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GSA Bulletin; February 2001; v. 113; no. 2; p. 163–181; 19 figures.

Proterozoic multistage (ca. 1.1 and 0.8 Ga) extension recordedin the Grand Canyon Supergroup and establishment

of northwest- and north-trending tectonic grainsin the southwestern United States

J. Michael Timmons*Karl E. KarlstromCarol M. DehlerJohn W. GeissmanDepartment of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA

Matthew T. HeizlerNew Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico 87801, USA

ABSTRACT

The Grand Canyon Supergroup recordsat least two distinct periods of intracratonicextension and sedimentation in the late Me-soproterozoic and Neoproterozoic. New40Ar/39Ar age determinations indicate thatthe Mesoproterozoic Unkar Group was de-posited between ca. 1.2 and 1.1 Ga. Basinsin which the Unkar Group was depositedand the related northwest-striking faultswere created by northeast-southwest exten-sion, which was contemporaneous with re-gional northwest-southeast ‘‘Grenville’’contraction. New U-Pb data indicate thatthe Neoproterozoic Chuar Group was de-posited between 800 and 742 Ma. Sedimen-tary and tectonic studies show that Chuardeposition took place during east-west ex-tension and resulting normal slip across theButte fault. This event is interpreted to bean intracratonic response to the breakup ofRodinia and initiation of the Cordilleranrift margin. Laramide monoclines of theGrand Canyon region have north andnorthwest trends, reactivate faults thatoriginated at the time of Unkar and Chuardeposition, and can be traced for great dis-tances (hundreds of kilometers) from theGrand Canyon. We use the distribution ofmonoclines in the Southwest to infer the ex-tent of Proterozoic extensional fault sys-tems. The 1.1 Ga northwest-trending struc-

*E-mail: [email protected].

tures and ca. 800–700 Ma north-trendingextensional structures created regionalfault networks that were tectonically in-verted during formation of the AncestralRocky Mountains and Laramide contrac-tion and reactivated during Tertiaryextension.

Keywords: Chuar Group, Grand Canyon,growth faults, intracratonic basins, Neopro-terozoic, Proterozoic rifting.

INTRODUCTION

The .5000-km-long Cordilleran miogeoclineformed as Laurentia was rifted from westerncontinents in the Neoproterozoic. Rift timing re-mains controversial; rifting may have been ini-tiated by 700 Ma (Stewart, 1972; Ross et al.,1989), but drift-phase thermal subsidence ofwestern North America does not seem to haveoccurred until ca. 600 Ma (Bond and Kominz,1984; Levy and Christie-Blick, 1991; Bond,1997). Several workers have proposed poly-phase Neoproterozoic extension in the Cordillera(Burchfiel et al., 1992; Prave, 1999). Uncertain-ties in the tectonic history persist in part becauseof the fragmentary record in the miogeocline(Fig. 1), a lack of good age control, and theoverprinting of the margin by several phases ofsubsequent tectonism.

The remarkably well-preserved Grand Can-yon Supergroup offers a refined perspectiveon the Proterozoic rifting history of westernNorth America. This paper examines the re-

cord of intracratonic extensional tectonismand sedimentation inboard of the plate mar-gins. We recognize at least two discrete epi-sodes of Proterozoic extension in Grand Can-yon, one at ca. 1100–900 Ma and another at800–700 Ma. Two different structural trendswere associated with these two episodes of ex-tension: northwest-striking faults are associ-ated with deposition and tilting of the UnkarGroup and north-striking faults were activeduring deposition of the Chuar Group (Fig. 2).We discuss the reactivation of Proterozoicstructures during Laramide tectonism and usethe orientation and distribution of Laramidestructures in the Colorado Plateau region asan indication of the regional extent of normalfaults along which motion first occurred dur-ing Unkar and Chuar deposition.

GEOLOGIC SETTING, BACKGROUND,AND PREVIOUS WORK

The Grand Canyon Supergroup is exposedexclusively in the eastern Grand Canyon (Fig.2). It rests with angular unconformity on theGranite Gorge Metamorphic Suite (Ilg et al.,1996). The 1.2–1.1 Ga Unkar Group (;2100m thick) is divided into five formations: BassLimestone, Hakatai Shale, Shinumo Quartzite,Dox Sandstone, and Cardenas Lavas (Fig. 3;Hendricks and Stevenson, 1990). The se-quence records both fluvial and shallow-ma-rine deposition, with one main unconformitybetween the Hakatai Shale and Shinumo

164 Geological Society of America Bulletin, February 2001

TIMMONS et al.

Figure 1. Index map of Proterozoic sedimentary rocks and inferred Neoproterozoic struc-tures palinspastically restored after Levy and Christie-Blick (1989). Outcrops of Neopro-terozoic rocks are shown in black. The Neoproterozoic north-trending tectonic grain isinferred from the trend of steeply-dipping Laramide (reactivated) structures (D—Defiancemonocline, GH—Grand Hogback, GW—Grand Wash fault, and H—Hurricane mono-cline) and other north-trending features (FR—Front Range and RGR—Rio Grande rift).Inset shows a proposed Neoproterozoic plate reconstruction (after Brookfield, 1993; Karls-trom et al., 1999; Burrett and Berry, 2000). Neoproterozoic sedimentary basins (900–600Ma) are shaded, and ages of mafic dikes are in billions of years (L—Laurentia, Aus—Australia, E.Ant.—East Antarctica, and B—Baltica).

Quartzite (Hendricks and Stevenson, 1990).The Unkar Group is exposed in isolated rem-nants in grabens and half grabens along theColorado River (Fig. 2). In general, Unkarrocks dip 108–308NE toward normal faultsthat dip 608SW (Sears, 1990).

Overlying the Unkar Group, the Nankow-eap Formation is a relatively thin (120 m) sec-tion of red sandstone, mudstone, and quartzarenite bounded by unconformities (Fig. 3).Elston (1993) also recognized a major uncon-formity within the section and proposed thatthe red beds of the lower Nankoweap For-mation represent a continuation of ‘‘Unkar-like’’ sedimentation. Intraformational faultswere recognized in previous studies (Elston,1989) and are discussed later in this paper.

The Chuar Group comprises ;1600 m oftilted and gently folded unmetamorphosedsedimentary rocks exposed over ;50 km2 intributary canyons of the Colorado River (Fig.4; Huntoon et al., 1996). It unconformablyoverlies the Nankoweap Formation and is inturn overlain by the Sixtymile Formation.Chuar Group exposures are bounded on theeast by the Proterozoic Butte fault. The north-ern and western limit of Chuar Group and Six-tymile Formation exposures is marked by theangular unconformity beneath flat-lying Cam-brian strata. Chuar Group sedimentary rocksalso have been encountered in subcrop in wellcuttings from oil exploratory wells in southernUtah proximal to the East Kaibab monocline(Rauzi, 1990); therefore, the Chuar basin ex-tended to the north and west of present ex-posures, but no exposures or subcrop areknown east of the Butte fault. The ChuarGroup is divided into the Galeros and Kwa-gunt Formations, which are further dividedinto seven members (Fig. 3; Ford and Breed,1973; Elston, 1989). The stratigraphic sectionis overwhelmingly fine-grained, predominant-ly mudrocks (variegated) with important lat-erally continuous and correlatable marker bedsof dolomite and sandstone that help definemembers and formations.

Our interpretation that normal faulting tookplace in two main events, before upper Nan-koweap deposition and during Chuar Groupdeposition, differs from the interpretations ofprevious workers who proposed a single fault-ing and tilting event (Noble, 1914) during de-position of the Sixtymile Formation. Thisevent was variously named the Grand Canyon‘‘revolution’’ (Maxson, 1961), ‘‘disturbance’’(Wilson, 1962; Elston and McKee, 1982), and‘‘orogeny’’ (Elston, 1979) and was envisionedas broadly analogous in style to Basin andRange faulting of the Western United States.Elston (1979) first proposed that dolomite

pinch-outs in the uppermost Chuar Group andthe coarse-grained sandstone, breccia, andslump blocks of the Sixtymile Formation werethe record of ‘‘marine emergence and uplift’’and that extensional deformation occurred‘‘principally, if not entirely during depositionof the Sixtymile Formation.’’

Several workers have documented Larami-

de contractional reactivation leading to reverseslip on the Butte fault, which folded Paleozoicand Mesozoic strata into the east-facing EastKaibab monocline (Fig. 2; Huntoon, 1971).The East Kaibab monocline reactivated theButte fault for most of its exposed length andresulted in as much as 800 m of west-side-upstratigraphic separation of Paleozoic strata.

Geological Society of America Bulletin, February 2001 165

PROTEROZOIC MULTISTAGE EXTENSION RECORDED IN THE GRAND CANYON SUPERGROUP

Figure 2. Proterozoic rocks and extensional faults of eastern Grand Canyon. Rocks of theUnkar Group and correlatives are preserved in grabens and half grabens bounded bynorthwest-trending normal faults. One of these, the Palisades fault, is truncated by thenorth-trending Butte fault. The Chuar Group was deposited during movement on theButte fault. Laramide monoclines reactivated both the northwest and north structuraltrends. Also shown are sample locations for thermochronologic specimens presented inthis paper.

Figure 3. Stratigraphic column of the Grand Canyon Supergroup showing general member lithology, thickness, and approximate age(modified from Elston, 1989).

The monocline exits the Grand Canyon to thesouth along the northwest-striking Palisadesfault. To the north of the study area, the mono-cline trends to the northwest and then bendsto a north-south orientation (Fig. 2).

AGE OF THE GRAND CANYONSUPERGROUP

Geochronologic information from theGrand Canyon Supergroup is limited owing tothe lack of suitable materials for age deter-minations in the sedimentary-dominated se-quence. New 40Ar/39Ar age determinationsfrom separates K-feldspar from four rocks inthe Granite Gorge Metamorphic Suite just be-neath Unkar Group strata yield relatively flatage spectra for ;90% of the total 39Ar re-leased (Fig. 5A). Samples K7–95.5 and K7–99–4 both give age gradients ranging betweenca. 1250 and 1350 Ma; sample K6–91.1 hasa gradient from ca. 1200 to 1300 Ma. SampleK7–115–3 has the flattest age spectrum andyields dates between ca. 1200 and 1250 Ma.On the basis of typical Ar kinetic diffusionparameters (cf. Lovera et al., 1997), these agespectra indicate that the basement cooledthrough 200 8C by 1250–1200 Ma and, there-fore, the Unkar Group strata are younger than1250–1200 Ma. Additionally, the fact that acooling rate of ;25 8C/m.y. at 1200 Ma iscalculated for K7–115–3 K-feldspar possiblysuggests relatively rapid denudation of the


TIMMONS et al.

Figure 4. Geologic map of the Chuar Group in eastern Grand Canyon. The Chuar Group is bounded on the east by the Butte faultand on the west by angular unconformity with Paleozoic rocks. The Chuar syncline parallels the trace of the Butte fault, has an axialplane that dips 608–708 to the east, and has variable plunge along its length. The East Kaibab monocline reactivated the Butte fault formost of its exposed length. River miles are measured downstream from Lee’s Ferry, Arizona. Locations for measured sections presentedin Figure 11 and 17 are shown.



Figure 5. (A) 40Ar/39Ar age spectra for K-feldspars from the Granite Gorge MetamorphicSuite, which lies just below the Unkar Group unconformity. The spectra reveal age gra-dients between 1200 and 1350 Ma, indicating that these rocks cooled through 200 8C by1200 Ma. Deposition of the Grand Canyon Supergroup thus postdates 1200 Ma. The younginitial ages (600 to 800 Ma) of the age spectra are consistent with low-temperature heatingfollowing Unkar Group deposition. (B) 40Ar/39Ar age spectra for three diabase dikes (K7–74–3, K6–70–2, K6–65–2) and one Cardenas Lavas (K6–68–3). The age spectra are com-plex and presumably relate to hydrothermal alteration at ca. 800 Ma. The ca. 1050 Maapparent ages for K7–74–3 (Lizard dike) indicate that the dike is at least this old and isconsistent with reported Rb/Sr age results for the Cardenas Lavas.

basement at this time and implies a deposi-tional age for lowermost Unkar strata of ca.1200 Ma. Lower Unkar Group strata are in-truded by thick diabase sills, similar to the 1.1Ga mafic intrusions throughout central andwestern Arizona (Howard, 1991). These sillsare similar in composition to dikes that cut theupper Unkar Group and to the Cardenas La-vas, and the dikes and sills may represent theplumbing system for the Cardenas Lavas(Hendricks and Lucchitta, 1974). The Carden-as Lavas yielded a Rb-Sr whole-rock date of1070 6 70 Ma (Elston and Mckee, 1982; Lar-son et al., 1994). Thus, the available data in-dicate that the Unkar Group was deposited be-tween ca. 1200 and ca. 1100 Ma.

It is interesting that the K/Ar dates for the

Cardenas Lavas span a wide interval, from700 to 900 Ma, and were postulated to reflectcooling during movement on the Butte fault,coincident with deposition of the SixtymileFormation and ‘‘Grand Canyon orogeny’’(Elston, 1979; Elston and McKee, 1982).However, Larson et al. (1994) suggested thatthe range in K/Ar dates records an alterationand/or heating event at low temperatures, lessthan 250 8C. To better define the age of theCardenas Lavas and associated dikes, we ob-tained 40Ar/39Ar age spectrum data from sev-eral samples (Fig. 5B). The age spectra arehighly disturbed and yield total gas dates (ca.770–988 Ma) that are consistent with previousK/Ar analyses. Sample K7–74–3 (from theLizard dike) yields steps as old as ca. 1050

Ma, which indicates that this dike is at leastthis old, (assuming no excess Ar; Fig. 5B).The complexity of the basalt and dike agespectra is presumably related to the alterationevent inferred by Larson et al. (1994). We pos-tulate that the dikes and Cardenas Lavas wereburied to depths of at least 2–3 km duringChuar deposition and were apparently alteredduring progressive rifting (800–742 Ma; dis-cussed subsequently) resulting in pervasive Arloss. A low temperature for the alterationevent is supported by the 600–800 Ma Ar lossvalues for the initial part of the basement K-feldspar age spectra, which indicates that thebasement was heated to ;150 8C followingUnkar deposition (Fig. 5A). Additionally, zir-con fission-track dates from upper UnkarGroup strata of ca. 1100 Ma (Naeser et al.,1989) suggest that the Unkar Group was notheated to temperatures in excess of 250 8Csince 1100 Ma.

New U-Pb dates from an ash bed providesthe first direct age of the Chuar Group. Theash was sampled within the uppermost Wal-cott Member of the Chuar Group, 1 m belowthe Sixtymile Formation. It yielded a U-Pbdate of 742 6 7 Ma based on seven zirconfractions, including four single grains (Karls-trom et al., 2000 ). This result provides a di-rect date on uppermost Chuar Group deposi-tion. The age of initiation of Chuar Groupdeposition remains unknown, but is likelyyounger than 800 Ma on the basis of conser-vative estimates of subsidence and deposition-al rates (2.5 cm/k.y., assuming the basin con-sidered was not underfilled) and the lack ofobvious unconformities in the Chuar Group.

Paleomagnetic data are important for lateMesoproterozoic and Neoproterozoic plate re-constructions and for correlation with strata ofcomparable age; therefore a short summary ofavailable paleomagnetic data is useful. Elston(1993) noted that paleomagnetic pole posi-tions from the Unkar Group define a counter-clockwise, north-northeast–elongated loop(Fig. 6) that resembles the loop defined byKeweenawan rocks of the Midcontinent riftsystem (e.g., Halls, 1974; Pesonen and Halls,1979). Elston proposed that the Unkar Groupwas deposited between ca. 1.25 Ga, based oncorrelation of Bass Limestone and HakataiShale poles with results from the Sudbury di-abase dikes of the Lake Superior region, and1.07 6 0.07 Ga, the inferred age of the Car-denas Lavas. The majority of the paleomag-netic data from the Dox Sandstone agree withdata from well-dated Keweenawan igneousrocks with ages between 1.098 and 1.087 Ga(Fig. 6). The unconformity-bounded Nankow-eap Formation is reported by Elston (1993) to


TIMMONS et al.

Figure 6. Orthographic projection, centered on lat 108N, long 1808E, showing paleomagnetic poles and associated projected cones of95% confidence from rocks of Mesoproterozoic, Neoproterozoic, and early Paleozoic age from the Grand Canyon and elsewhere (‘‘ref-erence’’ poles) in North America. ‘‘Reference’’ paleomagnetic data for North America (unfilled cones of confidence) are compiled inMeert et al. (1994) and Park et al. (1995). Heavy dashed line with arrows is the Keweenawan loop. Closed symbols for poles refer toresults of inferred uniform normal polarity; open symbols refer to results of inferred uniform reverse polarity. Ages are given in Ma.Grand Canyon paleomagnetic data (Unkar strata, gray, Chuar strata, dark gray) are from Elston (1989), Elston and Gromme (1974),and Elston (1993) and are labeled as follows. Unkar Group: BL l, u—Bass Limestone, lower, upper; HS l, u—Hakatai Shale, lower,upper; SQm—Shinumo Quartzite, middle; DS ul, lm, m, um, u—Dox Sandstone, upper lower, lower middle, middle, upper middle,upper; Clv—Cardenas Lavas; Css—Cardenas sandstone; Nf—Nankoweap Formation, ferruginous. Chuar Group: TM—Tanner Mem-ber, Galeros Formation; J1, Jupiter Member, site 1; J2—Jupiter Member, site 2; CC—Carbon Canyon Member; G—mean, GallerosFormation; CB—Carbon Butte Member, Kwagunt Formation. Sixty Mile Formation: SM l, u—lower, upper.

have been deposited between 1.05 Ga, on thebasis of correlation with paleomagnetic resultsfrom the Nonesuch and Freda Formations(Henry et al., 1977) and Jacobsville Sandstone(Roy and Robertson, 1978), and 0.98 Ga, onthe basis of paleomagnetic correlation to theChequamegon Sandstone of Lake Superior

(McCabe and Van der Voo, 1983). Both setsof data are also comparable with other datafrom igneous rocks of similar age in NorthAmerica (Harlan, 1993; Feig et al., 1994).

On the basis of paleomagnetic data, the hi-atus between the youngest preserved Nankow-eap Formation and deposition of the Chuar

Group was proposed by Elston (1993) to be,100 m.y.; deposition of the Chuar Groupwas proposed to have begun shortly after 0.9Ga. Elston’s (1993) data from his lower andupper members of the Sixtymile Formationare statistically indistinguishable from thoseof well-dated 780 Ma mafic dikes in western



Figure 7. Angular unconformity (;58) between the Cardenas Lavas (below) and the Nan-koweap Formation (above) documents a faulting history that predates Nankoweap depo-sition. View is to the west looking across Basalt Canyon and shows an apparent dipof ;28.

Figure 8. An interpretive sketch map of theoverprinting relationships between north-west-striking faults that were initiated dur-ing Unkar Group deposition and north-striking faults that were initiated duringChuar Group deposition. The variableplunge of the Chuar syncline is interpretedto reflect differential normal displacementalong the Butte fault, accommodated byburied faults that were the sites of move-ment during Unkar deposition. Also shownis the approximate stratigraphic separation(in meters) across parts of the Butte andPalisades faults and the relative timing ofmovement (U—during Unkar deposition,C—during Chuar and/or Sixtymile deposi-tion, and L—Laramide movement).

North America (Park et al., 1995) and theRapitan Group (Park, 1997). However, resultsfrom older Chuar rocks (e.g., Carbon Canyonand Carbon Butte sequences) as well as fromthe uppermost part of the Nankoweap For-mation resemble data from the overlying Mid-dle Cambrian Tapeats Sandstone, promptingthe permissible interpretation that many of theChuar rocks were remagnetized prior to orduring Tapeats deposition. This uncertaintyneeds to be resolved with field-based tests ofthe antiquity of the magnetization.

UNKAR GROUP EXTENSION (Ca. 1.1Ga): NORTHWEST-TRENDINGGRABENS AND HALF GRABENS

Several observations suggest that the maintilting and normal faulting of the Unkar Grouptook place before deposition of the Nankow-eap Formation. (1) Mutually crosscutting re-lationships between normal faults and diabasesills and dikes (Sears, 1973) suggest that maficmagmatism at ca. 1.1 Ga overlapped in timewith extensional faulting in the Unkar Group.(2) In the Tanner graben (Fig. 4), several nor-mal faults that were initiated during Unkar de-position die out up-section in the NankoweapFormation, and, near the Palisades fault, intra-formational faults die out in the Cardenas La-vas. (3) There is a 38–58 angular unconformitybetween the Unkar Group and NankoweapFormation (Fig. 7). Although this episode offaulting has largely been dismissed as minor,we associate it with activity on the northwest-striking normal faults that control the outcroppattern of the Unkar Group. We interpret this

episode of extension to have been significantin magnitude, regional in scale, and differentin structural style from extension during de-position of the Chuar Group (describedsubsequently).

Rocks of the Unkar Group generally dipnortheast toward southwest-dipping normalfaults (Fig. 2) and form coherent #208-dip-ping tilt blocks in half-graben geometries.Structures related to symmetrical or full gra-bens also are exposed at Basalt Canyon, atPhantom Ranch, and on a small scale in BassCanyon and Stone Creek. Small-scale conju-gate pairs of normal faults (,10 m displace-ment) cut Unkar Group strata and strike to thenorthwest, indicating that extension wasnortheast directed. This geometry is consistentwith data obtained from abundant 1.1 Ga di-abase sills and coeval northwest-striking dikeselsewhere in the Southwest, which imply sub-horizontal northwest-directed contraction andsubhorizontal northeast-directed extension(Howard, 1991).

A key fault for deciphering extension be-fore deposition of the Chuar Group is the Pal-isades fault (Fig. 4). The fault strikes 3108 anddips steeply to the southwest. Proterozoicstratigraphic separation across this structure is;1100 m down-to-the-southwest after ;300m of Laramide reverse slip is restored (Fig.8). Close examination of the intersection be-tween the Palisades fault and the Butte faultsuggest that the Butte fault truncates the Pal-isades fault (Fig. 4). South of the Palisadesfault, the Unkar Group (uppermost Dox For-mation and Cardenas Lavas) dips 108–158NEbeneath horizontal Tapeats Sandstone where-

as, north of the fault, Unkar Group strata (low-est member of the Dox Formation) are sub-horizontal beneath Tapeats Sandstone (Fig. 4).This difference in dip implies that the Pali-sades fault tilted and juxtaposed Cardenas La-vas against lower Dox Formation. In contrast,Chuar Group strata are not affected by the Pal-isades fault; rather, Chuar Group strata and theChuar syncline trend north across the trace ofthe Palisades fault. The continuity of Buttefault hanging-wall strata and discontinuity ofButte fault footwall strata across the Palisadesfault indicate that the Palisades fault is trun-cated by the Butte fault (Fig. 8) and that fault-ing and tilting of Unkar Group rocks predateButte fault movement and Chuar Groupdeposition.


TIMMONS et al.

Figure 9. View looking northwest at Tanner Rapids from Tanner Trail shows that faultsthat predate the Chuar Group in the Tanner graben cut the lower Nankoweap Formationand Unkar Group, but are covered by upper Nankoweap Formation and Chuar Group.

Figure 10. Intraformational fault in theCarbon Canyon Member in NankoweapCanyon documenting synsedimentary nor-mal faulting. View is to the southeast. Notethat displacement decreases up-section tozero.

NANKOWEAP EXTENSION:MULTIPLE UNCONFORMITIES?

Evidence for extensional deformation is re-corded by unconformities and intraformationalfaults in the Nankoweap Formation. Elstonand Scott (1973) and Elston (1993) reported amajor unconformity within the NankoweapFormation and suggested that faulting and ero-sion preceded deposition of the upper memberof that formation. Intraformational normalfaults within the Tanner graben are truncatedby strata of the upper Nankoweap Formationand Chuar Group (Fig. 9), suggesting exten-sion during early phases of Nankoweap de-position. Adjacent to major faults, such as theone in Basalt Canyon, sedimentary beds pinchout against the fault. Extension recorded with-in lower Nankoweap strata may be a contin-uation of Unkar-related extension, as support-ed by the similarity between red beds in theDox Formation, intraflow red beds in the Car-denas Lavas, and red beds of the lower Nan-koweap Formation (Elston, 1979). The dura-tion and tectonic importance of thedisconformities below, within, and at the topof the Nankoweap Formation remain poorlyunderstood.

CHUAR GROUP EXTENSION ANDBUTTE FAULT (Ca. 800–740 Ma):NORTH-TRENDING GROWTH FAULTAND GROWTH SYNCLINE

The Butte fault is a normal fault strikingnorth-northwest and dipping 608–708W thattruncates the east side of the Chuar Group for

an exposed strike length of ;18 km (Fig. 4).It records the largest displacement and longesthistory of tectonism of any extensional faultin the Grand Canyon. Proterozoic stratigraphicseparation across the Butte fault is as much as1800 m down-to-the-west just north of thePalisades fault (Fig. 8) (after restoring 300 mof Laramide west-side-up movement). Thismaximum Proterozoic stratigraphic separationcombines the effects of both Butte fault andPalisades fault normal slip.

Our interpretation of the fault relationshipsin the Palisades fault and Butte fault area isthat the Butte fault crosscuts a system ofnorthwest-striking normal faults and relatedtilted blocks of Unkar Group strata (Fig. 8).This interpretation is supported by character-istic northwest-stepping jogs along the traceof the Butte fault. These are found where faultslivers of Unkar Group strata (upper Dox For-mation and Cardenas Lavas) are present alongthe fault in Lava Chuar, Sixtymile, and Nan-koweap Canyons. We propose that thesenorthwestward deflections of the Butte faultfollow buried older faults like the Palisadesfault. This overprinting may also explain thevariable plunge of the Chuar syncline (Fig. 4).Arches along the syncline axis coincide withthe projection of the northwest-striking faults.Syncline troughs where Sixtymile Formationstrata are preserved are located over the hang-ing wall of hypothesized preexisting half gra-bens (Fig. 8).

One fault in the Chuar Group strikes at ahigh angle to the Butte fault (Fig. 4). Thisfault, in northernmost Lava Chuar Canyon,

dips to the north and has north-side-downsense of movement. Our mapping suggeststhat this fault and related northwest-strikingstructures accommodated differential exten-sion along the Butte fault, caused the doublyplunging nature of the Chuar syncline, andwere likely inherited from an older northwest-striking fault system that was initiated at thetime of Unkar deposition (Fig. 8).

Numerous subordinate normal faults in theChuar Group are consistent with one mainpopulation of conjugate faults striking parallelto the Butte fault. Proterozoic faults (over-lapped by Cambrian strata) invariably havenormal-sense stratigraphic separation and pre-dominantly dip slip. For example, in Nankow-eap Canyon, the Butte fault splits into multi-ple segments, all overlain by Cambrian strata(Fig. 4). Stratigraphic separation across indi-vidual faults increases to the west. This step-ping of the fault to the west can be tracedsouth to Sixtymile and Lava Chuar Canyonswhere remnants of the earlier Unkar tilt blocksare preserved in fault slivers near a majornorthwest-trending segment of the Butte fault(Fig. 8). Subordinate faults, which in somecases are intraformational (Fig. 10), documentsynsedimentary movement.



Figure 11. East-west cross sections and measured sections of the Carbon Butte Member of the Kwagunt Formation in (A) Lava ChuarCanyon and (B) Kwagunt Canyon. Measured section locations shown in Figure 4. The Chuar syncline is asymmetric, with an axialplane that dips to the east, toward the Butte fault. The syncline tightens with depth, as suggested by mapping and measured sections.Projected into both cross sections are the measured sections of the Carbon Butte Member (numbers 1–5 in cross section A and 6–12 incross section B), partial measured sections in the Carbon Canyon Member (shown in detail in Fig. 14), and intraformational faults inthe Carbon Canyon Member and Carbon Butte Member. Reactivation of the Butte fault during Laramide contraction inverted theChuar basin and folded Paleozoic strata into the east-facing East Kaibab monocline, shown with its present geometry.


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Figure 12. Schematic time slices illustrating the growth history of the Chuar syncline atkey intervals. (A) Truncation of the Tanner Member dolomite against a subordinate nor-mal fault that is overlapped by the Tanner Member shale, indicating that extensionalfaulting was synchronous with Tanner Member deposition. (B) Growth nature of theChuar syncline during deposition of the Carbon Butte Member; note the postulated de-pocenter in the syncline and thinning of units over the footwall of a ‘‘blind’’ Butte fault.(C) Our interpretation of syncline and fault relationships during deposition of the Sixtym-ile Formation. Note the pinch-out of dolomite facies in the Walcott Member, the apparenttruncation of the Chuar syncline by the Sixtymile Formation, and the tightening of thesyncline at depth. (D) The present fault and syncline geometry in Lava Chuar Canyon.

The Chuar syncline is an open asymmetricfold defined by Chuar Group strata just westof and parallel to the Butte fault (Fig. 4). Onthe east limb, adjacent to the fault, beddingdips steeply to the west (as steep as 758); onthe west limb, bedding dips shallowly to mod-erately to the east (Fig. 11, A and B). Theaxial plane of the syncline dips 608–708E. Thehinge line is doubly plunging along the traceof the Butte fault (Fig. 4). Parallelism betweenthe trace of the syncline’s axial plane and theButte fault argues for principally dip-slipmovement, in agreement with slickenlines,and for a genetic link between displacementalong the Butte fault and syncline develop-ment. The syncline seems to mark a changein structural style up-section above the Tannergraben (Fig. 4). The lowest unit of the ChuarGroup (Tanner Member dolomite), althoughnot obviously folded by the syncline, wastruncated by one strand of the Butte fault inNankoweap Canyon prior to burial of the faultby overlying mudstone. Hence, we infer thatthe Butte fault was active during deposition ofthe Tanner Member (Fig. 12A). The TannerMember and Nankoweap Formation are ex-posed in fault slivers east of the main Buttefault and Chuar syncline, whereas all overly-ing members of the Chuar Group are exposedsolely in the hanging wall of the Butte faultand are folded by the Chuar syncline. Thus,the Tanner Member seems to separate horst-and-graben–style deformation in lower for-mations from syncline development in upperstrata. Field observations and mapping showthat the syncline tightens with structuraldepth, such that lower sedimentary horizonshave steeper dips than upper horizons; thetightest segments of the Chuar syncline coin-cide with syncline troughs, and the most openparts of the syncline match syncline arches.The steepest beds dip parallel to the fault, con-sistent with drag on a fault. Together, theseobservations suggest that the Chuar synclinehas a growth history. None of the synclinalfeatures can be observed in the overlying Pa-leozoic cover, indicating that the syncline isNeoproterozoic in age and clearly unrelated toreverse-sense Laramide reactivation of theButte fault (Fig. 11, A and B).

The geometry and style of deformation pre-served in Chuar Group rocks and Butte faultresemble other rift basins. The El Qaa syn-cline of the Miocene Gulf of Suez rift wasinterpreted as a growth syncline that devel-oped adjacent to, and in the hanging wall of,a blind normal fault (Gawthorpe et al., 1997).Two distinct phases of structural developmentare preserved in the El Qaa syncline. The firstphase is marked by syntectonic deposition of

a fine-grained sedimentary package in agrowth syncline developing in the hangingwall of an extensional fault-propagation foldwhere the depocenter corresponds to the axisof the syncline. Phase two is marked by sur-face faulting and deposition of coarser detritusshed from an exposed scarp and footwall aswell as a shift of the depocenter toward thefault scarp. The scale of this modern analogue

is similar to the Chuar syncline. Each has anaxis ,1 km from the fault plane, and each hascomparable stratigraphic accommodation andsedimentary distribution (discussed subse-quently). Normal-fault and growth-synclinegeometries similar to the Chuar syncline alsohave been produced in sandbox and clay-mod-el experiments (Withjack et al., 1990; Hardyand McClay, 1999).



Figure 13. Facies correlation between two measured partial sections in the upper CarbonCanyon Member between the ‘‘polygonal’’ bed and Baicalia bed. Note lateral persistenceof thicker dolomite beds and lateral discontinuity of other facies. More abundant andthicker black shale units in the axis of the syncline suggest deposition concurrent withsyncline development. Location of sections shown in Figure 4.

SEDIMENTOLOGIC EVIDENCE FORSYNCHRONEITY OF CHUARSEDIMENTATION, FAULTING, ANDSYNCLINAL DEVELOPMENT

Measured sections from the middle to upperChuar Group show thickness variations acrossthe syncline and across subordinate faults in-dicating that sedimentation was synchronouswith normal faulting. Laterally continuoussandstone and carbonate marker beds allowcorrelation of sections within the field area.Some marker beds and (especially) interven-ing mudstone horizons markedly changethickness, with relatively thin sequences onthe east limb of the syncline and thicker se-quences in the syncline-axis region and on thewest limb.

Sections were measured in the CarbonButte Member of the Kwagunt Formation andthe upper Carbon Canyon and Duppa Mem-bers of the Galeros Formation. East-west crosssections in Lava Chuar Canyon and KwaguntCanyon were constructed from the availablemapping and measured sections (Fig. 11, Aand B). In the Lava Chuar cross section (Fig.11A), the base of each Carbon Butte measuredsection is the base of the lowermost sandstone,and the top of the section is the base of acontinuous bed of the stromatolite Boxonia inthe Awatubi Member. The Carbon ButteMember is composed of multiple, laterallycontinuous clastic facies ranging from medi-um-grained sandstone to variegated mudrocks.The entire member thickens ;35% from theeast limb to the axis and thins ;20% from theaxis to the farthest west-limb measurement(Fig. 11A). Thickness changes occur predom-inantly in the shale and mudstone units, butare also observed in the next to lowest sand-stone (thickest in section 4) and Awatubi basalstromatolite (thickest in section 3); thus all fa-cies show the thickness variations. These ob-servations also indicate a depocenter that ap-proximately coincided with the synclinal-axisregion.

In Kwagunt Canyon there is an overallwestward thickening away from the Buttefault (Fig. 11B). Again, the thinnest measuredsections are on the east limb, adjacent to theButte fault. There are no outcrops in the syn-cline axis, but the basal sandstone is thickestnear the syncline axis (section 11) and thinsto the west from section 11 to section 12. Par-tial measured sections 9 and 10 (Fig. 11B)show a thickness change of the upper sand-stone unit across a subordinate normal faulton the west limb. From these observations, weinterpret the Carbon Butte Member to recordnormal faulting (west-side-down) during sed-

imentation and development of the syncline(Fig. 12B).

Thickness variations are also observed instrata above and below the Carbon ButteMember, for example, in the Carbon Canyon

Member of the Galeros Formation (Fig. 13).On the east limb of the syncline, the partialsection measured in Kwagunt Canyon is ;30m thick; in the axial area, the same interval(measured in Carbon Canyon) is 47 m thick,


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Figure 14. Photograph of the Chuar syncline looking north toward Nankoweap Butte.Sandstone of the Carbon Butte Member, at right, dips steeply to the west, and the Six-tymile Formation caps Nankoweap Butte. Note that the synclinal axial plane dips to theeast (toward the Butte fault) and the syncline tightens with depth.

Figure 15. Rose diagram of trends of sym-metrical ripple crests from sandstones ofthe Carbon Canyon and Carbon ButteMembers. Parallelism between rippletrends and the Butte fault suggest that theshorelines paralleled the fault.

an increase of ;60% (Fig. 13). Thinning ofstrata on the east limb of the fold is also ob-served in the Walcott Member, where dolo-mite horizons thin and pinch out toward theButte fault (Elston, 1979; Cook, 1991; Fig.12C). Mapping and the tightening of the syn-cline with depth suggest that the AwatubiMember, although not directly measured, alsothins on the east limb (Fig. 14). Overall, thethinnest sections are on the east limb adjacentto the Butte fault, and the thickest deposits ofthe Carbon Canyon Member through WalcottMembers roughly coincide with the syncline-axis region.

The combined evidence has two importantimplications. First, slip across the Butte fault,syncline development, and sedimentationwere temporally and genetically linked. Sec-ond, the truncation of fine-grained sedimen-tary units by the Butte fault suggests thatChuar deposits were originally continuousacross the Butte fault and that sedimentationseems to have kept pace with faulting suchthat significant relief was not generated duringdeposition and syncline development. Thusthe Butte fault is interpreted to have been ablind normal fault during Chuar deposition,compatible with fault-propagation models.

Sedimentary structures and soft-sedimentdeformation features also are consistent withsynextensional sedimentation. Symmetricalripple crests in sandstone beds of the CarbonButte Member and Carbon Canyon Member(Fig. 15) trend subparallel to the Butte fault.We interpret the ripples to be wave-ripple setsthat developed parallel to shorelines, a situa-tion that suggests that shorelines paralleled theButte fault. Sandstone beds of the CarbonButte Member contain complex, high-ampli-tude, water-escape structures. These includecentimeter-scale flame-and-pillar structures aswell as meter-scale fluid-escape pipes (Fig.16A). These disturbed beds are laterally wide-spread and, in one case, show downslopemovement of material toward the axis of thesyncline (Fig. 16B). Although water-escapestructures alone do not indicate a tectonic sig-nature within the sediment, the presence of ac-tive (penecontemporaneous) faults and theproximity to the Butte fault both suggest a co-seismic origin for these deformation features.

Consequently, we infer that the Neoproter-ozoic Butte fault was an intrabasinal synse-dimentary normal fault and that the Chuarsyncline was a growth syncline in the hangingwall of a propagating normal fault. It is im-portant to emphasize that the overall shape ofthe Chuar basin is unknown, because the onlyknown outcrops are in the Grand Canyon.However, the presence of Chuar Group strata

in the subsurface to the north, importance ofthe north-trending Butte fault in influencingdeposition, and parallelism to the developingnorth-striking Cordilleran margin suggest alikely north-south orientation for the Chuarrift basin (Fig. 1). This orientation would beanalogous to intracratonic rift basins located asimilar distance (200 km) ‘‘inboard’’ of themiogeoclinal hinge during Mesozoic rifting ofeastern North America.

SIXTYMILE FORMATION ANDEXTENSION FOLLOWING CHUARDEPOSITION

The record of extension following ChuarGroup deposition has long been recognizedand is often cited as the principal extensionalevent in Grand Canyon Supergroup history.Our interpretation of a significant extensionalhistory recorded by the Chuar Group warrantsa reevaluation of the tectonic history recordedby the Sixtymile Formation.

The Sixtymile Formation is preserved onlyin four small areas in the axis of the Chuarsyncline (Fig. 4). The top of the formation isunconformably overlain by the Middle Cam-brian Tapeats sandstone, and thus the originalthickness is unknown. It has a maximum pre-served thickness of ;60 m. Elston (1979) pro-posed three members within the SixtymileFormation in Sixtymile Canyon. The uppertwo members are preserved in all four out-crops; the lower member of Elston (1979) isexposed only in Sixtymile Canyon (Fig. 17).

There are questions concerning whetherElston’s lower member should be groupedwith the Sixtymile Formation (Elston, 1979)

or the underlying Walcott Member (Ford andBreed, 1973; Cook, 1991; this study). In Six-tymile Canyon, the ‘‘lower member’’ occupiesapproximately the same position as the 742Ma ash at Nankoweap Butte. We agree withElston (1979) that this member displays evi-dence for slumping and sliding of large car-bonate blocks into black shale, boudinage ofa .2-m-thick dolomite horizon, and accom-panying chaotic deformation. At this same ho-rizon, a large block of dolomite, interpreted tobe an intraclast, appears to be ‘‘tumbled’’ (Els-ton, 1979), and bedding in the clast is at highangle to regional bedding (Fig. 17). Elston(1979) interpreted the boudinaged upper do-lomite and the ‘‘tumbled’’ block to have been



Figure 16. (A) Water-escape structures can be large in scale and are widespread in sandstone beds of the Carbon Butte Member. Theproximity of these deformational structures to the Butte fault and associated intraformational faults suggests that they may be seismicin origin. (B) At the top of the outcrop, fine-grained competent sandstone is folded into a syncline that parallels the Chuar syncline.The sandstone appears to have moved to the east (right), toward the syncline axis, accommodated by the contorted sandstone below.

Figure 17. Measured sections and schematic features of upper Walcott and Sixtymile Formation. View is toward the north, parallel tothe Butte fault and Chuar syncline. Both the east section from Nankoweap Butte (E) and the section from Sixtymile Canyon weremeasured in the syncline-hinge region. Sixtymile Formation appears to thin markedly toward the west summit (W) of Nankoweap Butte.

derived from a dolomite doublet in the Wal-cott Member that crops out ;60 m lower inthe section (Figs. 3 and 17). He suggested thatthis dolomite doublet was unroofed, then slid

westward off of the inferred Butte fault scarp.This scenario seems unlikely for two reasons.First, the doublet pinches out to zero thickness;500 m west of the fault and thus would not

likely have been present on the scarp to slide(Fig. 17). Second, the upper dolomite exhibitskarst features such as cavities infilled withsandstone and is unlike the dolomite doublet


TIMMONS et al.

Figure 18. Lower-hemisphere equal-area projection of minor faults in Paleozoic and Pro-terozoic strata, interpreted as Laramide in age; black dots represent poles to fault planes;great-circle fits were picked by eye after 1% contouring of fault-plane data (contouringnot shown). Contour shading shows distribution of slickenline data. (A) Conjugate thrusts(only measured in Paleozoic rocks) suggest that s1 is trending toward 0428. (B) Conjugatehigh-angle reverse faults appear to reactivate the Butte fault system, and slickenlinessuggest oblique slip with small dextral component. (C) Strike-slip faults accommodateoblique convergence on an older Butte fault. (D) Schematic drawing of mean fault ori-entations; inferred s1 stress direction during Laramide contraction comes from thrustconjugates shown in A.

in the Walcott (Cook, 1991). Thus the mag-nitude and direction of sliding of blocks inElston’s lower member remain unknown.

The middle member of the Sixtymile For-mation (siltstone member) is a thick sequenceof white to red, laminated, and thinly beddedsiltstone, with laminations of several milli-meters to several centimeters. The beds are ir-regularly disrupted by intraformational brec-ciation and disharmonic folding that probablydeveloped soon after deposition, yet after lith-ification of coarser-grained siliciclastic mate-rial. In particular, the base of this member inAwatubi Canyon is marked by decimeter-scaledecollement folds and nappe folds that suggestcontinued sliding and slumping. At Nankow-eap Butte, the base of the middle member ismarked by a 1-m-thick silicified layer thatcontains ovoid concretions at the base, inter-nal brecciation, and perhaps a scoured top.This layer is overlain by a succession of silic-ified marker layers that may represent tephradeposits, silicified carbonates, regoliths, orzones of hydrothermal alteration (Fig. 17).

The upper member (channel-fill member)contains channels filled with intraformationalbreccia derived from the middle member, thenby red fluvial sandstones. At NankoweapButte and Sixtymile Canyon, the channels are;15 m deep, have steep (to undercut) channelwalls, and trend 3408, subparallel to the Buttefault. Stratification becomes more ordered up-section in the channels (Fig. 17) and givesway to interbedded, intact, red sandstone andbreccia. Both trough cross-stratification andclast imbrication indicate flow toward 3408,parallel to the channel axis and the Butte fault.All clasts appear to be derived from siltstoneand chert of the underlying middle member,an observation that is contrary to the interpre-tations of Elston (1979), who reported exoticclasts. Some clasts are rounded, most are an-gular, and a few are meter-scale fragmentsfrom the channel walls. Both the rounding ofclasts and the steep channel walls imply somelithification of the middle member beforechannel incision such that the contact betweenmiddle and upper members is an unconformityof unknown hiatus. These channels and chan-nel fill may reflect localized tectonism alongthe Butte fault scarp and hence emergence ofthe fault. However, they may also representmore regional effects such as a period of base-level drop and channel cutting comparable toother Neoproterozoic sections within the Cor-dillera (Christie-Blick, 1997) and worldwide(Eyles, 1993; Hoffman et al., 1998).

Slip on the Butte fault after deposition ofthe Sixtymile Formation and prior to deposi-tion of the Tapeats Sandstone likely also oc-

curred. An unknown thickness of both ChuarGroup and Sixtymile Formation was removedfrom the footwall of the Butte fault such thatCambrian rocks now rest on the lower DoxFormation of the Unkar Group.

Overall, we agree with Elston (1979) andElston and McKee (1982) that the SixtymileFormation records a dramatic change in thecharacter of deposition from the underlyingChuar Group, and we concur that these rocksmay record one period of slip across the Buttefault. The dramatic change in sedimentarycharacter may reflect the propagation of thefault to the surface and development of aButte fault scarp. In addition, deposits may re-cord a sizable base-level drop as suggested byincised channels of the upper member. TheSixtymile Formation records continued exten-

sion in western North America associated withthe breakup of the western Laurentian margin,with accompanied change from marine to ter-restrial deposition, relative base-level drop,and incision of canyons. It is interesting tospeculate whether the record of base-leveldrop is a consequence of regional (perhapsglobal) Neoproterozoic glaciations and cli-mate change. However, no glacial depositshave been recognized in the Chuar Group orSixtymile Formation.

LARAMIDE CONTRACTION ANDREACTIVATION OF PROTEROZOICEXTENSIONAL FAULTS

To address the original extent of normal-fault systems active during both Unkar and



Figure 19. Index map of ca. 1300–1000 Ma sedimentary rocks and inferred late Meso-proterozoic structures. Outcrops and known subcrops of Late Mesoproterozoic sedimen-tary rocks are shown in black (A—Apache Group, LA—Las Animas Formation, P—Lower Pahrump Group, and U—Unkar Group). The Mesoproterozoic northwest-trendinggrain is continental in scale and suggests a period of continental extension during syn-chronous Grenville contraction. Northwest-trending structural elements include C—Cen-tral Basin platform, CC—Circle Cliffs, Mo—Mogollon Rim, Texas-Walker Lane, Mojave-Sonora, Uncompahgre uplift, and Wind River Range. Inset shows a proposed platereconstruction (after Brookfield, 1993; Karlstrom et al., 1999, and Burrett and Berry,2000) and late Mesoproterozoic basins (shaded) and tectonic elements; mafic dike ages arein billions of years(L—Laurentia, E. Ant—East Antarctica, B—Baltica, Aus-s—Australia[SWEAT], and Aus-a—Australia [AUSWUS]).

Chuar deposition, we need to understand howthese faults were reactivated by contraction inthe Laramide orogeny (ca. 70–50 Ma). Nu-merous minor faults have been measured inboth Precambrian and Paleozoic strata.Among these structures we observe three

main families of faults, all interpreted to haveformed during Laramide contraction: (1) low-angle conjugate thrust faults, (2) high-angleconjugate reverse faults parallel to the Buttefault, and (3) steeply dipping strike-slip faultsthat strike 0648, oblique to the Butte fault. A

comparative study of these structures suggeststhat they are all mutually crosscutting and areinferred to record Laramide shortening, tec-tonic reversal of movement along the Buttefault, and formation of the East Kaibab mono-cline (Huntoon, 1971; Fig. 18).

The conjugate low-angle thrust faults arecompatible with Anderson’s theory for neo-formed thrusts during horizontal compression,with northeast-directed horizontal shortening(inferred s1 orientation of 0428; Fig. 18A),subvertical s3, and horizontal s2 trending to3158. The local shortening direction deter-mined from these faults is broadly consistentwith that obtained by previous workers whoreported a more east-northeast–oriented short-ening direction of ;0658 (Reches, 1978). Thehigh-angle reverse faults have moderate tosteep dips and form conjugate sets that havestrikes subparallel to the Butte fault and donot conform to simple faulting theory (Fig.18B). Slickenlines on fault planes in Paleozoicstrata suggest that movement was primarilydip slip and reverse. The structures suggestthat there must have been preexisting zones ofweakness (Butte fault) that were opportunis-tically exploited by Laramide contraction.This hypothesis is supported by the observa-tion that the density of these structures in-creases toward the Butte fault, perhaps be-cause of fracturing in response to localizedstresses around a reactivated fault. The con-jugate strike-slip faults are subvertical, havesubhorizontal slickenlines, and strike 0648 and0258 (smaller data set, thus difficult to dis-criminate on the stereonet), oblique to theButte fault (Fig. 18C). Movement sense on themain population (0648) is uncertain, becausekinematic indicators of relative movementsense are rare. Dextral strike-slip movementhas been observed for faults striking 0258. Theinferred s1 orientation from the conjugatestrike-slip faults is ;0468, subparallel to re-sults from the thrust conjugates. The faults areinterpreted to accommodate oblique shorten-ing across the Butte fault.

From this analysis, we infer that the Lar-amide reactivation of the Butte fault and for-mation of the East Kaibab monocline recordnortheast-directed horizontal shortening, witha dominant dip-slip component and a minorcomponent of dextral strike-slip (Fig. 18D).Because the Butte fault was a preexistingstructure, the orientation of the Butte faultwith respect to the regional shortening direc-tion played a strong role in how deformationwas accommodated across the structure.Northwest-striking segments of the Butte faultshow nearly pure dip slip; north-trending seg-ments are oblique to the shortening direction


TIMMONS et al.

and have more subsidiary strike-slip faults.North of the Grand Canyon and in Utah, theEast Kaibab monocline bends to the northeastand records a still larger component of obliquedextral shear during shortening (Tindall andDavis, 1999).

INFERRED REGIONAL EXTENT OFPROTEROZOIC FAULT SYSTEMS

Because Laramide contraction in the south-western United States clearly reactivated lateMesoproterozoic and Neoproterozoic normalfaults (Huntoon, 1971; Sears, 1973; Huntoonet al., 1996), it may be possible to infer extentand geometry of Proterozoic fault systemsfrom the distribution of Laramide monoclinesand reverse faults in the Colorado Plateau re-gion. Common structural grains, observed onand adjacent to the Colorado Plateau, includenorthwest-trending Phanerozoic lineamentsand Laramide monoclines (Fig. 19) and north-trending Laramide monoclines and uplifts(Fig. 1). Laramide monoclines and uplifts(e.g., Uncompahgre, Circle Cliffs, and perhapsWind River), as well as some Phanerozoic lin-eaments like the Mojave-Sonora, Texas-Walk-er Lane, and Mogollon Rim, have similartrends and dips to known faults and dikeswhose age matches that of the Unkar Group.In fact, the strong northwest trend observedover much of Laurentia (Grand Canyon, Cen-tral Basin platform, Sudbury dikes, and Mac-kenzie dikes; cf. Fahrig and West, 1986; Fig.19 inset) apparently records regional northeastextension, which overlaps in time with Gren-villian northwest-directed contraction. Wesuggest that the northwest-trending structuralgrain was first established in the late Meso-proterozoic during the late stages of Grenvilleorogenesis and records a period of continen-tal-scale northeast-directed extension.

The other common structural grain on theColorado Plateau and surrounding region is anorth trend, similar to that of the Neoproter-ozoic Butte fault (Fig. 1). It is interesting that,like the East Kaibab monocline (Fig. 1), manymonoclines step from northwest- to north-trending segments, suggesting a linked net-work of reactivated faults (e.g., Defiance upliftand Grand Hogback). Other north-trendingstructural features include the Front Range ofColorado, the eastern and western edges of theColorado Plateau, and the Rio Grande rift. Wesuggest that the north-trending structural grainobserved on the Colorado Plateau and sur-rounding region was established during riftingof western Laurentia in the Neoproterozoic.Previous workers have also attributed thestructural grains observed on the Colorado

Plateau and surrounding region to a generalfabric that was established in the late Meso-proterozoic to Neoproterozoic (Marshak andPaulsen, 1996; Karlstrom and Humphreys,1998), but the data for different ages of north-west- (1.1 Ga) versus north-trending faults(800–742 Ma) presented in this paper suggestthat each structural grain represents differentProterozoic extensional events.

DISCUSSION OF REGIONALTECTONIC IMPLICATIONS

Tectonic models for the late Mesoprotero-zoic and Neoproterozoic structural and sedi-mentary history of western North America re-main controversial. Harrison et al. (1974)postulated that sedimentary rocks of the Beltbasin, Uinta basin, and the Grand Canyon–Apache–Pahrump basin were deposited in au-lacogens developed at high angles to the Cor-dilleran miogeocline, the so-called ‘‘UdderHypothesis.’’ However, new U-Pb geochro-nology indicates that the lower Belt Super-group accumulated between 1470 and 1430Ma and the upper by 1370 Ma (Link et al.,1993; Aleinikoff, 1996). The Unkar Group(and parts of the Apache Group) are 1.2–1.1Ga (Elston and McKee, 1982; Elston, 1993;this study), and the Chuar Group (and UintaMountain Supergroup?) are Neoproterozoic(800–742 Ma; Link et al., 1993; this study).Thus, it is clear that these different Mesopro-terozoic and Neoproterozoic basins were notdeposited synchronously in aulacogens (Els-ton, 1993). There is no reason to believe thatsedimentation spanned hundreds of millionsof years in each basin. Rather, sedimentary se-quences record local punctuated depositionaland extensional episodes.

Our interpretation of the mechanisms driv-ing extensional deformation and basin for-mation are as follows. Incipient rifting of thecontinental interior was accompanied by the1.1 Ga mafic magmatic event, perhaps facili-tated by mantle insulation by a stable crust(Hoffman, 1989), but also related to far-fieldstresses and intracratonic rifting (1130–1080Ma in the Midcontinent). Rift basins and as-sociated magmatism seem to overlap in timewith late stages of Grenville contraction andmay be kinematically linked to Grenville oro-genesis (Lambeck, 1983; DeRito et al., 1983;Karlstrom and Humphreys, 1998; Fig 19).Neoproterozoic (age of Chuar deposition) ex-tension apparently reflects east-west extensionduring breakup of the supercontinent Rodiniaand initiation of the Cordilleran miogeoclinealong the western margin of North America(Fig. 1). Thus, emerging models point toward

a prolonged history of multistage extension inwestern North America in the latePrecambrian.

This model may help resolve the long-standing controversy regarding timing of rift-ing in western Laurentia. Mesoproterozoic andNeoproterozoic basins both record continen-tal-scale extension, with important ‘‘events’’at ca. 1.1 Ga and 0.80–0.74 Ga. In the GrandCanyon, the ca. 800–742 Ma Chuar Group re-cords east-west extension that is broadly syn-chronous with other previously identified ex-tensional basins along the Cordillera: the 770Ma Little Dal Group (Young et al., 1979; Els-ton and McKee, 1982); the .723 Ma RapitanGroup (Link et al. 1993); and the pre–syn-Sturtian Kingston Peak Formation (Prave,1999). The record of ca. 750 Ma extensionobserved in sedimentary and igneous rocks allalong the western Cordillera (Ross et al.,1989; Ross, 1991) and Grand Canyon (thisstudy) documents a major, continental-scalerifting event (thousands of kilometers inlength and hundreds of kilometers in width),presumably related to the separation of west-ern continents from western North America.

Australia may have been part of this west-ern continent (Karlstrom et al., 1999), and pa-leomagnetic data for North America and Aus-tralia suggest that by 755 Ma, the twocontinents were separated (Wingate and Gid-dings, 2000). Alternative rift models that pro-pose younger rifting, based on subsidencemodels for rifting (Bond and Kominz, 1984;Levy and Christie-Blick, 1991), rely heavilyon the Paleozoic record and may not be sen-sitive to a complex rift history. Complexities,such as multiple rift episodes or an asymmet-ric rift geometry, could conceivably disturbthe thermal subsidence history of the Cordil-leran margin. Thus, a single rift event alongthe western margin of North America is toosimplistic, and emerging models for multiplerift episodes seem to better describe the Neo-proterozoic extensional history of theCordillera.

Understanding the multiphase extensionalhistory, and importance of inherited structuralgrain, may ultimately help to refine models forthe positioning of Laurentia’s Proterozoic con-tinental neighbors by identifying structuralgrains that were preferred during rifting versusthose that were transforms (Fig. 1). Brookfield(1993) proposed that northwest- and north- tonortheast-trending segments (reconstructedcoordinates) on both Laurentia and Australiawere once a continuous rift-transform systemduring rifting of Rodinia. The northwest-trending segments, such as the Koonenberryfault zone in Australia (Burrett and Berry,



2000) and the Mojave-Sonora and Texas-Walker Lane lineaments in southern Laurentia,may have been preexisting structures estab-lished during Grenvillian intracratonic riftingand basin development. These were apparent-ly rift segments during the initial accumula-tion of the Adelaidian sequences in Australia(Preiss, 1987), but may have evolved intotransforms at later stages of a complex pro-gressive rifting (Brookfield, 1993; Karlstromet al., 1999; Burrett and Berry, 2000). GrandCanyon Supergroup data suggest that, by 740Ma, extension was oriented east-west in west-ern Laurentia, with probable initiation of anorth-trending Cordilleran miogeocline by thistime.

SUMMARY

Strata of the Grand Canyon Supergroup re-cord at least two distinct episodes of intracra-tonic rifting and basin formation in south-western North America, one related to the 1.1Ga Grenville collision and the second to theca. 850–742 Ma incipient rifting of Rodinia.The first event is recorded by sedimentary andigneous rocks of the Mesoproterozoic UnkarGroup (1200–1100 Ma). These rocks are ex-posed in tilt blocks and grabens that recordnortheast-directed extension across northwest-striking normal faults prior to deposition ofthe unconformity-bounded Nankoweap For-mation with an inferred age of 900 Ma. ThisMesoproterozoic extension is marked by a,58 angular unconformity between the UnkarGroup and overlying Nankoweap Formationand Chuar Group and by intraformationalfaults in Nankoweap strata. Extension waspossibly active during, but definitely after, 1.1Ga mafic magmatism, with renewed or contin-ued extension during Nankoweap deposition(ca. 900 Ma).

The second event is recorded by the Neo-proterozoic Chuar Group (ca. 800–742 Ma).The Chuar Group records east-west extensionand formation of the north-striking, west-dip-ping Butte fault that both truncates and locallyreactivates northwest-striking Mesoprotero-zoic faults. The Butte fault is the eastern limitof Chuar Group exposures and accommodates;1800 m of Proterozoic stratigraphic separa-tion. The Chuar syncline is just west of andparallel to the trace of the Butte fault. Thisgeometry, plus tightening of the syncline atdepth and sedimentary evidence, suggests thatthe syncline is a growth structure. The syn-cline fold axis is doubly plunging along thestrike of the fault, suggesting along-strike var-iations in normal displacement across thefault. The Paleozoic cover is not folded, in-

dicating that the syncline is Neoproterozoicand unrelated to the reverse slip (300 m) alongthe Butte fault during Laramide contraction.

Thickness variations of the Chuar Group(especially Carbon Canyon through WalcottMembers) indicate synchronous deposition,normal faulting, and syncline development.Siliciclastic strata tend to be thicker in thesynclinal-axis region and thinner on the eastlimb proximal to the fault. Abrupt thicknessand facies changes are also observed acrosssubordinate normal faults within the Buttefault system. Sedimentary and soft-sedimentdeformational structures in the Chuar Groupare also consistent with synchronous deposi-tion, faulting, and syncline development. Con-volute bedding, bedding-parallel slip surfaces,pinch-out of units toward the fault, and intra-formational breccia collectively indicate thatButte fault movement and syncline develop-ment continued during deposition of the upperChuar and Sixtymile strata. Thus, rather thana Grand Canyon ‘‘disturbance’’ after ChuarGroup deposition, we interpret extension toalso have been concurrent with Chuar Groupdeposition (ca. 800–742 Ma).

Laramide contractional faulting locally re-activated both Mesoproterozoic and Neopro-terozoic structures. The East Kaibab mono-cline in the Grand Canyon mainly follows thenorth-trending Neoproterozoic Butte fault, ex-cept where it follows older Mesoproterozoicnorthwest-trending structures such as the Pal-isades fault. Many monoclines have north-and northwest-trending segments, and wespeculate that many of the monoclines of theColorado Plateau and Rocky Mountains alsoreactivated faults of Neoproterozoic age. Like-wise, many northwest-trending Phanerozoicstructures, like the Mogollon Rim and Un-compahgre uplift, may reflect inheritance fromMesoproterozoic trends. Thus, new data fromthe Grand Canyon may provide a critical cluefor piecing together two regional extensionalevents, at ca. 1.1 Ga and ca. 800–700 Ma, thatwere responsible for forming two major tec-tonic ‘‘grains’’ in the crust of the WesternUnited States.

ACKNOWLEDGMENTS

This work was made possible by National Sci-ence Foundation (NSF) grant EAR 9706541 to KarlKarlstrom, John Geissman, and Maya Elrick. Fur-ther thanks go to NSF for the ED-MAP grant toKarlstrom for mapping in the Grand Canyon Su-pergroup. We also thank Grand Canyon NationalPark for our research agreement and sampling per-mit. Further support came from Ben Donegan (con-sulting geologist), Conoco Inc., and SchlumbergerInc. We thank Michael Wells, Paul Link, and as-sociate editor John Bartley for their thoughtful re-

views of the manuscript. Maya Elrick, Laurie Cros-sey, Gary Smith, and Gene Humphreys helped usimprove the manuscript. The following individualsassisted us in the Grand Canyon: Adam Read, MarySimmons, Colin Shaw, Brad Ilg, Mike Doe, JakeArmour, Casey Cook, Sarah Tindall, Arlo Weil, andPaul Bauer.

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