane. Volcanic blocks comprise approxi-mately one-half to two-thirds of the mlangesurface area, but the proportion of serpen-tinite is locally much higher.

The mlange matrix consists of intenselysheared, slickensided serpentinite. Shear surfaces dip steeply and trend sub-parallel tothe ophiolite [serpentinite] belt, but indetail these surfaces conform to the enclosedtectonic blocks. Commonly, the serpentinitecuts up through some of the larger blocks indike-like sheets; here the serpentinite hasclearly intruded or been plastically squeezedin along faults or shear zones. Locally, two or more subparallel serpentinite intrusionsisolate slices of volcanic rock between them,and those slices become progressively dis-

rupted into boudin-like blocks. Finally, zonesof serpentinite 5 to 10 m wide contain trainsof disrupted volcanic blocks or only isolatedblocks. It is evident that a once-intact volca-nic formation has been broken by faulting andshearing, and then pervasively invaded and


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internally disrupted by tectonically mobileserpentinite, creating a mlange.

The serpentinite is composed of chrysotile-lizardite serpentine and brucite, indicative of formation at low temperature (Coleman,

1971b). It originated mainly from harzburg-ite, for serpentinized harzburgite is the ubiq-uitous type of native mlange block.

Pillow lava is abundant among the volcanicmlange blocks. One large block contains 7 m

FIG. 3. Outcrop photographs of typical rocks within the northern Tehama-Colusa (Round Mountain) serpentinitemlange. A. Typical serpentinite mlange southwest of Paskenta. Dark rock is serpentinite matrix; light-hued rocks areweathered basaltic lava mlange blocks. B. Serpentinite mlange at Black Diamond Glade northwest of Stonyford, show-ing the characteristic sparse vegetation with abundant manzanita; low rounded brushy hill in the center is a basalticmlange block. C. Pillow lava within a basaltic mlange block; Tehama (Round Mountain) serpentinite mlange west of Crowfoot Point. The pillows, resting on dark basaltic mud, are upside down. D. Pillow lava within a basaltic mlangeblock; Tehama (Round Mountain) serpentinite mlange northwest of Bennett Creek. E. Radiolarian ribbon chert within acomposite basalt-chert mlange megablock; Tehama serpentinite mlange west of Crowfoot Point. F. Part of a weatheredbasaltic mlange block (lower left) capped by radiolarian ribbon chert (dark rock, partly behind tree); Tehama mlangealong Road M2 near Round Mountain.


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of thin-bedded radiolarian chert resting indepositional contact upon pillows. Petro-graphically, these lavas are spilites derivedchiefly from aphyric and plagioclase-micro-phyric basalt. Primary igneous textures,including variolitic quench textures, are wellpreserved, but alteration has producedmainly secondary assemblages of albite-chlorite-epidote-sphene-calcite quartz.Clinopyroxene is the main surviving volcanicphase.

The tectonic block assemblage within themlange and also the blocks themselves (Fig. 3)resemble Franciscan basaltic lava and chert, and

differ significantly from rocks in the CRO. Compar-isons are deferred to later sections.Red Mountain, at the southern end of the Tehama

serpentinite mlange belt west of Chrome (Fig. 2),exposes massive serpentinized harzburgite withoutmlange blocks. Jayko and Blake (1986, Fig. 3) andJayko et al. (1987, Fig. 6) draw a contact that sepa-rates this harzburgitic area from the adjacentserpentinite mlange and show it as CRO. But weregard it instead as a change from serpentinized

mantle peridotite mixed with blocks of oceaniccrusti.e., the mlangeto more massive serpenti-nized peridotite withoutblocks. This may provide aglimpse of once-deeper mantle below the levelof admixed crustal blocks. Such a transition fromserpentinite mlange to massive serpentinized peri-dotite (without mlange blocks) is common farther south, in the Colusa mlange.

The TCSM belt in Glenn County (Fig. 1) consistsof a narrow (0 to 250 m) strip of serpentinite,

bounded on both sides by steep faults (Jennings andStrand, 1960; Blake et al., 1992). Its appearencewas described by Hopson et al. (1981, p. 458):

A typical section is well exposed where thisbelt is crossed by the south fork of Elk Creek.Here the ophiolite is represented by only20 to 40 m of strongly sheared and pulverizedserpentinite. The eastern contact is a verticalfault (Stony Creek fault) that truncated thebedding of steeply dipping Tithonian mud-

stones belonging to the lower Great ValleySequence. The western contact is another vertical fault that sets the serpentinite againstdark slaty argillite and metagraywacke of theSouth Fork Mountain Schist (FranciscanComplex). This fault is sharp, lacks imbrica-

tion, and does not noticeably disturb the adja-cent bedded metasedimentary sequence.

Basaltic mlange blocks are locally present(Blake et al., 1992) but inconspicuous in this narrow

serpentinitic septum, where weathering and poor exposure obscures details. We concur with Jaykoand Blake (Jayko and Blake, 1986; Jayko et al.,1987; Blake et al., 1992) that this narrow strip isa continuation of the Round Mountain (Tehama)serpentinite mlange.

Colusa serpentinite mlangeThe name change from Tehama to Colusa seg-

ment of the TCSM belt is arbitrarily placed near theColusa County line, where the belt turns westward tocurve around the Stonyford volcanic complex (Fig.1). The belt southward from here crops out in ColusaCounty. This southern segment of the ultramafic beltexposes serpentinite-matrix mlange similar to theTehama mlange, but with patchy distribution of mlange blocks and large areas of serpentinizedperidotite devoid of blocks. Our findings come fromthree widely spaced traverses across the belt: (1) theDry CreekBlack Diamond Ridge road north of Stonyford: (2) the Goat Mountain Road through

Little Stony Creek canyon; and (3) the BartlettSprings Road from Bear Valley to Little IndianValley Reservoir.

The Dry CreekBlack Diamond Ridgesectionexposes chiefly sheared serpentinite at lower eleva-tions with blocks increasing in frequency up theeastern flank of Black Diamond Ridge. Severalbasaltic blocks up to 250 m or more across occur inthe Black Diamond Glade area. One of these (Fig.3B) is rimmed (overlain) by >20 m of dark red rib-bon chert, then by dark argillite soil. Three other slabs of dark-grey slaty argillite, each tens of meterslong, crop out lower along this traverse. Thismlange changes westward into more massive,less serpentinized peridotite tectonite along BlackDiamond Ridge itself.

The Little Stony Creek Canyontraverse (GoatMountain Road) through the TCSM reveals a similar pattern. Here too, crustal mlange blocks in shearedserpentinite are found only along the eastern side of the belt, changing to blocky serpentininized peridot-

itelocally sheared but devoid of mlangeblocksthrough the interior and western side. Per-haps the oceanic mantle is more deeply exposedhere, beneath the level of disrupted oceanic crust.

The geologic map of the Stonyford area (in Sher-vais et al., in press, a, b), spanning the N-S interval


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from Black Diamond Ridge to Little Stony Creek,also shows an eastern belt of sheared serpentinitewith mlange blocks and a western zone of massive,partly serpentinized harzburgite. Our observationsaccord with theirs. Also, two megablocks of volcan-ogenic sandstone are shown within the serpentinitesouth of Stonyford, on the Shervais map. But weregard them instead as remnants of ophiolitic sand-stone (from former basal GVG thrust over theserpentinite) that were let down as erosional lagblocks upon the TCSM surface. We find a similar sandstone block resting upon (not within) thesheared serpentinite nearby at Little Stony Creek.

The mlange is best developed on the south,along the Bartlett Springs Road, partly within the1:24,000-scale geologic map of the Little IndianValleyWilbur Springs area (McLaughlin et al.,1989). Here sheared to massive serpentiniteencloses tectonic blocks up to 200 m or more thatcomprise approximately 20% (locally up to 40%) of the bedrock. Most of the mlange blocks are basalticsubmarine lava. The lavas are strongly altered, andcommonly sheared internally with chlorite coatingthe slip surfaces. Pillow structure is preservedlocally. Sedimentary blocks are rare; slaty argillite is

exposed at one roadside locality.The complex structural relations of the serpen-tinite belt from here southward to its terminationnear Wilbur Springs are well shown on the1:24,000-scale geologic map and cross sections of McLaughlin et al. (1989). Although their 1989 geo-logic map shows the Colusa serpentinite mlange asserpentinized peridotite (Jsp) of the Coast Rangeophiolite, and a 3 km long slab of basalt faultedagainst Jsp as also part of the CRO (following Baileyet al., 1970, and Hopson et al., 1981), our reassess-ment of red radiolarian chert within that basalt(Table 1, and discussion in the section on mlangeblocks) now identify the chert-bearing basalt as aTCSM mlange block.

Our investigation of the TCSM belt ends at Wil-bur Springs (Fig. 1), but its apparent continuation(i.e., direct alignment) with the large serpentinitemasses farther south in Lake and Napa counties (toLake Berryessa) requires comment. Moiseyev(1966, 1970) pointed out that those serpentinites are

of two types: intact masses derived from peridotite,and detrital serpentinitesthe latter describedlater in a classic paper on Ophiolitic olistostromesin the basal Great Valley sequence, Napa County by S. P. Phipps (1984). He noted that a thick (upto 1 km), laterally extensive serpentinous chaotic

unit lies directly above the serpentinite thatrepresents the Coast Range ophiolite and below thewell bedded Great Valley sequence of Upper Juras-sic and Cretaceous age (p. 103). And also:Throughout the study area the chaotic unit liesdirectly above serpentinite that represents the CoastRange ophiolite (p. 121). No mafic section ever intervenes (p. 121). From this and other evidencediscussed later, we infer that the large serpentinitemasses below the chaotic serpentinous olistostromeunit is notthe CRO but rather a southward extensionof the TCSM belt. The serpentinite was derived fromharzburgite tectonite (Phipps, 1984) but evidently withoutmlange blocks. Perhaps the shallow ser-pentinite-mlange component was never present

this far south. Or, the mlange carapace may haveslid off and its remnants mixed within the ophi-olitic olistostrome unit. The significance of theolistostrome unit is discussed later.

SummaryThe entire length of the Tehama-Colusa ultrama-

fic belt (TCSM) is serpentinite-matrix mlange, atleast in part, with mlange blocks composed chieflyof oceanic basalt, minor radiolarian ribbon chert,

plus rare plutonic rocks and dark slaty argillite. Theserpentinite host was derived from peridotite tecto-nite (harzburgite > dunite). The mlange is bestdeveloped on the north (Tehama serpentinitemlange) but is recognizable (at least locally) allalong the ultramafic belt.

From Chrome southward, the mlange blocksamid sheared serpentinite are more common alongthe eastern side of the TCSM belt, and large areas of massive, less-serpentinized harzburgite withoutmlange blocks crop out on the western side. This isseen west of Chrome (Fig. 2; also Jayko and Blake,1986) and across the TCSM belt at Black DiamondRidge and in Little Stony Creek Canyon (Shervais etal., in press, a, b). If the block-bearing mlange isindicative of strongly disrupted and pervasively ser-pentinized oceanic crust and uppermost mantle, andthe massive, less serpentinized harzburgite once laydeeper, then the TCSM is a warped oceanic blocktilted eastward, exposing deeper mantle levels to thewest.

Metagraywacke-slate-greenstone slices along theCoast Range fault

The high-angle Coast Range fault brought upthe once-deeply buried South Fork Mountainschist (SFMS) on the west against the Coast Range


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TABLE 1. Biostratigraphic Data from TCSM Radiolarian Cherts

Sample PAS-03-B2: dark grey, non-tuffaceous radiolarian chert

Geologic occurrence: Near base of 14 m ribbon chert succession capping weathered basaltic lava, composite mlange blockin northern TCSM belt (Round Mountain serpentinite mlange).Sample location: Forest Road M2 (Toomes Camp Rd.), 4.3 miles (road distance) beyond Crowfoot Point, SE 1/4 of Sect. 28,T24N, R7W. GPS = N3954' 20.4"; W12239' 3.7".

Collected by: Emile Pessagno Jr., June 2003. Processed August 2004.

Radiolarian assemblage: Ristola proceraPessagno, Praecaneta turpiculaPessagno, Praecaneta decoraPessagno, Pantanel-lium josephinensePessagno, Blome, and Hull, Pantanellium meraceibaenseGroup Pessagno, Bernoulliussp., Archaeospon- goprunumsp., Spongocapsulumsp.

Biostratigraphic determination: Superzone 1, Zone 1I, and possibly also Zone 2, Subzone 2 .

Chronostratigraphic assignment: Middle Jurassic (upper Callovian) to Upper Jurassic (lower Oxfordian to possibly middleOxfordian). Paleobiogeographic determination: Central Tethyan. Fauna contains abundant members of the Pantanellium meraceibaense Pessagno Group. Members of this group are abundant in Tethyan strata of east-central Mexico and central Mexico. More-over, they are quite abundant in the volcanopelagic succession that overlies the Josephine ophiolite. Virtually no specimensof Praeparvicingula(with horn) are present in this assemblage. Clearly Central Tethyan.

Sample PK 14-2. Red, non-tuffaceous radiolarian chert

Geologic occurrence: Near top of 14 m ribbon chert succession capping weathered basaltic lava, composite mlange block innorthern TCSM (Round Mountain serpentinite mlange).

Sample location: Forest Road M2 (Toomes Camp Rd.), 4.3 miles (road distance) beyond Crowfoot Point, SE 1/4 of Sect. 28,T24N, R7W. GPS = N3954' 20.4"; W12239' 3.7".Collected by: Cliff Hopson, Sept. 1976. Processed 1977. Re-examined April 2003.

Radiolarian assemblage: Ristola proceraPessagno, Praecaneta turpiculaPessagno and Whalen, Praecaneta decoraPessa-gno, Hsuum brevicostata Ozvoldova.

Biostratigraphic determination: Superzone 1, Zone 1I (based on occurrence of R. proceraand P turpicula).

Chronostratigraphic assignment: Middle Jurassic (upper Callovian) to Late Jurassic (lower Oxfordian).

Paleobiogeographic determination: Central Tethyan (criteria of Pessagno and Blome, 1986; Pessagno et al., 1993).

Sample DF-PK-1. Red, nontuffaceous radiolarian chert

Geologic occurrence: Ribbon chert mlange block in northern TCSM belt (Round Mountain serpentinite mlange).

Sample location: Forest Road M2 (Toomes Camp Rd.) between Crowfoot Point and Round Mountain saddle (exact locationnot recorded).

Collected by: Debra Fritz (M.A. thesis, University of Texas, Austin, Texas, 1974). Re-examined by E. Pessagno, April 2003.

Radiolarian assemblage: Praecaneta decoraPessagno and Whelan. The only diagnostic taxon identified. The faunalacks E. pyctumand Mirifusus. These latter taxa are both easy to recognize and the abundance and preservation are goodenough to determine that both fauna are clearly absent.

Biostratigraphic determination: Superzone 1, Zone 1F to Zone 1I (lower part).

Chronostratigraphic assignment: Middle Jurassic (upper Bathonian to upper Callovian). Paleobiogeographic determination: Central Tethyan (criteria of Pessagno and Blome, 1986; Pessagno et al., 1993).

Table continues


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ophiolite and the serpentinite mlange on the east,during extensional exhumation of the Eastern Fran-ciscan belt in the Late Cretaceous (Jayko et al.,

1987; Blake et al., 1988). About ten fault-boundedslabs of slate, metagraywacke, and greenstone cropout along a 50 km segment of the Coast Range faultfrom near Yolla Bolly junction (southern end of thewestern Klamath Mountains) on the north to near Elk Creek on the south (Jayko and Blake, 1986, Fig.3). The seven southernmost slabs occur along theSFMSserpentinite mlange contact (Fig. 2), inmost cases occuring slightly within the serpentinitewhere they might locally be mistaken for mlangeblocks. A detailed study by Jaydo and Blake (1986)clearly demonstrated, however, that these slices areexotic, i.e., low-grade (prehnite-pumpellyite facies)metamorphic rocks whose metamorphic mineralogy,fabric, sandstone petrology, and K/Ar age (~158 Ma)are quite different from the SFMS, the TCSM

mlange blocks, the CRO rocks, and the GVG stratathat bound them on both sides. Their remarkableresemblance to rocks of the Late Jurassic (Oxford-

ian) Galice Formation of the western and southernKlamath Mountains is pointed out, along with possi-ble emplacement scenarios along the Coast Rangefault (Jayko and Blake, 1986).

The main point made here is that these slate/graywacke greenstone slices are not mlangeblocks within the TCSM ultramafic belt, eventhough some of them occur within serpentinite closeto the present Coast Range fault. That is, they werenot part of the oceanic crustal assemblagenowdisruptedthat once lay atop the TCSM upper man-tle peridotite. The composition of these slivers, their low metamorphic grade, and especially their occurence along 50 km of the Coast Range fault,suggests to us that these were Franciscan rocks thatonce lay structurally above the higher-grade South

TABLE 1. (Continued)

Louvion-Trellu, 1986 determinations

Data source:Louvion-Trellu, 1986: DEA thesis, Universite de Bretagne Occidentale, Brest, France. Quoted in Huot andMaury, 2002.Geologic occurrence: Ribbon chert mlange blocks in Round Mountain serpentinite mlange, northern TCSB belt.

Sample locations: Along (and near?) Forest Road M2 (Toomes Camp Rd.) between Crowfoot Point and Round Mountainsaddle (exact locations not given in Huot and Maury, 2002).

Biostratigraphy: Faunal assemblages and zonal data not given in Huot and Maury (2002).

Chronostratigraphic assignment: Middle Jurassic (Callovian) and Late Jurassic (upper Oxfordian).

Sample NCF 997A, B, C. Red, manganiferous (non-tuffaceous) radiolarian chert

Geologic occurrence: Red manganiferous chert interbedded with vesicular [amygdaloidal] basalts in 3 km long mass of submarine basalt faulted against serpentinite (southernmost TCSM belt) on the north and juxtaposed discordantly (faulted?)against terriginous clastic strata of Great Valley Group on the south.

Sample location: Outcrop on ridge west of Sulphur Creek east fork, 0.58 km S69W of Eagle Rock, USGS Wilbur Springs7.5' quadrangle; Section 18, T14N, R5W.

Collected by: Emile Pessagno, 1977. Processed in 1977 and initial results reported in Micropaleontology, no. 1, p. 104,1977. Results reported below are from a re-examination in Sept. 2004.

Radiolarian assemblage: Ristola altissima(Ruest), Mirifusus baileyi(Pessagno, Mirifusus guadalupensisPessagno, Mirifusismediodilata(Ruest), Parvicingula turrita(Ruest), Caneta hsui(Pessagno), Podobursasp., Saitoum pageiPessagno, Eucryr-tidium ptyctum(Riedel and Sanfilippo), Archaeodictyomitra rigidaPessagno, Pantanelliumsp., Acanthocircus variabilisSquinabol.

Biostratigraphic determination: Zone 3, Subzone 3. (Concurrence of Mirifusus baileyi, Caneta hsui, and M. guadalupensis pin the age of this sample).

Chronostratigraphic assignment: Upper Jurassic (uppermost Kimmeridgian to lower Tithonian).

Paleobiogeography: Tethyan Realm, Northern Tethyan Province.


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Fork Mountain schist, and have since been dis-placed against it. Extensional exhumation of theSFMS in the Late Cretaceous (Jayko et al., 1987)might have left slivers of those less deeply buriedroof rocks clinging to the serpentinite as the SFMSblock was brought up against it. This explanationaccords best with the tectonic history of the TCSMterrane, proposed in a later section.

Mlange Blocks

Tectonic blocks within the Tehama-Colusa ser-pentinite mlange belt include native blocks of ser-pentinized peridotite and oceanic crustal blocks of submarine basaltic lava, radiolarian ribbon chert,andin the Colusa sector of TCSMslabs of slatyargillite, all enclosed in a matrix of variablydeformed serpentinite. The occurrence of tectonicblocks of garnet amphibolite and their possible rela-tionship to the TCSM belt are also discussed.

Serpentinized peridotiteResistant blocks of massive serpentinized peri-

dotite weather out in relief above softer serpentinitehost rock, which is commonly deformed to slicken-

tite (spaced shear surfaces), schistose serpentinite,or soft gouge. These are nativemlange blocks, notexternally derived but standing out because of resis-tance to weathering and erosion.

The original character of the serpentinized peri-dotite is best shown in the relatively massive blocks.Protolithic harzburgite is distinguished from duniteby the presence of serpentinized pyroxenes(bastite), conspicuous in outcrop and thinsections. But closer scrutiny (Huot and Maury,2002) reveals further important details. Their workin the Round Mountain area (north end of Tehamamlange) found that:

Despite their extreme serpentinization(>65%) and commonly highly tectonizedcharacter, the studied examples of ultramaficrocks can still be identified as clinopyroxeneharzburgite, harzburgite senso stricto (s.s.),and orthopyroxene dunite. In most samples,olivine, orthopyroxene (1025 modal %), andrare clinopyroxene (


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in ophiolites) is dunite s.s., resorbed-enstatitedunite, cpx (pl) harzburgite (abyssal peridotite),and harzburgite s.s. (depleted upper mantle),respectively. Those rocks above the harzburgite s.s.are commonly streaked with gabbroic (pl + cpx + ol)impregnations from entrapped picritic meltsexpelled from deeper mantle (Nicolas and Prinz-hofer, 1983; Ceuleneer and Rabinowicz, 1992;Hekinian et al., 1993; Boudier and Nicolas, 1995;Hopson et al., in prep.). One of us has observedwell-exposed gradational sequences from dunite (amelt-extraction cumulate) downward throughresorbed-enstatite dunite to cpx ( pl) harzburgitetectonite and then harzburgite s.s. in the WadiGideah East traverse across the mantle-crust transi-

tion zone (MTZ) of the Oman ophiolite, and also thesame succession with abundant gabbroic impregna-tions (pl + cpx + ol) in four traverses across theOman ophiolite mantle-crust transition zone, espe-cially in the bare mountainside east of Khaffifah(Pallister and Hopson, 1981).

Analyzed clinopyroxene harzburgite mlangeblocks in the Tehama (Round Mountain) serpen-tinite mlange also correspond geochemically toabyssal peridotite from the Mid-Atlantic Ridge and

from Hess Deep near the East Pacific Rise (Huotand Maury, 2002, Fig. 7). The common occurence of abyssal peridotite (including cpx-pl-impregnatedharzburgite) and enstatite dunite in the serpentinitemlange block assemblage is strongly suggestive of uppermostoceanic mantle, including the duniticMoho transition zone (MTZ) occurring just beneathbasaltic/gabbroic oceanic crust. Such a peridotiteassemblage is unlikely in deeper sections of theoceanic mantle, including the hanging walls of subduction zones.

Basaltic lavaBasaltic mlange blocks range in size from kilo-

meter-scale down to meter scale (Fig. 3A). Jaykoand Blake (1986) and Blake et al. (1992) mappedbasaltic mlange blocks up to 4 km long, with sev-eral having kilometer-scale dimensions (Fig. 2). TheRound Mountain basaltic block in the Tehamamlange west of Paskenta measures 2.5 km by 1 km,and a lesser but still immense basaltic slab at

Thomes Creek gorge is at least 1200 m by 500 m by>200 m (Hopson et al., 1981). Most blocks, how-ever, are less than 250 m across. The basaltic blockscommonly weather out in topographic relief abovethe less resistant serpentinite. These are readilyapparent in the moderately vegetated Tehama

mlange but less obvious in the brushier Colusamlange. Traverses through the latter commonlyfind the basaltic blocks standing out as craggy pin-nacles. But many basaltic blocksweathered tosoilform only low mounds densely covered withbrush. Yet, these mounds stand out from the enclos-ing manzanita-cloaked serpentinite by their dis-tinctive vegetation cover, i.e., dense brush withabundant dark green chamise ( Adenostoma fascicu-latum) (Fig. 3B) whose tan-hued blooms give anoverall orange appearence when viewed fromnearby.

Pillow lava, massive lava, and basaltic breccias(including submarine talus breccia) are found in themlange blocks (Figs. 3C and 3D). Pillow and brec-

cia structure, however, are commonly deformed or obliterated by shearing and/or weathering. Thedeformation and low-grade alteration partly maskthe original composition, textures, and structures of the mafic lavas.

Most of the lavas are aphyric megascopically andintersertal microscopically. Microphyric basaltshave plagioclase or clinopyroxene-plagioclasemicrophenocrysts; phyric olivinealtered to greenphyllosilicates or carbonateis less easily recog-

nized. Groundmass glass is ubiquitously trans-formed into variable proportions of authigenic clays,chlorite, and pumpellyite (Huot and Maury, 2002,p. 114). Internal shear surfaces, along which therocks break, are coated with chlorite or other greenphyllosilicates.

Immobile trace element geochemistry, however,sees through the mask of rock alteration and defor-mation. Shervais and Kimbrough (1985a, 1987)analyzed basaltic lavas from three Paskentamlange blocks (i.e., Tehama serpentinite mlangewest of Paskenta) and compared them with volcanicrocks from the Coast Range ophiolite, the Stonyfordseamont lavas, and some Franciscan lavas. Theyreported, In contrast [to CRO lavas], volcanic rocksfrom the Stonyford seamount and the Paskentamlange blocks are transitional subalkaline basaltsthat are compositionally similar to enriched MORBor ocean-island tholeiites (Shervais and Kim-brough, 1985a, p. 37, 1987, Fig. 5, p. 176). Jayko etal. (1987, p. 481) concluded that This difference in

chemistry between the ophiolite remnant and blockswithin the Round Mountain (Tehama) serpentinitemlange implies that the mlange blocks were notderived from the ophiolite.

Huot and Maury (2002), with a larger data baseof 14 analyzed samples of basalt and diabase from


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the mlange blocks, and employing the immobiletrace element classification of Winchester andFloyd (1977), found that all samples have subalka-line affinities and their compositions range frombasalt to basaltic andesite. Furthermore, Themajority of the mafic blocks define a trend markedby a TiO2 enrichment along the liquid line of descent. A detailed discussion of the trace-elementgeochemistry led to the following conclusion:

Only one main magmatic series of subalka-line affinity is represented in the RoundMountain mafic blocks. This conclusion isessentially supported by TiO 2, Zr, REE abun-dances, and LREE-depleted patterns similar to N-MORB. (Huot and Maury, 2002, p. 118)

However:

Sample 38, with a LREE-enriched patternand the highest (Zr/Y) ratio of our set, isthought to be the only sampled transitionaloceanic tholeiite. Such transitional MORBsare commonly reported in mafic lithologiesfrom the Franciscan assemblages (MacPher-son, 1983; Jayko, 1984; Shervais and Kim-brough, 1987; Shervais and Hanan, 1989;Shervais, 1990). (Huot and Maury, 2002, p.118)

Two of the three mlange-block lavas studied byShervais and Kimbrough (1985a, 1987) are of thislatter type, leading to their conclusion that themlange mafic lavas are like those at Franciscanlocalities. Plotting all 14 of their basaltic samples ona TiO2 vs 100 Mg/(Mg +Fe2+) diagram, Huot andMaury (2002) showed four in the Franciscanmetabasalt field (which is weighted toward high-TiO2 seamount lavas) and most of the others clus-tered just beneath it, partly overlapping the CROlavas, which have lower TiO2. We conclude tenta-tively that the Round Mountain mlange-block lavas(northern TCSM) are similar to Franciscan spread-ing-center lavas but not Franciscan seamounts.Perhaps an even more telling basis for comparingthe crustal mlange blocks with the Franciscanversus the CRO is the contrast between their subvol-canic and plutonic assemblages, noted next.

Plutonic and subvolcanic rocksDespite the remarkable abundance and size of

basaltic lava blocks in the Tehama/Colusa serpen-tinite mlange, blocks of diabase and especiallyplutonic rocks are rare. Hopson et al. (1981) and

Jayko et al. (1987) both mentioned the rarity of dia-base and gabbro blocks in the mlange. Huot andMaury (2002) found rare diabase but only one blockof gabbro. Diorite, clinopyroxenite, and wehrlitemlange blocks are reported from one local area(Shervais et al., in press, b). The appearence of minor wehrlite/clinopyroxenite in the mlange is notsurprising, inasmuch as they commonly occur withcpx-impregnated dunite in the transition zone(MTZ) at the top of oceanic upper mantle (Nicolasand Prinzhofer, 1983; Juteau et al., 1988; Boudier and Nicolas, 1995). Related cpx-impregnateduppermost mantle (MTZ) rocks have already beendescribed elsewhere in the TCSM belt (see serpenti-nized peridotite, above).

Franciscan igneous assemblages throughout theCalifornia Coast Ranges comprise chiefly basalticlavas (basaltic greenstone) and serpentinizedperidotite; diabase and gabbro are rare (e.g., Baileyet al., 1964; Blake et al., 1985). In contrast, diabasicdike and sill complexes, isotropic and layered(cumulus) gabbros, and clinopyroxenite/wehrlite areprominent components of well-preserved CROcrustal successions (e.g., Bailey et al., 1970; Baileyand Blake, 1974; Hopson et al., 1981, 1996, and in

prep.; Hopson, 2002; Shervais, 2001; Shervais etal., 2004). Thus, igneous/meta-igneous blocks in theTehama-Colusa serpentinite mlange match theFranciscan but are quite unlikethe Coast Rangeophiolite igneous assemblage.

Radiolarian ribbon chertRadiolarian chert comprises only a small propor-

tion (


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and tuffaceous radiolarian mudstones that overliethe CRO (Hopson et al., 1981, 1996, in prep.; Pes-sagno et al., 2000; Robertson, 1989).

The mlange cherts occur both alone (as

deformed slabs of ribbon chert) and as part of com-posite blocks with chert beds attached to basalticlava (Fig. 3F). Such composite blocks are inferred tohave been pelagic radiolarite beds deposited atopbasaltic oceanic crust. Most of the chert remnantsare only as few meters thick, with parts evidentlysheared off. A 25 m ribbon chert succession restingon basaltic lava along the Toomes Camp road (SE1/4 of Sect. 28, T24N, R7W), is the thickest continu-ous section of chert observed.

Work in progress indicates that the mlange-block cherts are late Middle Jurassic in age, extend-ing into the Late Jurassic (Table 1). The radiolarianassemblage once considered to be OxfordianKim-meridgian (Pessagno, in Hopson et al., 1981) is nowreassigned to the Callovian to upper Oxfordian.Dark gray ribbon chert (Fig. 3E) capping a basalticblock, and red ribbon chert from higher in that 14 msuccession, both have Zone 1I radiolarian assem-blages, assigned to the Callovian to lower Oxfordian(Table 1). The radiolarians from another chert

remnant nearby span Zones 1F to lower 1I (MiddleJurassic: upper Bathonian to Callovian). Louvion-Trellu (1986, cited in Huot and Maury, 2002) alsoreported Callovian to early Oxfordian radiolarianages for some of the mlange-block cherts. Based onthese data, the radiolarites deposited atop TCSMbasaltic oceanic crust range in age from late MiddleJurassic (upper Bathonian and Callovian) to earliestLate Jurassic (lower Oxfordian). The underlyingoceanic basalt, therefore, is Middle Jurassic.

Younger late Jurassic radiolarian chert occursinterbedded with oceanic basalt, inferred to be vol-canic seamounts that grew atop the TCSM oceaniclithosphere. The most complete such remnant is theStonyford volcanic complex (Shervais et al., in pressa, and references cited therein), described in a later section. Another example is the red, non-tuffaceousradiolarian chert interbedded in a thick 3 km longremnant of submarine basalt cropping out 34 kmnorthwest of Wilbur Spring at the southern end of the TCSM belt (Fig. 1). This basalt, formerly mis-

taken for a Coast Range ophiolite remnant (Bailey etal., 1970; Hopson et al., 1981; McLaughlin et al.,1989), is regarded now as a TCSM mlangemegablock within the southern TCSM belt. Thisreassessment is based on the red, nontuffaceous,Franciscan-like appearence of the chert and its

occurrence withinthe lava massas at SFVCinstead of resting on top. Thus, it is unlikethe green-ish-gray tuffaceous volcanopelagic (VP) chert stratathat overlie the CRO basalts. The abundant, well-preserved radiolarians belong to Zone 3, Subzone3, assigned to the uppermost Kimmeridgian tolower Tithonian stages of the Upper Jurassic (Table1, sample NSF 997A, B, C). The basaltic megablockis faulted against (originally erupted upon?) theTCSM serpentinized peridotite (shown as CROserpentinite by McLaughlin et al., 1989), and juxta-posed discordantly (faulted?) against Great ValleyGroup strata.

The full span of TCSM radiolarite depositionMiddle and Late Jurassiclies withinthe EarlyJurassic through Early Cretaceous age span of Fran-ciscan radiolarian cherts (Murchey, 1984; Sedlockand Isozaki, 1990), allowing their correlation. TheTCSM chert only partly overlaps the depositionalspan of the tuffaceous radiolarian cherts (VP suc-cession) that overlie the Coast Range ophiolite. TheVP cherts are chiefly Late Jurassic, i.e., early Oxfor-dian to late Tithonian (Hull et al., 1993; Pessagno,Hull, Munoz, and Blome in Hopson et al., 1996),although basal chert at two CRO-VP remnants may

be late Callovian (Hopson et al., in prep.). In con-clusion, the TCSM mlange-block cherts physicallymatch the nontuffaceous Franciscan ribbon chertsand fall within their temporal range. In contrast,they differ physically and are partly older than thetuffaceous radiolarian cherts and mudstones that lieatop the Coast Range ophiolite.

Present limited evidence (Table 1) suggests thatthe TCSB radiolarians were warm-water dwellersfrom the Central Tethyan (paleoequatorial) Provinceof the Middle Jurassic ocean, using the criteria of Pessagno and Blome (1986, 1990), Pessagno et al.(1991, 1993, 1999), Hull (1995, 1997), Hull et al.,(1997), and Pessagno and Martin (2003). The radi-olarian cherts are therefore exotic to their presentlocation near 3940N (within the Southern BorealProvince)i.e., they were transported northwardfrom an original paleoequatorial setting. They maybe similar in this regard to the Jurassic Franciscanradiolarian cherts, whose oceanic basaltic substrateevidently originated south of the paleoequator

(Alvarez et al., 1980; Murchey and Jones, 1984).Both the TCSM and Franciscan ribbon cherts recordslow pelagic deposition on the ocean floor at depthsgreater than the calcium carbonate compensationdepth. The depositional conditions and diageneticdevelopment of ribbon chert from radiolarite at ODP


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sites 800 and 801 in the equatorial Pacific (Ogg etal., 1992) seem applicable here.

Argillite

Dark slaty argillite, enclosed by serpentinite, isthe only other sedimentary rock type present in theTehama-Colusa serpentinite mlange belt. We haveobserved these only in the Colusa mlange, wherethey occur as small slabs (30 mlong) composed wholly of dark argillite. The lack of silty or sandy interbeds suggests that the originaldeposit may have been abyssal clay (Kennett, 1982;Berger and Winterer, 1974).

Original stratigraphic relations come from a sin-

gle locality northwest of Stonyford: a huge chamise-covered basaltic block crossed by the road at BlackDiamond Glade, just east of the spring (Fig. 3B).Here poorly exposed ribbon chert >10 m thickdirectly abuts the weathered basalt, presumablydeposited on its surface. Bordering the opposite sideof the chert is a narrow zone of dark soil with argil-lite fragments. Evidently argillite overlay the chertthat rested on basalt. It appears that radiolarite dep-osition on basaltic sea floor was followed by abyssal

clay, a succession common on the deep-sea floor dueto progressive subsidence beneath the silica com-pensation depth with sea-floor spreading away froma ridge (Berger and Winterer, 1974; Ingersoll, 1988,Fig. 4). Alternatively, the argillite may have beenterriginous mud beyond reach of sandy turbidites.

Garnet amphibolite?Rare blocks of amphibolite [and] garnet

amphibolite in the serpentinite mlange are also

reported (Jayko and Blake, 1986, p. 1059). No sig-nificance is attached to the amphibolite, which is acommon foliated variant of uralitic gabbro in oce-anic crustal assemblages. But garnet amphibolite isa mafic metamorphic rock that forms at high pres-sure. The single block of garnet amphibolite knownto us in the TCSM belt occurs where Thomes Creekcrosses the eastern margin of the serpentinitemlange (Hopson et al., 1981). How the block gotthere is enigmatic, but it is not visibly embedded in

the serpentinite. John Shervais reported another block of garnet amphibolite (eclogite?) on the westflank of Black Diamond Ridge and perhaps otherslittering the slope nearby (pers. commun., August,2004). We doubt that the garnet amphibolites foundalong the TCSM were blocks withinthe serpentinite

mlange, because of their apparent surficialoccurence.

Small serpentinite bodies (diapirs?, tectonicslabs?, serpentinitic debris-flow remnants?) within

(or lying on?) GVG Upper Jurassic mudstone near the south fork of Elder Creek (Fig. 2) contain smallblocks of greenstone, chert, amphibolite, and a gar-net amphibolite (Blake et al., 1987). The origin of those serpentinite bodies is speculative, and their postulated connection with the serpentinite mlangebelt (Jayko and Blake, 1986)several kilometersaway, beyond the intervening PaskentaElder CreekCRO remnantis tenuous.

The amphibolite and garnet amphibolite blocks

on the TCSM would seem to have some connectionwith the mid to Late Jurassic (~165140 Ma)amphibolite, garnet amphibolite, and other high-P/Tmetamorphic rocks (blueschist, eclogite) that occur widely as blocks and slabs in the Franciscan Com-plex (e.g., Bailey et al., 1964; Coleman and Lan-phere, 1971; Suppe and Foland, 1978; Blake et al.,1988; Cowan and Bruhn, 1992; Wakabayashi, 1990,1999), a topic beyond the scope of this paper. In thecontext of their rare occurence on the TCSM belt,however, suffice it to say that we believe that theseare from CRO oceanic crust metamophosed in themid to Late Jurassic Great Valley subduction zone(see later section on Mesozoic Tectonic History).Fragments from exhumed slabs of once deeply sub-ducted crust were released into the coarse clasticproxymal facies of uppermost Jurassic and lower-most Cretaceous Great Valley Group. Then, follow-ing upheaval beneath the eastern Great Valley zonein the late Early Cretaceous, they were transportedwestward into the Franciscan depositional realm assubmarine debris flows and slide blocks. Suchtransport westward by subaqueous mass flowage,followed later by erosional removal of their host sed-iments, left heavy amphibolitic and other high-grademetamorphic lag blocks resting on the surface of the TCSM and adjacent terranes, where some of them still remain. During uplift of the TCSM/CROforearc ridge in the Early Cretaceous (see later sec-tion on Mesozoic Tectonic History) many of themetamorphic lag blocks were carried farther west-

ward into the Franciscan Central belt; others movedeastward a short distance into the new forearc basin,carried by serpentinitic debris flows that interfinger with distal terriginous strata of the Great ValleyGroup near Wilbur Springs (Carlson, 1981a,1981b).


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Origin of the Mlange

The protolith of the serpentinite mlange wasbasaltic oceanic crust overlying upper mantleperidotite and transition-zone dunite. Radiolarianribbon chert up to 25 m thick lay atop seafloor basalt, and thin argillaceous mudstone or claylocally overlay the chert. Terriginous sandstonesand/or volcaniclastic strata were lacking, indicatingan open-ocean setting, far from a continent marginor active volcanic arc. This oceanic crust-mantlesuccession was pervasively disrupted, hydrated, andphysically mixed by a large-scale tectonic eventbeneath the deep ocean floor, forming areally exten-sive submarine serpentinite-matrix mlange. Itescaped the later trench sedimentation, subduction,and high P/T metamorphism that characterize theFranciscan rocks farther west in the Coast Ranges.

Insight into the disruption mechanism comesfrom the following description of basaltic block-ser-pentinite structural relationships, viewed along theToomes Camp Road (Rd. M2) west of Crowfoot Point(Paskenta section of Tehama serpentinite mlange):

The transition from large intact masses of vol-

canic rock into serpentinite-matrix mlangeshows relationships that reveal the melangingmechanism. Beginning in the volcanic unit,massive or pillowed lavas are locally brokenby narrow fault zones that are occupied bysheared, pulverized serpentinite. As the adja-cent mlange is approached, the serpentinitefault slices within the volcanics become wider and more abundant, until they gradually iso-late masses of volcanic rock between them.

This grades into mlange as the proportion of serpentinite (host) to volcanic rock (isolatedmasses and blocks) increases. Finally,mlange grades into barren serpentinite asthe volcanic blocks become fewer and thendisppear altogether. Thus, a complete transi-tion occurs, from volcanic rock throughserpentinite-matrix mlange to serpentinite.The melanging evidently resulted from (1)faulting and tectonic disruption of the volca-

nic unit, (2) the intrusion of incompetent,plastically flowing serpentinite into the faultzones and disrupted brittle masses of volcanicrock, and (3) further disruption and mixing bypervasive shearing and plastic flowage. (Hop-son et al., 1981, p. 461462)

The timing of serpentinization relative to oceaniccrust-mantle disruption and mixing is not well con-strained. The outcrop relations described in thequotation above suggest mobility of the ultramaficmaterial at low temperature and shallow depthsbeneath the sea floor. Thus, the invasive ultramaficmaterial had already been converted to incompetentserpentinite or was being convertedto serpentinite.The latter possibility seems more attractive: the per-vasive disruption of oceanic crust and underlyingperidotite would have facilitated large-scale entry of seawater, launching serpentinization. Thus, we infer that deformation and serpentinization proceededtogether.

What was the setting and cause of the Tehama-Colusa oceanic lithosphere disruption? An oceanic fracture zone(Shervais and Kimbrough, 1987) is anattractive possibility. Saleeby (1984), reviewing theoccurrence of oceanic serpentinites, noted that Themajority of these ocean floor serpentinites wererecovered from fracture zones. The relationship toserpentinite mlange is explained:

Tectonic disruption of newly formed oceaniccrust along transform segments of fracture

zones in conjuction with, and followed by, thesolid state injection of serpentinitic rocksleads to the development of serpentinite-matrix mlange as a primary constituent of oceanic crust. (Saleeby, 1984, p. 155)

The process described here corresponds nicelywith our interpretation of structural relationships inmlange exposed in traverses through the northernTCSM belt, such as the one west of Crowfoot Pointdescribed above. The large scaleof oceanic fracturezones also seems adequate to match the TCSMmelanging. Quoting again from Saleeby (1984): the total length of fracture zone crust on theocean floors far exceeds the length of subductionand collision zones, and fracture zones arecommonly on the order of 50 km wide Thisexplanation requires that alignment of the FZ coin-cide approximately with the N-S alignment (presentcoordinates) of the ~100 km long Tehama-Colusaserpentinite mlange belt (Fig. 1).

Close similarities are noted with the Upper Trias-sic serpentinite mlange that comprises thedisrupted oceanic basement of the RattlesnakeCreek terrane in the southwestern KlamathMountains. Here Wright and Wyld (1994, p. 1033)concluded that:


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the basement mlange was derived by dis-ruption of oceanic crust and upper mantle ina setting far removed from either an arc or ter-riginous landmass and are most consistentwith an interpretation that mlange formationoccurred in an oceanic fracture zone.

Their detailed discussion of this and other possibil-ities provide helpful insights and data sources.

Other early Mesozoic and Paleozoic serpentinitemlanges in the western Sierra Nevada (Saleeby,1979, 1981; Sharp, 1988; Edelman et al., 1989) arecomplicated by multistage histories and/or by oro-genic deformation, metamorphism, and plutonism,veiling their primary origin and setting.

Another possible cause of a broad swath of per-vasive lithosphere disruption in a deep-sea, open-ocean setting is a migrating deformation zone (i.e.,migrating transform fault zone) between propagatingand failing rift tips where ocean-ridge jumpingoccurred. Here spreading center abandonment,paired with a new spreading center that propagatesthrough slightly older oceanic crust alongside thefailing rift, leaves disrupted crust between them. Amodern example along the western Galapagosspreading center was described by Kleinrock andHey (1989). This process could accord with the per-vasive disruption and serpentinization of oceaniclithosphere moving progressively through a verylarge area, like the TCSM. Arguing against thisscenario, however, is the thick cover of ribbon chert(up to 25 m), perhaps capped by abyssal clay (i.e.,the argillite mlange slabs). Such a sediment cover suggests old ocean crust, perhaps carried by spread-ing to abyssal depths, away from expected sites of rift propagation. But whatever the cause of TCSM

mlanging, it evidently occurred in an open-oceansetting, away from a source of clastic detritus.

A different, deep-seatedorigin of the serpentinitemlange proposed by Jayko et al. (1987) involves itsformation in a structural setting below the maficigneous rocks of the Coast Range ophiolite andabove a subducting plate. They explain:

We infer that serpentinite mlange formswithin a zone below the oceanic crust of theupper plate [i.e., the CRO] and above the sub-ducting oceanic lithosphere. This thick zoneof tectonized harzburgite is partially serpenti-nized by dewatering of the subduction crust (Jayko et al., 1987, p. 481).

Further:

It is also an environment in which one mightexpect to find a lithologic association domi-nated by the upper part (chert and basalt) andthe lower part (harzburgite) of ocean crustwithout intervening plutonic and cumulatelayers. We infer that the alkalic volcanicrocks and chert were derived from the sub-ducting oceanic plate rather than from theupper plate (Coast Range ophiolite) and thatthe serpentinized harzburgite composing themlange matrix is primarily derived from thehanging wall of the subduction zone. Volcanicrocks that occur as blocks in the FranciscanComplex tend to be alkalic like the volcanicrocks found in the Round Mountain serpen-

tinite mlange (Bailey et al., 1964; MacPher-son, 1983; Jayko, 1984; Shervais andKimbrough, 1985). We infer that the ophioliteremnant was juxtaposed against the RoundMountain mlange along a detachment fault(the Beehive Flat fault, Fig. 6). (Jayko et al.,1987, p. 482)

This attractive hypothesis explains several fea-tures of the mlange. It is rejected, however, for thewant of an adequate water source (i.e., the basalt-

chert subducting crust) for large-scale serpentiniza-tion of hanging-wall peridotite, and especially for lack of evidence for subduction. The latter includes:(1) the basaltic and chert mlange blocks show nohint of high P/T metamorphism; (2) the serpentinitelacks coarse metamorphic antigorite; and (3) nosandy trench sediments occur as mlange blocks.Instead, the serpentinite is mostly the low-T chryso-tile-lizardite-brucite variety; the basaltic blocksshow mainly low-grade, subgreenschist-facies alter-ation; the cherts are unrecrystallized with good pres-ervation of radiolarians; and the absence of terriginous clastic or volcaniclastic sandy mlangeblocks all favor development of the mlange byshallow disruption and hydration of oceanic crustand uppermost mantle in a nonsubduction setting.So too does the abundance of abyssal peridotite (cpx pl harzburgite) and enstatite dunite in someserpentinite mlange blocksrocks indicative of uppermost oceanic mantle and crust-mantle transi-tion zone (MTZ), not the deep underside of a mantle

wedge.Huot and Maury (2002) followed Jayko et al.

(1987) in proposing an origin for the serpentinitemlange involving subduction of Franciscanoceanic lithosphere beneath the CRO, inferred bythem to be backarc crust. They too believe that


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melanging occurred in a subduction setting withtectonic accretion of the uppermost part of thedowngoing slab to the lowest part of the overridingplate Specifically:

We propose, in agreement with Shervais andKimbrough (1987) and Jayko et al. (1987),that such a mechanism of accretion is respon-sible for the formation of the Round Mountainserpentinite mlange. The Franciscansupracrustal rocks (represented by the vol-cano-sedimentary lithologies) would thushave been tectonically underplated beneaththe overriding plate (represented by the

sheared serpentinite-matrix and the ultrama-fic blocks). (Huot and Maury, 2002, p. 121)

They continue:

Accreted crustal slivers [i.e., the chert,basalt, and diabase] are devoid of high-pres-sure, low-temperature minerals, a feature thatsuggests a shallow accretion of the units of the Round Mountain serpentinite mlange.(Huot and Maury, 2002, p. 121)

But this interpretation remains confronted withformidable problems. One is the source of the vastvolumes of water necessary to achieve serpentiniza-tion of peridotite throughout the huge TCSM belt(Fig. 1). A subducting TCSM slab capped only bybasaltic lava and radiolarian chert (plus minor argil-lite) is inadequate as a water source, as mentionedabove. Also, the style of mlange mixing, describedearlier, seems incompatible with tectonic slicing,duplexing, and/or ductile deformation where adowngoing slab and its roof interacted dynamically.The absence of tectonite fabrics in the cherts andbasalts is especially telling.

Still another problem is the absence of terrigi-nous sediment (graywacke and mudstone) on thedowngoing oceanic slab (inferred to be Fran-ciscan by those authors) when mlanging occurred.Edgar H. Bailey, a prime authority on the northernCoast Range Franciscan (e.g., Bailey et al., 1964)forcefully told one of us (CAH, 1977) that morethan 90 percent of the Franciscan is graywacke

[including metagraywacke + mudstone]. Its hugeabundance is borne out by much work in the north-ern Coast Ranges (e.g., Bailey et al., 1964; Suppe,1973; Maxwell, 1974; McLaughlin, 1978; Worrall,1981; Jayko, 1984; McLaughlin and Ohlin, 1984;Blake et al., 1982, 1985, 1988, 1992; McLaughlin

et al., 1989). This terriginous sediment componentof the Franciscan, deposited initially as a clasticapron (Blake and Jones, 1974; McLaughlin andOhlin, 1984) and then as trench sediment along theNorth American continent margin (e.g., Page, 1981;Dickinson et al., 1982), began in the latest Jurassic(Jones et al., 1969; Blake and Jones, 1974). But, theserpentinite mlanging disrupted Middle Jurassicand earliest Late Jurassicoceanic lithosphere, either prior to burial beneath those terriginous clastics or beyond their reach. We return to the question of whereand whenmlanging took place in the later section on Mesozoic Tectonic History.

In conclusion, the TCSM mlanging is explainedhere as the widespread tectonic disruption and ser-pentinization of basaltic oceanic crust and upper-most mantle beneath the sea floor, perhaps alongone or more oceanic fracture zones. Serpentinizationmarks the entry of enormous volumes of seawater during pervasive deformation of the upper mantleperidotite. Swelling, plastically flowing serpentiniteinvaded, wedged apart, and engulfed blocks of thedisrupted crustal rocks, forming mlange. An openocean setting, away from continent-margin sedimen-tation or active arc volcanism, is inferred from theabsence of terriginous clastic or volcaniclastic sedi-ment cover when crustal disruption took place.Radiolarian ribbon chert, perhaps capped by abys-sal clay, was the chief sediment cover involved inthe crustal disruption. Also involved locally was along-lived submarine volcano (Jurassic seamount)growing atop the basaltic oceanic crust during radi-olarite deposition, considered next.

Stonyford Volcanic Complex

An important feature closely associated with theTCSM belt occurs near Stonyford, where the serpen-tinite mlange wraps around a large mass of intactsubmarine lavas, hyaloclastites, and minor inter-stratified radiolarian cherts. This assemblage, crop-ping out as an ovoid mass approximately 8 5km in plan [with] estimated stratigraphic thicknessat least 1 km and possibly as much as 2.5 km(Brown, 1964a), was once called the Stonyford sea-

mount (Hopson et al., 1981; Shervais andKimbrough, 1985a) and the Stonyford volcanics(MacPherson and Phipps, 1985), but was later renamed the Stonyford volcanic complex(Shervaisand Hanan, 1989). Its chief features were summa-rized by Zoglman and Shervais (1991, p. p. A395):


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The Stonyford volcanic complex (SFVC) is athick sequence of submarine volcanic rocksstructurally juxtaposed between high P/Tmetamorphic rocks of the Franciscan assem-blage and unmetamorphosed sediments of theGreat Valley sequence, in the northern CoastRanges of California. The SFVC consists pri-marily of pillow lava, with subordinate mas-sive lava flows, diabase dikes, fragmentaleruptive breccias, and hyaloclastite breccias.The hyaloclastite breccias, interpreted assubmarine fire fountain deposits, containunaltered volcanic glass which has beendated by 39Ar/40Ar at 164 1 m.y. (Hanan etal., 1991). Radiolarian chert and minor lime-

stone are found intercalated with pillow lavathroughout the sequence; chert forms beddedsequences up to 10 m thick as well as interp-illow fillings.

The lavas and hyaloclastites include OIB tholei-ites, low-Ti basalts, andhigh in the pilealkalibasalts (Shervais, 1993). A small brecciatedmicroplagiogranite body crops out at the northernmargin of the volcanic pile, evidently an SFVCsilicic intrusive sheet (Hopson observation, June2004). Further details of the geology, geochemistry,Pb isotope composition, and age of the SFVC aregiven by Shervais and Kimbrough (1987), Shervaisand Hanan (1989), Hanan et al. (1991), Zoglmanand Shervais (1991), and Shervais et al. (2002, andin press, b).

The SFVC is regarded as a relict seamount(Shervais and Kimbrough, 1987; Zoglman and Sher-vais, 1991; Shervais, 1993; Shervais et al., in press,b; also Hopson et al., 1981). The thickness of this

local submarine lava pile, the chemistry of its lavas,and the absence of cogenetic plutonic rocks beneathit all argue for its origin as a seamount, built onproto-TCSM oceanic crust and mantle.

Three lines of evidence connect the SFVC withoceanic crustal rocks (now mlange blocks) of theserpentinite-matrix mlange: (1) the partly matchingimmobile trace-element chemistry of their basalticlavas (Shervais and Kimbrough, 1985a, 1987); (2)their physically matching radiolarian ribbon cherts;and (3) the approximate corresponding age of thosecherts. Typical SFVC chert has been described as:

Reddish-brown manganiferous (nontuf-faceous) radiolarian ribbon chert [that] islocally interbedded within the pillow lavas.Along Stony Creek near the campground

(Sec. 35) chert units 15 and 30 m thick areinterstratified within the basalt. (Hopson etal., 1981, p. 457).

And also:

Chert forms a prominent horizon about 10 mthick intercalated with basalt in the westernpart of the complex; lenses of Mn-rich umber up to 2 m thick grade laterally into red jasper-oid breccias. Radiolaria are common in theupper 45 m of the banded ribbon chert thatoverlies the umbers and jasperoid breccias.(Shervais and Hanan, 1989, p. 510511)

The radiolarian cherts within the SFVC sea-mount and those occurring as mlange blocks in theTehama-Colusa serpentite mlange belt are twins,physically. Both are nontuffaceous ribbon cherts,typically in dark red-brown hues but dark greylocally. Both are indistinguishable physically fromthe Franciscan radiolarian ribbon cherts (Bailey etal., 1964, Figs. 26, 28, and 29).

The age of the TCSM oceanic crustal cherts andthe lenses of similar chert occuring low stratigraph-ically in the SFVC appear to span about the samerange, whereas the higher SFVC cherts are some-

what younger. Specifically, the ages of TCSMmlange-block ribbon cherts measured thus far (Table 1) range from Middle Jurassic (upper Batho-nian through Callovian) to early Late Jurassic (lower to possibly middle Oxfordian), using the radiolarianzonal system of Pessagno et al. (1987, 1993). Bycomparison, radiolarians from thick chert lensesintercalated with basalt in the SFVC range fromMiddle Jurassic (Bathonian, corresponding to Uni-tary Association Zone 6-6 of Baumgartner et al.,1995) near the base of the complex to Late Callovianto early Kimmeridgian (UAZ 8-10) in the upper part(B.L. Murchey, in Shervais et al., in press, b).

Comparison of the TCSM chert ages in Table 1with the SFVC chert ages listed in Shervais et al. (inpress, b) is not precise, since the former correspondto the radiolarian zonation scheme of Pessagno et al.(1987, 1993) and the latter to the UAZ zonal systemof Baumgartner et al. (1995), which assign older ages for comparable taxa. The differences, dis-cussed by Pessagno and Hull (1996), Hull et al.,

(1997), Pessagno et al. (1999, 2003), and byMurchey (1997, and in Shervais et al., in press, b),are beyond the scope of this paper.

Despite unresolved chert age differences itseems clear that the Stonyford submarine volcano(SFVC) experienced its early growth (Middle Juras-


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sic) upon TCSM basaltic ocean crust duringradiolarite deposition on that surface, and that itsgrowth lasted well into the Late Jurassic. A corre-sponding TCSM basaltic megamlange block withinterbeddedUpper Jurassic red radiolarian chert(Table 1) occurs ~25 km farther south, near Wilbur Springs (Fig. 1). This too is probably the remnantof a seamount that grew upon TCSM oceanic litho-sphere.

Why then did not TCSM radiolarite deposition onthe deep-ocean floor also continue into the mid-LateJurassic? Perhaps it did: the uppermost TCSMcherts abovethe basalt have not yet been identifiedwithin the mlange. Or, speculatively, seafloor spreading may have carried the TCSM ocean crustto depths below the silica compensation depth(SCD)i.e., where radiolarite deposition ceasedand only abyssal clay slowly accumulatedwhereasthe seamounts rose above the SCD. The thin slabs of argillite in the Colusa serpentinite mlange could beremnants of that abyssal clay.

Paleobiogeographically, the TCSM sea-floor cherts studied by Pessagno are Central Tethyan(Table 1), indicating radiolarian growth in pale-oequatorial warm waters. In the Stonyford seamount,

B. L. Murchey (in Shervais et al., in press, b) notesthat A major faunal change occurs within theLocality B section, wherein the relatively small-sized, polytaxitic radiolarian faunas in the lower part of the section give way to very robust, oligotax-itic nassilarian-dominated faunas that include Praeparvicingula spp. A similar change occurs inthe upper volcanipelagic succession atop the Jose-phine ophiolite in the Smith River section (westernKlamath Mountains), where Pessagno et al. (1993)noted the disappearence upsection of pantanellidsand influx of Praeparvicingula, inferred to indicatetransport from Central Tethyan to Southern Borealpaleolatitudes during Jurassic sea-floor spreading(Pessagno and Blome, 1986; Pessagno et al., 1993,2000). The similar faunal succession at Stonyford(reported in Shervais et al., in press, b) thereforesuggests that initial growth of the long-lived subma-rine volcano began in warm paleoequatorialwatersas did the TCSM oceanic crustand com-pleted its growth farther north in cooler, higher-lati-

tude waters. This holds also for the Wilbur Springsseamount(?) basalt that contains Upper Jurassic redchert with a Northern Tethyan fauna (Table 1).

These observations favor a model in whichduring the late Middle Jurassicthe Stonyford vol-canic complex was growing atop basaltic oceanic

crust and peridotite upper mantle during the periodof radiolarite deposition. BathonianCallovian radi-olarite (transformed authigenically to ribbon chertas described by Ogg et al., 1992) was accumulatingas a thin blanket atop the basaltic sea floor, andatabout the same timealso as intercalations withinthe growing SFVC basaltic seamount. Both wereaffected by the subsequent widespread tectonicevent, but to different degrees. The Tehama-Colusabelt of oceanic crust and mantle was pervasivelydisrupted and hydrated, forming serpentinite-matrixmlange. The overlying thick pile of seamount lavaswas doubtless also deformed but not mlanged,being separated from the underlying mobile serpen-tinite. Faulting and other internal deformation of the

Stonyford seamount doubtless occurred, e.g., notethe brecciation of the plagiogranite body mentionedabove, and especially the invasion of basaltic SFVClavas by the underlying serpentinite, mapped byShervais et al. (in press a, b). But, most of the Stony-ford seamount survived oceanic deformation,remaining largely intact (Shervais et al., in press, band references cited therein).

In conclusion, we concur with Shervais and Kim-brough (1985b, 1987) that the serpentinite mlange

terrane is a Franciscan feature, and also with Sher-vais and others (Shervais and Kimbrough, 1985a,1987; Shervais and Hanan, 1989) that the SFVCwas a FranciscanJurassic seamount. This holds alsofor the basaltic megablock near Wilbur Springs (Fig.1) with Franciscan-like radiolarian chert. We rejectthe more recent interpretation of the SFVC as part of the Coast Range ophiolite (Shervais et al., 2002, andin press, b), which is strikingly different lithologi-cally, pseudostratigraphically (Hopson et al., 1981,and in prep.), geochemically (Shervais and Kim-brough, 1985a, 1985b), and the overlying tuffaceousradiolarian cherts (volcanopelagic succession) of which are younger (mostly Late Jurassic) than theTCSM cherts and reflect a quite different deposi-tional setting (Hopson et al., 1996, in prep.; Pessa-gno et al., 2000). The proto-TCSM oceanic mantleand basaltic crust (Bathonian or perhaps older)overlaps and partly predates the CRO remnants(161168 Ma; J. M. Mattinson, pers. commun.,2004), it lacks the abundant plutonic rocks, and key

trace elements in its lava are different. Moreover,the major tectonic event that mlanged the TCSMbelt left the CRO uneffected.

The Stonyford seamount evidently began itsgrowth at more or less the same time as the CROoceanic crust (late Middle Jurassic; Hopson et al., in


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prep.) but in a different, distant tectonic setting, asreflected by their contrasting radiolarian cherts andother differences. Plate tectonic movements thenbrought them together along the Stony CreekBee-hive Flat fault system, a topic addressed next.

Mesozoic Tectonic History:Tehama-Colusa Serpentinite Mlange

and Adjacent Terranes

The importance of the Tehama-Colusa serpen-tinite mlange (TCSM), tectonically, is the role thatit and the adjacent Coast Range ophiolite (CRO)played in development of the forearc ridge that sep-arated the inboard Great Valley forearc basin fromthe outboard Franciscan subduction/accretion com-plex during the Cretaceous. This section deals withthe Jurassic prehistory of the forearc ridge, itsabrupt rise in the Early Cretaceous, and related sub-sequent Cretaceous events.

The TCSM and CRO are juxtaposed along the Beehive Flat fault,a northern continuation of theStony Creek fault(Fig. 2). This subvertical faultsystem separates the TCSM on the west from theCRO and conformably overlying Upper Jurassic

basal strata of the Great Valley Group (GVG) on theeast. The Stony Creek fault s.s., extending fromWilbur Springs north to Mill Creek (west of Paskenta), separates the serpentinite mlange fromGVG Upper Jurassic strata (Fig. 1), presumablyunderlain by unexposed CRO. The small, NW-trending Mill Creek fault west of Paskenta dextrallyoffsets the Stony Creek fault and lifts up the rocks onthe east side, exposing CRO (its Digger CreekElder Creek remnant) beneath the GVG strata.Beyond this small offset the Stony Creek fault con-tinues as the Beehive Flat fault (Fig. 2), whichplaces the serpentinite mlange visibly against CROand diagonally truncates its map units. The BeehiveFlat fault is cut off by the late Cretaceous CoastRange fault ~13 km farther north (Jayko and Blake,1986; Blake et al., 1992).

Brown (1964a, 1964b), working in the Stonyfordarea, described the Stony Creek fault (Fig. 1) as athrust fault. He interpreted the Stonyford volcaniccomplex (SFVC) as a klippe above a low-dipping

thrust fault that flattens westward from the otherwisesteep Stony Creek fault (Brown, 1964b, Fig. 123.2).Shervais and Schuman mapped the SFVC basal con-tact as complexly faulted but not as a low-anglethrust (Shervais et al., in press, a, b). For most of itslength, however, we accept Browns interpretation of

the Stony Creek fault as a steep reverse fault thatbrings up the rocks on the east. McLaughlin et. al.(1984, 1989) also showed the Stony Creek fault as athrust fault in the Wilbur Springs area.

It seems likely, however, that the original bound-ary between the TCSM and CRO terranesnow theStony CreekBeehive Flat fault systemwas a relicttransform fault with very large strike-slip displace-ment. The evidence comes from the contrastingJurassic tectonic histories of the TCSM and theCRO, the two Jurassic oceanic terranes that it sepa-rates. Neither terrane was ever deeply buried, andno amount of dip-slip displacement can account for their different Jurassic tectonic histories. Detailedcomparison of their contrasting Jurassic histories isbeyond the scope of this paper, but a brief summaryof some important differences is offered.

The Coast Range ophiolite (CRO)1 is a compositebody encompassing remnants of Middle Jurassicoceanic lithosphere, an Upper Jurassic abyssal vol-canopelagic sediment cover, an ophiolitic breccia(OB) at northern CRO remnants, Late Jurassic intru-sive sheets, and an ophiolite remnant that invadesand displaces the VP and underlying CRO. Themid-Jurassicophiolite (now dismembered) origi-nally comprised an axial crust-mantle sequence(tholeiitic submarine lava, sheeted diabase, diorite/gabbro, cumulus gabbro, and impregnatedharzburgite tectonite members), and a coeval off-axis sequence of upper submarine lava (from basal-tic/ankaramitic small volcanoes), a dunite-wehrlite-clinopyroxenite intrusive complex (Moho TransitionZone, MTZ), and a wehrlitic-gabbroic-noritic dikeswarmrooted in the MTZthat fed the upper lava(Hopson, 2002; Hopson et al., in prep.). This mid-

Jurassic ophiolite with well-preserved primarystructures and textures, and with minimal internaldeformation, implies a robust thermal budget andthus a fast spreading center, similar to the modernEast Pacific Rise (Dilek et al., 1998). In the LateJurassic (Kimeridgian/Tithonian), an unstableoceanic ridge system (with ridge-jumping) propa-gated through the mid-Jurassic oceanic lithosphere,sending intrusive sheets into the still-accumulating

1Many of our conclusions concerning the CRO come from alarge manuscript entitled California Coast Range ophiolite:

Composite Middle and Late Jurassic oceanic lithosphere,whose preparation is nearly complete. This manuscript, onwhich we draw heavily and believe to be the best currentsource of information on the ophiolite, is cited here and else-where in this paper as Hopson et al. (in prep.).


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VP soft sediments, locally displaced them withnewly formed mafic crust, and generated fragmentaldetritus (ophiolitic breccia and mafic clastic sedi-ment sheets) at northern Coast Range CRO locali-ties (Hopson et al., in prep.).

In contrast, the Tehama-Colusa serpentinitemlange belt comprised only thin basaltic crustcomposed largely of submarine lavas with sparsediabase and only rare plutonic remnants (diorite,gabbro, pyroxenite/wehrlite). That successionreflects a low thermal budget with conspicuousdeformation of primary features (Dilek et al., 1998;Thy and Dilek, 2000; Robinson et al., 2000; alsoSinton and Detrick, 1992), perhaps including ridge-axis core complexing that brought up serpentinizedmantle peridotite while stripping off basaltic crust,analogous to the slow-spreading Mid-Atlantic Ridge(Cannat, 1993; Blackman et al., 1998; Cann et al.,1998). The Jurassic CRO and TCSM oceanic litho-sphere formed at about the same timelate MiddleJurassic and Late Jurassicat paleoequatorial mid-ocean ridges (Hopson et al., 1996, in prep.), butthose ridges were widely separated, with contrastingthermal budgets and spreading rates.

Other important differences between the CRO

and TCSM oceanic lithosphere involve the contrastsin their Jurassic sedimentary histories. The CRO-VPplate stratigraphy reveals an upward successionfrom intralava pelagic limestone within the mid-Jurassic ophiolite, to a CRO-VP disconformity (39m.y. hiatus), to a thin succession of tuffaceous radi-olarian chert (VP distal facies), to a thicker assem-blage (at only two of 22 CRO remnants) of radiolarian-bearing tuff, volcaniclastic turbidites,and submarine volcanic debris-flow deposits (VPproxymal facies). This upward succession, spanning1822 m.y., is incompatible with an arc-related set-ting in which the oceanic basement remains close tothe arc. Instead, this stratigraphy reflects CRO plateformation in an open-ocean setting far from an arc,followed by a progressive approach to an active arcthrough zones of sub-CCD calcareous sediment star-vation, then radiolarian ooze/fallout-tuff deposition(distal facies), and finally proxymal-facies deposi-tion (forearc volcaniclastic apron lobes, reachingtwo CRO segments), together spanning most of

the Late Jurassic (Hopson et al., 1996, in prep.;Pessagno et al., 2000). The CRO oceanic litho-sphere was being drawn progressively closer to aneast-dipping subduction zone in front of the mid- tolate Jurassic (Rogue-Logtown Ridge) continent-fringing intraoceanic arc. VP sedimentation

declined and was overwhelmed by terriginous mudsand distal turbidites (basal Great Valley Group) inthe latest Jurassic (Hopson et al., 1981, 1996), fol-lowing the inboard Nevadan orogeny (Harper et al.,1994; Sharp, 1988) and erosion of the new contrac-tional mountain belt. Mafic sands, grits, and gravelsspread locally around high-standing sea-floor remnants of the CRO ophiolitic breccia unit, inter-fingering with Late Jurassic GVG terriginous sedi-ments from the distant Nevadan orogen.

The results of paleomagnetic and biostrati-graphic studies document CRO formation within afew degrees of the mid-Jurassic paleoequator (Luy-endyk and Hornafias, 1982; Beebe and Luyendyk,1983; Beebe, 1986) and a progression from pale-oequatorial warm-water to higher-latitude cooler-water radiolarian faunas during VP sedimentation(Pessagno et al., 1993, 1999, 2000; Hull, 1995,1997; Hopson et al., 1996; Hull et al., 1997; Pessa-gno and Martin, 2003). Together these record anorthward component of CRO plate motion duringthe Late Jurassic. The quite different interpretion of CRO petrogenesis and Jurassic history by Shervaiset al (2004) is recognized but not accepted (Hopsonet al., in prep.).

In contrast, the TCSM oceanic lithosphereremained in an abyssal deep-sea environment inwhich radiolarian ooze and clay accumulated belowthe calcium carbonate compensation depth (i.e.,forming radiolarite that changed diagenetically toribbon chert) mainly during the late Middle Jurassicand early Late Jurassic (Table 1). Greater thicknessof the Callovianupper Oxfordian ribbon cherts mayreflect a higher accumulation rate of radiolariandetritus while seafloor spreading carried the basal-tic substrate across the equatorial zone of pelagichigh productivity (Berger and Winterer, 1974; Ken-nett, 1982; Ingersoll, 1988). The TCSM ribboncherts, unlikethe VP succession atop the CRO, arelargely Franciscan-like ribbon cherts devoid of vol-caniclastic components such as tuff (from airbornedust and ash), shards, and calc-alkaline volcanicmicrolites (Huot and Maury, 2002 note a singleexception). A paleoequatorial origin for the TCSMcherts, like that of Early Jurassic Franciscan radi-olarian cherts (Murchey, 1984; Murchey and Jones,

1984), seems likely in view of their Central Tethyanradiolaria (Table 1).Differences between the VP tuffaceous cherts

(atop CRO) and the nontuffaceous TCSM (and Fran-ciscan) ribbon cherts reflects their different Jurassichistories. Although both have paleoequatorial


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(Central Tethyan) radiolarian faunas in their lower portions, their transport northward followed differ-ent paths or had different timing. The VP tuffaceouschertsaccumulated while being drawn obliquelynorth-northeastward toward the subduction zone infront of the active Jurassic arcfollowing the falloutzone of the volcanic dust and ash carried south andsouthwest by the trade winds (Hopson et al., 1996,Fig. 3). Thus, radiolarian ooze was mixed with air-borne tephra that settled to the abyssal ocean floo

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