Proterozoic multistage (ca. 1.1 and 0.8 Ga) extension recorded in the Grand Canyon Supergroup and establishment of northwest- and north-trending tectonic grains in the southwestern United States

J. Michael Timmons* Karl E. Karlstrom Carol M. Dehler John W. Geissman Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA Matthew T. Heizler New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico 87801, USA

ABSTRACT tures and ca. 800±700 Ma north-trending cord of intracratonic extensional tectonism extensional structures created regional and sedimentation inboard of the plate mar- The Grand Canyon Supergroup records fault networks that were tectonically in- gins. We recognize at least two discrete epi- at least two distinct periods of intracratonic verted during formation of the Ancestral sodes of Proterozoic extension in Grand Can- extension and sedimentation in the late Me- Rocky Mountains and Laramide contrac- yon, one at ca. 1100±900 Ma and another at soproterozoic and Neoproterozoic. New tion and reactivated during Tertiary 800±700 Ma. Two different structural trends 40Ar/39Ar age determinations indicate that extension. were associated with these two episodes of ex- the Mesoproterozoic Unkar Group was de- tension: northwest-striking faults are associ- posited between ca. 1.2 and 1.1 Ga. Basins Keywords: Chuar Group, Grand Canyon, ated with deposition and tilting of the Unkar in which the Unkar Group was deposited growth faults, intracratonic basins, Neopro- Group and north-striking faults were active and the related northwest-striking faults terozoic, Proterozoic rifting. during deposition of the Chuar Group (Fig. 2). were created by northeast-southwest exten- We discuss the reactivation of Proterozoic sion, which was contemporaneous with re- INTRODUCTION structures during Laramide tectonism and use gional northwest-southeast ``Grenville'' the orientation and distribution of Laramide contraction. New U-Pb data indicate that The Ͼ5000-km-long Cordilleran miogeocline structures in the Colorado Plateau region as the Neoproterozoic Chuar Group was de- formed as Laurentia was rifted from western an indication of the regional extent of normal posited between 800 and 742 Ma. Sedimen- continents in the Neoproterozoic. Rift timing re- faults along which motion ®rst occurred dur- tary and tectonic studies show that Chuar mains controversial; rifting may have been ini- ing Unkar and Chuar deposition. deposition took place during east-west ex- tiated by 700 Ma (Stewart, 1972; Ross et al., tension and resulting normal slip across the 1989), but drift-phase thermal subsidence of GEOLOGIC SETTING, BACKGROUND, Butte fault. This event is interpreted to be western North America does not seem to have AND PREVIOUS WORK an intracratonic response to the breakup of occurred until ca. 600 Ma (Bond and Kominz, Rodinia and initiation of the Cordilleran 1984; Levy and Christie-Blick, 1991; Bond, rift margin. Laramide monoclines of the 1997). Several workers have proposed poly- The Grand Canyon Supergroup is exposed Grand Canyon region have north and phase Neoproterozoic extension in the Cordillera exclusively in the eastern Grand Canyon (Fig. northwest trends, reactivate faults that (Burch®el et al., 1992; Prave, 1999). Uncertain- 2). It rests with angular unconformity on the originated at the time of Unkar and Chuar ties in the tectonic history persist in part because Granite Gorge Metamorphic Suite (Ilg et al., deposition, and can be traced for great dis- of the fragmentary record in the miogeocline 1996). The 1.2±1.1 Ga Unkar Group (ϳ2100 tances (hundreds of kilometers) from the (Fig. 1), a lack of good age control, and the m thick) is divided into ®ve formations: Bass Grand Canyon. We use the distribution of overprinting of the margin by several phases of Limestone, Hakatai Shale, Shinumo Quartzite, monoclines in the Southwest to infer the ex- subsequent tectonism. Dox Sandstone, and Cardenas Lavas (Fig. 3; tent of Proterozoic extensional fault sys- The remarkably well-preserved Grand Can- Hendricks and Stevenson, 1990). The se- tems. The 1.1 Ga northwest-trending struc- yon Supergroup offers a re®ned perspective quence records both ¯uvial and shallow-ma- on the Proterozoic rifting history of western rine deposition, with one main unconformity *E-mail: [email protected]. North America. This paper examines the re- between the Hakatai Shale and Shinumo

GSA Bulletin; February 2001; v. 113; no. 2; p. 163±181; 19 ®gures.

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Quartzite (Hendricks and Stevenson, 1990). The Unkar Group is exposed in isolated rem- nants in grabens and half grabens along the Colorado River (Fig. 2). In general, Unkar rocks dip 10Њ±30ЊNE toward normal faults that dip 60ЊSW (Sears, 1990). Overlying the Unkar Group, the Nankow- eap Formation is a relatively thin (120 m) sec- tion of red sandstone, mudstone, and quartz arenite bounded by unconformities (Fig. 3). Elston (1993) also recognized a major uncon- formity within the section and proposed that the red beds of the lower Nankoweap For- mation represent a continuation of ``Unkar- like'' sedimentation. Intraformational faults were recognized in previous studies (Elston, 1989) and are discussed later in this paper. The Chuar Group comprises ϳ1600 m of tilted and gently folded unmetamorphosed sedimentary rocks exposed over ϳ50 km2 in tributary canyons of the Colorado River (Fig. 4; Huntoon et al., 1996). It unconformably overlies the Nankoweap Formation and is in turn overlain by the Sixtymile Formation. Chuar Group exposures are bounded on the east by the Proterozoic Butte fault. The north- ern and western limit of Chuar Group and Six- tymile Formation exposures is marked by the angular unconformity beneath ¯at-lying Cam- brian strata. Chuar Group sedimentary rocks also have been encountered in subcrop in well cuttings from oil exploratory wells in southern Utah 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 are known east of the Butte fault. The Chuar Group is divided into the Galeros and Kwa- gunt Formations, which are further divided into seven members (Fig. 3; Ford and Breed, 1973; Elston, 1989). The stratigraphic section Figure 1. Index map of Proterozoic sedimentary rocks and inferred Neoproterozoic struc- is overwhelmingly ®ne-grained, predominant- tures palinspastically restored after Levy and Christie-Blick (1989). Outcrops of Neopro- ly mudrocks (variegated) with important lat- terozoic rocks are shown in black. The Neoproterozoic north-trending tectonic grain is erally continuous and correlatable marker beds inferred from the trend of steeply-dipping Laramide (reactivated) structures (DÐDe®ance of dolomite and sandstone that help de®ne monocline, GHÐGrand Hogback, GWÐGrand Wash fault, and HÐHurricane mono- members and formations. cline) and other north-trending features (FRÐFront Range and RGRÐRio Grande rift). Our interpretation that normal faulting took Inset shows a proposed Neoproterozoic plate reconstruction (after Brook®eld, 1993; Karls- place in two main events, before upper Nan- trom et al., 1999; Burrett and Berry, 2000). Neoproterozoic sedimentary basins (900±600 koweap deposition and during Chuar Group Ma) are shaded, and ages of ma®c dikes are in billions of years (LÐLaurentia, AusÐ deposition, differs from the interpretations of Australia, E.Ant.ÐEast Antarctica, and BÐBaltica). previous workers who proposed a single fault- ing and tilting event (Noble, 1914) during de- position of the Sixtymile Formation. This pinch-outs in the uppermost Chuar Group and de contractional reactivation leading to reverse event was variously named the Grand Canyon the coarse-grained sandstone, breccia, and slip on the Butte fault, which folded Paleozoic ``revolution'' (Maxson, 1961), ``disturbance'' slump blocks of the Sixtymile Formation were and Mesozoic strata into the east-facing East (Wilson, 1962; Elston and McKee, 1982), and the record of ``marine emergence and uplift'' Kaibab monocline (Fig. 2; Huntoon, 1971). ``orogeny'' (Elston, 1979) and was envisioned and that extensional deformation occurred The East Kaibab monocline reactivated the as broadly analogous in style to Basin and ``principally, if not entirely during deposition Butte fault for most of its exposed length and Range faulting of the Western United States. of the Sixtymile Formation.'' resulted in as much as 800 m of west-side-up Elston (1979) ®rst proposed that dolomite Several workers have documented Larami- stratigraphic separation of Paleozoic strata.

164 Geological Society of America Bulletin, February 2001 PROTEROZOIC MULTISTAGE EXTENSION RECORDED IN THE GRAND CANYON SUPERGROUP

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

AGE OF THE GRAND CANYON SUPERGROUP

Geochronologic information from the Grand Canyon Supergroup is limited owing to the lack of suitable materials for age deter- minations in the sedimentary-dominated se- quence. New 40Ar/39Ar age determinations from separates K-feldspar from four rocks in the Granite Gorge Metamorphic Suite just be- neath Unkar Group strata yield relatively ¯at age spectra for ϳ90% of the total 39Ar re- leased (Fig. 5A). Samples K7±95.5 and K7± 99±4 both give age gradients ranging between ca. 1250 and 1350 Ma; sample K6±91.1 has a gradient from ca. 1200 to 1300 Ma. Sample K7±115±3 has the ¯attest age spectrum and yields dates between ca. 1200 and 1250 Ma. Figure 2. Proterozoic rocks and extensional faults of eastern Grand Canyon. Rocks of the On the basis of typical Ar kinetic diffusion Unkar Group and correlatives are preserved in grabens and half grabens bounded by parameters (cf. Lovera et al., 1997), these age northwest-trending normal faults. One of these, the Palisades fault, is truncated by the spectra indicate that the basement cooled north-trending Butte fault. The Chuar Group was deposited during movement on the through 200 ЊC by 1250±1200 Ma and, there- Butte fault. Laramide monoclines reactivated both the northwest and north structural fore, the Unkar Group strata are younger than trends. Also shown are sample locations for thermochronologic specimens presented in 1250±1200 Ma. Additionally, the fact that a this paper. cooling rate of ϳ25 ЊC/m.y. at 1200 Ma is calculated for K7±115±3 K-feldspar possibly suggests relatively rapid denudation of the

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

Geological Society of America Bulletin, February 2001 165 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 fault and on the west by angular unconformity with Paleozoic rocks. The Chuar syncline parallels the trace of the Butte fault, has an axial plane that dips 60؇±70؇ to the east, and has variable plunge along its length. The East Kaibab monocline reactivated the Butte fault for most of its exposed length. River miles are measured downstream from Lee's Ferry, Arizona. Locations for measured sections presented in Figure 11 and 17 are shown.

166 Geological Society of America Bulletin, February 2001 PROTEROZOIC MULTISTAGE EXTENSION RECORDED IN THE GRAND CANYON SUPERGROUP

Ma, which indicates that this dike is at least this old, (assuming no excess Ar; Fig. 5B). The complexity of the basalt and dike age spectra is presumably related to the alteration event inferred by Larson et al. (1994). We pos- tulate that the dikes and Cardenas Lavas were buried to depths of at least 2±3 km during Chuar deposition and were apparently altered during progressive rifting (800±742 Ma; dis- cussed subsequently) resulting in pervasive Ar loss. A low temperature for the alteration event is supported by the 600±800 Ma Ar loss values for the initial part of the basement K- feldspar age spectra, which indicates that the basement was heated to ϳ150 ЊC following Unkar deposition (Fig. 5A). Additionally, zir- con ®ssion-track dates from upper Unkar Group strata of ca. 1100 Ma (Naeser et al., 1989) suggest that the Unkar Group was not heated to temperatures in excess of 250 ЊC since 1100 Ma. New U-Pb dates from an ash bed provides the ®rst direct age of the Chuar Group. The ash was sampled within the uppermost Wal- cott Member of the Chuar Group, 1 m below the Sixtymile Formation. It yielded a U-Pb date of 742 Ϯ 7 Ma based on seven zircon fractions, including four single grains (Karls- trom et al., 2000 ). This result provides a di- rect date on uppermost Chuar Group deposi- 40 39 Figure 5. (A) Ar/ Ar age spectra for K-feldspars from the Granite Gorge Metamorphic tion. The age of initiation of Chuar Group Suite, which lies just below the Unkar Group unconformity. The spectra reveal age gra- deposition remains unknown, but is likely -dients between 1200 and 1350 Ma, indicating that these rocks cooled through 200 ؇Cby younger than 800 Ma on the basis of conser 1200 Ma. Deposition of the Grand Canyon Supergroup thus postdates 1200 Ma. The young vative estimates of subsidence and deposition- initial ages (600 to 800 Ma) of the age spectra are consistent with low-temperature heating al rates (2.5 cm/k.y., assuming the basin con- 40 39 following Unkar Group deposition. (B) Ar/ Ar age spectra for three diabase dikes (K7± sidered was not under®lled) and the lack of 74±3, K6±70±2, K6±65±2) and one Cardenas Lavas (K6±68±3). The age spectra are com- obvious unconformities in the Chuar Group. plex and presumably relate to hydrothermal alteration at ca. 800 Ma. The ca. 1050 Ma Paleomagnetic data are important for late apparent ages for K7±74±3 (Lizard dike) indicate that the dike is at least this old and is Mesoproterozoic and Neoproterozoic plate re- consistent with reported Rb/Sr age results for the Cardenas Lavas. constructions and for correlation with strata of comparable age; therefore a short summary of available paleomagnetic data is useful. Elston (1993) noted that paleomagnetic pole posi- basement at this time and implies a deposi- Cardenas Lavas span a wide interval, from tions from the Unkar Group de®ne a counter- tional age for lowermost Unkar strata of ca. 700 to 900 Ma, and were postulated to re¯ect clockwise, north-northeast±elongated loop 1200 Ma. Lower Unkar Group strata are in- cooling during movement on the Butte fault, (Fig. 6) that resembles the loop de®ned by truded by thick diabase sills, similar to the 1.1 coincident with deposition of the Sixtymile Keweenawan rocks of the Midcontinent rift Ga ma®c intrusions throughout central and Formation and ``Grand Canyon orogeny'' system (e.g., Halls, 1974; Pesonen and Halls, western Arizona (Howard, 1991). These sills (Elston, 1979; Elston and McKee, 1982). 1979). Elston proposed that the Unkar Group are similar in composition to dikes that cut the However, Larson et al. (1994) suggested that was deposited between ca. 1.25 Ga, based on upper Unkar Group and to the Cardenas La- the range in K/Ar dates records an alteration correlation of Bass Limestone and Hakatai vas, and the dikes and sills may represent the and/or heating event at low temperatures, less Shale poles with results from the Sudbury di- plumbing system for the Cardenas Lavas than 250 ЊC. To better de®ne the age of the abase dikes of the Lake Superior region, and (Hendricks and Lucchitta, 1974). The Carden- Cardenas Lavas and associated dikes, we ob- 1.07 Ϯ 0.07 Ga, the inferred age of the Car- as Lavas yielded a Rb-Sr whole-rock date of tained 40Ar/39Ar age spectrum data from sev- denas Lavas. The majority of the paleomag- 1070 Ϯ 70 Ma (Elston and Mckee, 1982; Lar- eral samples (Fig. 5B). The age spectra are netic data from the Dox Sandstone agree with son et al., 1994). Thus, the available data in- highly disturbed and yield total gas dates (ca. data from well-dated Keweenawan igneous dicate that the Unkar Group was deposited be- 770±988 Ma) that are consistent with previous rocks with ages between 1.098 and 1.087 Ga tween ca. 1200 and ca. 1100 Ma. K/Ar analyses. Sample K7±74±3 (from the (Fig. 6). The unconformity-bounded Nankow- It is interesting that the K/Ar dates for the Lizard dike) yields steps as old as ca. 1050 eap Formation is reported by Elston (1993) to

Geological Society of America Bulletin, February 2001 167 TIMMONS et al.

Figure 6. Orthographic projection, centered on lat 10؇N, long 180؇E, showing paleomagnetic poles and associated projected cones of 95% con®dence 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 (un®lled cones of con®dence) are compiled in Meert et al. (1994) and Park et al. (1995). Heavy dashed line with arrows is the Keweenawan loop. Closed symbols for poles refer to results 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, uÐBass Limestone, lower, upper; HS l, uÐHakatai Shale, lower, upper; SQmÐShinumo Quartzite, middle; DS ul, lm, m, um, uÐDox Sandstone, upper lower, lower middle, middle, upper middle, upper; ClvÐCardenas Lavas; CssÐCardenas sandstone; NfÐNankoweap Formation, ferruginous. Chuar Group: TMÐTanner Mem- ber, Galeros Formation; J1, Jupiter Member, site 1; J2ÐJupiter Member, site 2; CCÐCarbon Canyon Member; GÐmean, Galleros Formation; CBÐCarbon Butte Member, Kwagunt Formation. Sixty Mile Formation: SM l, uÐlower, upper.

have been deposited between 1.05 Ga, on the (McCabe and Van der Voo, 1983). Both sets Group was proposed by Elston (1993) to be basis of correlation with paleomagnetic results of data are also comparable with other data Ͻ100 m.y.; deposition of the Chuar Group from the Nonesuch and Freda Formations from igneous rocks of similar age in North was proposed to have begun shortly after 0.9 (Henry et al., 1977) and Jacobsville Sandstone America (Harlan, 1993; Feig et al., 1994). Ga. Elston's (1993) data from his lower and (Roy and Robertson, 1978), and 0.98 Ga, on On the basis of paleomagnetic data, the hi- upper members of the Sixtymile Formation the basis of paleomagnetic correlation to the atus between the youngest preserved Nankow- are statistically indistinguishable from those Chequamegon Sandstone of Lake Superior eap Formation and deposition of the Chuar of well-dated 780 Ma ma®c dikes in western

168 Geological Society of America Bulletin, February 2001 PROTEROZOIC MULTISTAGE EXTENSION RECORDED IN THE GRAND CANYON SUPERGROUP

-Figure 7. Angular unconformity (ϳ5؇) 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 dip .of ϳ2؇

North America (Park et al., 1995) and the episode of extension to have been signi®cant Rapitan Group (Park, 1997). However, results in magnitude, regional in scale, and different Figure 8. An interpretive sketch map of the from older Chuar rocks (e.g., Carbon Canyon in structural style from extension during de- overprinting relationships between north- and Carbon Butte sequences) as well as from position of the Chuar Group (described west-striking faults that were initiated dur- the uppermost part of the Nankoweap For- subsequently). ing Unkar Group deposition and north- mation resemble data from the overlying Mid- Rocks of the Unkar Group generally dip striking faults that were initiated during dle Cambrian Tapeats Sandstone, prompting northeast toward southwest-dipping normal Chuar Group deposition. The variable the permissible interpretation that many of the faults (Fig. 2) and form coherent Յ20Њ-dip- plunge of the Chuar syncline is interpreted Chuar rocks were remagnetized prior to or ping tilt blocks in half-graben geometries. to re¯ect differential normal displacement during Tapeats deposition. This uncertainty Structures related to symmetrical or full gra- along the Butte fault, accommodated by needs to be resolved with ®eld-based tests of bens also are exposed at Basalt Canyon, at buried faults that were the sites of move- the antiquity of the magnetization. Phantom Ranch, and on a small scale in Bass ment during Unkar deposition. Also shown Canyon and Stone Creek. Small-scale conju- is the approximate stratigraphic separation UNKAR GROUP EXTENSION (Ca. 1.1 gate pairs of normal faults (Ͻ10 m displace- (in meters) across parts of the Butte and Ga): NORTHWEST-TRENDING ment) cut Unkar Group strata and strike to the Palisades faults and the relative timing of GRABENS AND HALF GRABENS northwest, indicating that extension was movement (UÐduring Unkar deposition, northeast directed. This geometry is consistent CÐduring Chuar and/or Sixtymile deposi- Several observations suggest that the main with data obtained from abundant 1.1 Ga di- tion, and LÐLaramide movement). tilting and normal faulting of the Unkar Group abase sills and coeval northwest-striking dikes took place before deposition of the Nankow- elsewhere in the Southwest, which imply sub- eap Formation. (1) Mutually crosscutting re- horizontal northwest-directed contraction and as, north of the fault, Unkar Group strata (low- lationships between normal faults and diabase subhorizontal northeast-directed extension est member of the Dox Formation) are sub- sills and dikes (Sears, 1973) suggest that ma®c (Howard, 1991). horizontal beneath Tapeats Sandstone (Fig. 4). magmatism at ca. 1.1 Ga overlapped in time A key fault for deciphering extension be- This difference in dip implies that the Pali- with extensional faulting in the Unkar Group. fore deposition of the Chuar Group is the Pal- sades fault tilted and juxtaposed Cardenas La- (2) In the Tanner graben (Fig. 4), several nor- isades fault (Fig. 4). The fault strikes 310Њ and vas against lower Dox Formation. In contrast, mal faults that were initiated during Unkar de- dips steeply to the southwest. Proterozoic Chuar Group strata are not affected by the Pal- position die out up-section in the Nankoweap stratigraphic separation across this structure is isades fault; rather, Chuar Group strata and the Formation, and, near the Palisades fault, intra- ϳ1100 m down-to-the-southwest after ϳ300 Chuar syncline trend north across the trace of formational faults die out in the Cardenas La- m of Laramide reverse slip is restored (Fig. the Palisades fault. The continuity of Butte vas. (3) There is a 3Њ±5Њ angular unconformity 8). Close examination of the intersection be- fault hanging-wall strata and discontinuity of between the Unkar Group and Nankoweap tween the Palisades fault and the Butte fault Butte fault footwall strata across the Palisades Formation (Fig. 7). Although this episode of suggest that the Butte fault truncates the Pal- fault indicate that the Palisades fault is trun- faulting has largely been dismissed as minor, isades fault (Fig. 4). South of the Palisades cated by the Butte fault (Fig. 8) and that fault- we associate it with activity on the northwest- fault, the Unkar Group (uppermost Dox For- ing and tilting of Unkar Group rocks predate striking normal faults that control the outcrop mation and Cardenas Lavas) dips 10Њ±15ЊNE Butte fault movement and Chuar Group pattern of the Unkar Group. We interpret this beneath horizontal Tapeats Sandstone where- deposition.

Geological Society of America Bulletin, February 2001 169 TIMMONS et al.

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

NANKOWEAP EXTENSION: an exposed strike length of ϳ18 km (Fig. 4). MULTIPLE UNCONFORMITIES? It records the largest displacement and longest Figure 10. Intraformational fault in the history of tectonism of any extensional fault Carbon Canyon Member in Nankoweap Canyon documenting synsedimentary nor- Evidence for extensional deformation is re- in the Grand Canyon. Proterozoic stratigraphic mal faulting. View is to the southeast. Note corded by unconformities and intraformational separation across the Butte fault is as much as that displacement decreases up-section to faults in the Nankoweap Formation. Elston 1800 m down-to-the-west just north of the zero. and Scott (1973) and Elston (1993) reported a Palisades fault (Fig. 8) (after restoring 300 m major unconformity within the Nankoweap of Laramide west-side-up movement). This Formation and suggested that faulting and ero- maximum Proterozoic stratigraphic separation sion preceded deposition of the upper member combines the effects of both Butte fault and dips to the north and has north-side-down of that formation. Intraformational normal Palisades fault normal slip. sense of movement. Our mapping suggests faults within the Tanner graben are truncated Our interpretation of the fault relationships that this fault and related northwest-striking by strata of the upper Nankoweap Formation in the Palisades fault and Butte fault area is structures accommodated differential exten- and Chuar Group (Fig. 9), suggesting exten- that the Butte fault crosscuts a system of sion along the Butte fault, caused the doubly plunging nature of the Chuar syncline, and sion during early phases of Nankoweap de- northwest-striking normal faults and related were likely inherited from an older northwest- position. Adjacent to major faults, such as the tilted blocks of Unkar Group strata (Fig. 8). one in Basalt Canyon, sedimentary beds pinch striking fault system that was initiated at the This interpretation is supported by character- out against the fault. Extension recorded with- time of Unkar deposition (Fig. 8). istic northwest-stepping jogs along the trace in lower Nankoweap strata may be a contin- Numerous subordinate normal faults in the of the Butte fault. These are found where fault uation of Unkar-related extension, as support- Chuar Group are consistent with one main slivers of Unkar Group strata (upper Dox For- ed by the similarity between red beds in the population of conjugate faults striking parallel mation and Cardenas Lavas) are present along Dox Formation, intra¯ow red beds in the Car- to the Butte fault. Proterozoic faults (over- the fault in Lava Chuar, Sixtymile, and Nan- denas Lavas, and red beds of the lower Nan- lapped by Cambrian strata) invariably have koweap Canyons. We propose that these koweap Formation (Elston, 1979). The dura- normal-sense stratigraphic separation and pre- tion and tectonic importance of the northwestward de¯ections of the Butte fault dominantly dip slip. For example, in Nankow- disconformities below, within, and at the top follow buried older faults like the Palisades eap Canyon, the Butte fault splits into multi- of the Nankoweap Formation remain poorly fault. This overprinting may also explain the ple segments, all overlain by Cambrian strata understood. variable plunge of the Chuar syncline (Fig. 4). (Fig. 4). Stratigraphic separation across indi- Arches along the syncline axis coincide with vidual faults increases to the west. This step- CHUAR GROUP EXTENSION AND the projection of the northwest-striking faults. ping of the fault to the west can be traced BUTTE FAULT (Ca. 800±740 Ma): Syncline troughs where Sixtymile Formation south to Sixtymile and Lava Chuar Canyons NORTH-TRENDING GROWTH FAULT strata are preserved are located over the hang- where remnants of the earlier Unkar tilt blocks AND GROWTH SYNCLINE ing wall of hypothesized preexisting half gra- are preserved in fault slivers near a major bens (Fig. 8). northwest-trending segment of the Butte fault The Butte fault is a normal fault striking One fault in the Chuar Group strikes at a (Fig. 8). Subordinate faults, which in some north-northwest and dipping 60Њ±70ЊW that high angle to the Butte fault (Fig. 4). This cases are intraformational (Fig. 10), document truncates the east side of the Chuar Group for fault, in northernmost Lava Chuar Canyon, synsedimentary movement.

170 Geological Society of America Bulletin, February 2001 PROTEROZOIC MULTISTAGE EXTENSION RECORDED IN THE GRAND CANYON SUPERGROUP

Figure 11. East-west cross sections and measured sections of the Carbon Butte Member of the Kwagunt Formation in (A) Lava Chuar Canyon and (B) Kwagunt Canyon. Measured section locations shown in Figure 4. The Chuar syncline is asymmetric, with an axial plane 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 in cross section B), partial measured sections in the Carbon Canyon Member (shown in detail in Fig. 14), and intraformational faults in the Carbon Canyon Member and Carbon Butte Member. Reactivation of the Butte fault during Laramide contraction inverted the Chuar basin and folded Paleozoic strata into the east-facing East Kaibab monocline, shown with its present geometry.

Geological Society of America Bulletin, February 2001 171 TIMMONS et al.

The Chuar syncline is an open asymmetric fold de®ned by Chuar Group strata just west of and parallel to the Butte fault (Fig. 4). On the east limb, adjacent to the fault, bedding dips steeply to the west (as steep as 75Њ); on the west limb, bedding dips shallowly to mod- erately to the east (Fig. 11, A and B). The axial plane of the syncline dips 60Њ±70ЊE. The hinge line is doubly plunging along the trace of the Butte fault (Fig. 4). Parallelism between the trace of the syncline's axial plane and the Butte fault argues for principally dip-slip movement, in agreement with slickenlines, and for a genetic link between displacement along the Butte fault and syncline develop- ment. The syncline seems to mark a change in structural style up-section above the Tanner graben (Fig. 4). The lowest unit of the Chuar Group (Tanner Member dolomite), although not obviously folded by the syncline, was truncated by one strand of the Butte fault in Nankoweap Canyon prior to burial of the fault by overlying mudstone. Hence, we infer that the Butte fault was active during deposition of the Tanner Member (Fig. 12A). The Tanner Member and Nankoweap Formation are ex- posed in fault slivers east of the main Butte fault and Chuar syncline, whereas all overly- ing members of the Chuar Group are exposed solely in the hanging wall of the Butte fault and 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 upper strata. Field observations and mapping show that the syncline tightens with structural depth, such that lower sedimentary horizons have steeper dips than upper horizons; the tightest segments of the Chuar syncline coin- cide with syncline troughs, and the most open Figure 12. Schematic time slices illustrating the growth history of the Chuar syncline at parts of the syncline match syncline arches. key intervals. (A) Truncation of the Tanner Member dolomite against a subordinate nor- The steepest beds dip parallel to the fault, con- mal fault that is overlapped by the Tanner Member shale, indicating that extensional sistent with drag on a fault. Together, these faulting was synchronous with Tanner Member deposition. (B) Growth nature of the observations suggest that the Chuar syncline Chuar syncline during deposition of the Carbon Butte Member; note the postulated de- has a growth history. None of the synclinal pocenter in the syncline and thinning of units over the footwall of a ``blind'' Butte fault. features can be observed in the overlying Pa- (C) Our interpretation of syncline and fault relationships during deposition of the Sixtym- leozoic cover, indicating that the syncline is ile Formation. Note the pinch-out of dolomite facies in the Walcott Member, the apparent Neoproterozoic in age and clearly unrelated to truncation of the Chuar syncline by the Sixtymile Formation, and the tightening of the reverse-sense Laramide reactivation of the syncline at depth. (D) The present fault and syncline geometry in Lava Chuar Canyon. Butte fault (Fig. 11, A and B). The geometry and style of deformation pre- served in Chuar Group rocks and Butte fault a ®ne-grained sedimentary package in a is similar to the Chuar syncline. Each has an resemble other rift basins. The El Qaa syn- growth syncline developing in the hanging axis Ͻ1 km from the fault plane, and each has cline of the Miocene Gulf of Suez rift was wall of an extensional fault-propagation fold comparable stratigraphic accommodation and interpreted as a growth syncline that devel- where the depocenter corresponds to the axis sedimentary distribution (discussed subse- oped adjacent to, and in the hanging wall of, of the syncline. Phase two is marked by sur- quently). Normal-fault and growth-syncline a blind normal fault (Gawthorpe et al., 1997). face faulting and deposition of coarser detritus geometries similar to the Chuar syncline also Two distinct phases of structural development shed from an exposed scarp and footwall as have been produced in sandbox and clay-mod- are preserved in the El Qaa syncline. The ®rst well as a shift of the depocenter toward the el experiments (Withjack et al., 1990; Hardy phase is marked by syntectonic deposition of fault scarp. The scale of this modern analogue and McClay, 1999).

172 Geological Society of America Bulletin, February 2001 PROTEROZOIC MULTISTAGE EXTENSION RECORDED IN THE GRAND CANYON SUPERGROUP

SEDIMENTOLOGIC EVIDENCE FOR SYNCHRONEITY OF CHUAR SEDIMENTATION, FAULTING, AND SYNCLINAL DEVELOPMENT

Measured sections from the middle to upper Chuar Group show thickness variations across the syncline and across subordinate faults in- dicating that sedimentation was synchronous with normal faulting. Laterally continuous sandstone and carbonate marker beds allow correlation of sections within the ®eld area. Some marker beds and (especially) interven- ing mudstone horizons markedly change thickness, with relatively thin sequences on the east limb of the syncline and thicker se- quences in the syncline-axis region and on the west limb. Sections were measured in the Carbon Butte Member of the Kwagunt Formation and the upper Carbon Canyon and Duppa Mem- bers of the Galeros Formation. East-west cross sections in Lava Chuar Canyon and Kwagunt Canyon were constructed from the available mapping and measured sections (Fig. 11, A and B). In the Lava Chuar cross section (Fig. 11A), the base of each Carbon Butte measured section is the base of the lowermost sandstone, and the top of the section is the base of a continuous bed of the stromatolite Boxonia in the Awatubi Member. The Carbon Butte Member is composed of multiple, laterally continuous clastic facies ranging from medi- um-grained sandstone to variegated mudrocks. The entire member thickens ϳ35% from the east limb to the axis and thins ϳ20% from the axis to the farthest west-limb measurement (Fig. 11A). Thickness changes occur predom- inantly in the shale and mudstone units, but are also observed in the next to lowest sand- stone (thickest in section 4) and Awatubi basal stromatolite (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-axis region. In Kwagunt Canyon there is an overall westward thickening away from the Butte fault (Fig. 11B). Again, the thinnest measured Figure 13. Facies correlation between two measured partial sections in the upper Carbon sections are on the east limb, adjacent to the Canyon Member between the ``polygonal'' bed and Baicalia bed. Note lateral persistence Butte fault. There are no outcrops in the syn- of thicker dolomite beds and lateral discontinuity of other facies. More abundant and cline axis, but the basal sandstone is thickest thicker black shale units in the axis of the syncline suggest deposition concurrent with near the syncline axis (section 11) and thins syncline development. Location of sections shown in Figure 4. to 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- imentation and development of the syncline Member of the Galeros Formation (Fig. 13). stone unit across a subordinate normal fault (Fig. 12B). On the east limb of the syncline, the partial on the west limb. From these observations, we Thickness variations are also observed in section measured in Kwagunt Canyon is ϳ30 interpret the Carbon Butte Member to record strata above and below the Carbon Butte m thick; in the axial area, the same interval normal faulting (west-side-down) during sed- Member, for example, in the Carbon Canyon (measured in Carbon Canyon) is 47 m thick,

Geological Society of America Bulletin, February 2001 173 TIMMONS et al. an increase of ϳ60% (Fig. 13). Thinning of strata on the east limb of the fold is also ob- served in the Walcott Member, where dolo- mite horizons thin and pinch out toward the Butte fault (Elston, 1979; Cook, 1991; Fig. 12C). Mapping and the tightening of the syn- cline with depth suggest that the Awatubi Member, although not directly measured, also thins on the east limb (Fig. 14). Overall, the thinnest sections are on the east limb adjacent to the Butte fault, and the thickest deposits of the Carbon Canyon Member through Walcott Members roughly coincide with the syncline- axis region. The combined evidence has two important implications. First, slip across the Butte fault, syncline development, and sedimentation Figure 14. Photograph of the Chuar syncline looking north toward Nankoweap Butte. were temporally and genetically linked. Sec- Sandstone of the Carbon Butte Member, at right, dips steeply to the west, and the Six- ond, the truncation of ®ne-grained sedimen- tymile Formation caps Nankoweap Butte. Note that the synclinal axial plane dips to the tary units by the Butte fault suggests that east (toward the Butte fault) and the syncline tightens with depth. Chuar deposits were originally continuous across the Butte fault and that sedimentation seems to have kept pace with faulting such in the subsurface to the north, importance of that signi®cant relief was not generated during the north-trending Butte fault in in¯uencing deposition and syncline development. Thus deposition, and parallelism to the developing the Butte fault is interpreted to have been a north-striking Cordilleran margin suggest a blind normal fault during Chuar deposition, likely north-south orientation for the Chuar compatible with fault-propagation models. rift basin (Fig. 1). This orientation would be Sedimentary structures and soft-sediment analogous to intracratonic rift basins located a deformation features also are consistent with similar distance (200 km) ``inboard'' of the synextensional sedimentation. Symmetrical miogeoclinal hinge during Mesozoic rifting of ripple crests in sandstone beds of the Carbon eastern North America. Butte Member and Carbon Canyon Member (Fig. 15) trend subparallel to the Butte fault. SIXTYMILE FORMATION AND We interpret the ripples to be wave-ripple sets EXTENSION FOLLOWING CHUAR that developed parallel to shorelines, a situa- DEPOSITION tion that suggests that shorelines paralleled the Figure 15. Rose diagram of trends of sym- Butte fault. Sandstone beds of the Carbon The record of extension following Chuar metrical ripple crests from sandstones of Butte Member contain complex, high-ampli- Group deposition has long been recognized the Carbon Canyon and Carbon Butte tude, water-escape structures. These include and is often cited as the principal extensional Members. Parallelism between ripple centimeter-scale ¯ame-and-pillar structures as event in Grand Canyon Supergroup history. trends and the Butte fault suggest that the well as meter-scale ¯uid-escape pipes (Fig. Our interpretation of a signi®cant extensional shorelines paralleled the fault. 16A). These disturbed beds are laterally wide- history recorded by the Chuar Group warrants spread and, in one case, show downslope a reevaluation of the tectonic history recorded movement of material toward the axis of the by the Sixtymile Formation. or the underlying Walcott Member (Ford and syncline (Fig. 16B). Although water-escape The Sixtymile Formation is preserved only Breed, 1973; Cook, 1991; this study). In Six- structures alone do not indicate a tectonic sig- in four small areas in the axis of the Chuar tymile Canyon, the ``lower member'' occupies nature within the sediment, the presence of ac- syncline (Fig. 4). The top of the formation is approximately the same position as the 742 tive (penecontemporaneous) faults and the unconformably overlain by the Middle Cam- Ma ash at Nankoweap Butte. We agree with proximity to the Butte fault both suggest a co- brian Tapeats sandstone, and thus the original Elston (1979) that this member displays evi- seismic origin for these deformation features. thickness is unknown. It has a maximum pre- dence for slumping and sliding of large car- Consequently, we infer that the Neoproter- served thickness of ϳ60 m. Elston (1979) pro- bonate blocks into black shale, boudinage of ozoic Butte fault was an intrabasinal synse- posed three members within the Sixtymile a Ͼ2-m-thick dolomite horizon, and accom- dimentary normal fault and that the Chuar Formation in Sixtymile Canyon. The upper panying chaotic deformation. At this same ho- syncline was a growth syncline in the hanging two members are preserved in all four out- rizon, a large block of dolomite, interpreted to wall of a propagating normal fault. It is im- crops; the lower member of Elston (1979) is be an intraclast, appears to be ``tumbled'' (Els- portant to emphasize that the overall shape of exposed only in Sixtymile Canyon (Fig. 17). ton, 1979), and bedding in the clast is at high the Chuar basin is unknown, because the only There are questions concerning whether angle to regional bedding (Fig. 17). Elston known outcrops are in the Grand Canyon. Elston's lower member should be grouped (1979) interpreted the boudinaged upper do- However, the presence of Chuar Group strata with the Sixtymile Formation (Elston, 1979) lomite and the ``tumbled'' block to have been

174 Geological Society of America Bulletin, February 2001 PROTEROZOIC MULTISTAGE EXTENSION RECORDED IN THE GRAND CANYON SUPERGROUP

Figure 16. (A) Water-escape structures can be large in scale and are widespread in sandstone beds of the Carbon Butte Member. The proximity of these deformational structures to the Butte fault and associated intraformational faults suggests that they may be seismic in origin. (B) At the top of the outcrop, ®ne-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 to the Butte fault and Chuar syncline. Both the east section from Nankoweap Butte (E) and the section from Sixtymile Canyon were measured 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- westward off of the inferred Butte fault scarp. likely have been present on the scarp to slide cott Member that crops out ϳ60 m lower in This scenario seems unlikely for two reasons. (Fig. 17). Second, the upper dolomite exhibits the section (Figs. 3 and 17). He suggested that First, the doublet pinches out to zero thickness karst features such as cavities in®lled with this dolomite doublet was unroofed, then slid ϳ500 m west of the fault and thus would not sandstone and is unlike the dolomite doublet

Geological Society of America Bulletin, February 2001 175 TIMMONS et al. in the Walcott (Cook, 1991). Thus the mag- nitude and direction of sliding of blocks in Elston's lower member remain unknown. The middle member of the Sixtymile For- mation (siltstone member) is a thick sequence of white to red, laminated, and thinly bedded siltstone, with laminations of several milli- meters to several centimeters. The beds are ir- regularly disrupted by intraformational brec- ciation and disharmonic folding that probably developed soon after deposition, yet after lith- i®cation of coarser-grained siliciclastic mate- rial. In particular, the base of this member in Awatubi Canyon is marked by decimeter-scale deÂcollement folds and nappe folds that suggest continued sliding and slumping. At Nankow- eap Butte, the base of the middle member is marked by a 1-m-thick silici®ed layer that contains ovoid concretions at the base, inter- nal brecciation, and perhaps a scoured top. This layer is overlain by a succession of silic- i®ed marker layers that may represent tephra deposits, silici®ed carbonates, regoliths, or zones of hydrothermal alteration (Fig. 17). The upper member (channel-®ll member) contains channels ®lled with intraformational breccia derived from the middle member, then by red ¯uvial sandstones. At Nankoweap Butte and Sixtymile Canyon, the channels are ϳ15 m deep, have steep (to undercut) channel walls, and trend 340Њ, subparallel to the Butte Figure 18. Lower-hemisphere equal-area projection of minor faults in Paleozoic and Pro- fault. Strati®cation becomes more ordered up- terozoic strata, interpreted as Laramide in age; black dots represent poles to fault planes; section in the channels (Fig. 17) and gives great-circle ®ts were picked by eye after 1% contouring of fault-plane data (contouring way to interbedded, intact, red sandstone and not shown). Contour shading shows distribution of slickenline data. (A) Conjugate thrusts breccia. Both trough cross-strati®cation and only measured in Paleozoic rocks) suggest that ␴1 is trending toward 042؇. (B) Conjugate) clast imbrication indicate ¯ow toward 340Њ, high-angle reverse faults appear to reactivate the Butte fault system, and slickenlines parallel to the channel axis and the Butte fault. suggest oblique slip with small dextral component. (C) Strike-slip faults accommodate All clasts appear to be derived from siltstone oblique convergence on an older Butte fault. (D) Schematic drawing of mean fault ori- and chert of the underlying middle member, entations; inferred ␴1 stress direction during Laramide contraction comes from thrust an observation that is contrary to the interpre- conjugates shown in A. tations of Elston (1979), who reported exotic clasts. Some clasts are rounded, most are an- gular, and a few are meter-scale fragments curred. An unknown thickness of both Chuar sion in western North America associated with from the channel walls. Both the rounding of Group and Sixtymile Formation was removed the breakup of the western Laurentian margin, clasts and the steep channel walls imply some from the footwall of the Butte fault such that with accompanied change from marine to ter- lithi®cation of the middle member before Cambrian rocks now rest on the lower Dox restrial deposition, relative base-level drop, channel incision such that the contact between Formation of the Unkar Group. and incision of canyons. It is interesting to middle and upper members is an unconformity Overall, we agree with Elston (1979) and speculate whether the record of base-level of unknown hiatus. These channels and chan- Elston and McKee (1982) that the Sixtymile drop is a consequence of regional (perhaps nel ®ll may re¯ect localized tectonism along Formation records a dramatic change in the global) Neoproterozoic glaciations and cli- the Butte fault scarp and hence emergence of character of deposition from the underlying mate change. However, no glacial deposits the fault. However, they may also represent Chuar Group, and we concur that these rocks have been recognized in the Chuar Group or more regional effects such as a period of base- may record one period of slip across the Butte Sixtymile Formation. level drop and channel cutting comparable to fault. The dramatic change in sedimentary other Neoproterozoic sections within the Cor- character may re¯ect the propagation of the LARAMIDE CONTRACTION AND dillera (Christie-Blick, 1997) and worldwide fault to the surface and development of a REACTIVATION OF PROTEROZOIC (Eyles, 1993; Hoffman et al., 1998). Butte fault scarp. In addition, deposits may re- EXTENSIONAL FAULTS Slip on the Butte fault after deposition of cord a sizable base-level drop as suggested by the Sixtymile Formation and prior to deposi- incised channels of the upper member. The To address the original extent of normal- tion of the Tapeats Sandstone likely also oc- Sixtymile Formation records continued exten- fault systems active during both Unkar and

176 Geological Society of America Bulletin, February 2001 PROTEROZOIC MULTISTAGE EXTENSION RECORDED IN THE GRAND CANYON SUPERGROUP

comparative study of these structures suggests that they are all mutually crosscutting and are inferred to record Laramide shortening, tec- tonic reversal of movement along the Butte fault, and formation of the East Kaibab mono- cline (Huntoon, 1971; Fig. 18). The conjugate low-angle thrust faults are compatible with Anderson's theory for neo- formed thrusts during horizontal compression, with northeast-directed horizontal shortening

(inferred ␴1 orientation of 042Њ; Fig. 18A),

subvertical ␴3, and horizontal ␴2 trending to 315Њ. The local shortening direction deter- mined from these faults is broadly consistent with that obtained by previous workers who reported a more east-northeast±oriented short- ening direction of ϳ065Њ (Reches, 1978). The high-angle reverse faults have moderate to steep dips and form conjugate sets that have strikes subparallel to the Butte fault and do not conform to simple faulting theory (Fig. 18B). Slickenlines on fault planes in Paleozoic strata suggest that movement was primarily dip slip and reverse. The structures suggest that there must have been preexisting zones of weakness (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 localized stresses around a reactivated fault. The con- jugate strike-slip faults are subvertical, have subhorizontal slickenlines, and strike 064Њ and 025Њ (smaller data set, thus dif®cult to dis- criminate on the stereonet), oblique to the Butte fault (Fig. 18C). Movement sense on the main population (064Њ) is uncertain, because kinematic indicators of relative movement sense are rare. Dextral strike-slip movement Figure 19. Index map of ca. 1300±1000 Ma sedimentary rocks and inferred late Meso- has been observed for faults striking 025Њ. The proterozoic structures. Outcrops and known subcrops of Late Mesoproterozoic sedimen- inferred ␴1 orientation from the conjugate tary rocks are shown in black (AÐApache Group, LAÐLas Animas Formation, PÐ strike-slip faults is ϳ046Њ, subparallel to re- Lower Pahrump Group, and UÐUnkar Group). The Mesoproterozoic northwest-trending sults from the thrust conjugates. The faults are grain is continental in scale and suggests a period of continental extension during syn- interpreted to accommodate oblique shorten- chronous Grenville contraction. Northwest-trending structural elements include CÐCen- ing across the Butte fault. tral Basin platform, CCÐCircle Cliffs, MoÐMogollon Rim, Texas-Walker Lane, Mojave- From this analysis, we infer that the Lar- Sonora, Uncompahgre uplift, and Wind River Range. Inset shows a proposed plate amide reactivation of the Butte fault and for- reconstruction (after Brook®eld, 1993; Karlstrom et al., 1999, and Burrett and Berry, mation of the East Kaibab monocline record 2000) and late Mesoproterozoic basins (shaded) and tectonic elements; ma®c dike ages are northeast-directed horizontal shortening, with in billions of years(LÐLaurentia, E. AntÐEast Antarctica, BÐBaltica, Aus-sÐAustralia a dominant dip-slip component and a minor [SWEAT], and Aus-aÐAustralia [AUSWUS]). component of dextral strike-slip (Fig. 18D). Because the Butte fault was a preexisting structure, the orientation of the Butte fault Chuar deposition, we need to understand how main families of faults, all interpreted to have with respect to the regional shortening direc- these faults were reactivated by contraction in formed during Laramide contraction: (1) low- tion played a strong role in how deformation the Laramide orogeny (ca. 70±50 Ma). Nu- angle conjugate thrust faults, (2) high-angle was accommodated across the structure. merous minor faults have been measured in conjugate reverse faults parallel to the Butte Northwest-striking segments of the Butte fault both Precambrian and Paleozoic strata. fault, and (3) steeply dipping strike-slip faults show nearly pure dip slip; north-trending seg- Among these structures we observe three that strike 064Њ, oblique to the Butte fault. A ments are oblique to the shortening direction

Geological Society of America Bulletin, February 2001 177 TIMMONS et al. and have more subsidiary strike-slip faults. Plateau and surrounding region to a general a prolonged history of multistage extension in North of the Grand Canyon and in Utah, the fabric that was established in the late Meso- western North America in the late East Kaibab monocline bends to the northeast proterozoic to Neoproterozoic (Marshak and Precambrian. and records a still larger component of oblique Paulsen, 1996; Karlstrom and Humphreys, This model may help resolve the long- dextral shear during shortening (Tindall and 1998), but the data for different ages of north- standing controversy regarding timing of rift- Davis, 1999). west- (1.1 Ga) versus north-trending faults ing in western Laurentia. Mesoproterozoic and (800±742 Ma) presented in this paper suggest Neoproterozoic basins both record continen- INFERRED REGIONAL EXTENT OF that each structural grain represents different tal-scale extension, with important ``events'' PROTEROZOIC FAULT SYSTEMS Proterozoic extensional events. at ca. 1.1 Ga and 0.80±0.74 Ga. In the Grand Canyon, the ca. 800±742 Ma Chuar Group re- Because Laramide contraction in the south- DISCUSSION OF REGIONAL cords east-west extension that is broadly syn- western United States clearly reactivated late TECTONIC IMPLICATIONS chronous with other previously identi®ed ex- Mesoproterozoic and Neoproterozoic normal tensional basins along the Cordillera: the 770 faults (Huntoon, 1971; Sears, 1973; Huntoon Tectonic models for the late Mesoprotero- Ma Little Dal Group (Young et al., 1979; Els- et al., 1996), it may be possible to infer extent zoic and Neoproterozoic structural and sedi- ton and McKee, 1982); the Ͼ723 Ma Rapitan and geometry of Proterozoic fault systems mentary history of western North America re- Group (Link et al. 1993); and the pre±syn- from the distribution of Laramide monoclines main controversial. Harrison et al. (1974) Sturtian Kingston Peak Formation (Prave, and reverse faults in the Colorado Plateau re- postulated that sedimentary rocks of the Belt 1999). The record of ca. 750 Ma extension gion. Common structural grains, observed on basin, Uinta basin, and the Grand Canyon± observed in sedimentary and igneous rocks all and adjacent to the Colorado Plateau, include Apache±Pahrump basin were deposited in au- along the western Cordillera (Ross et al., northwest-trending Phanerozoic lineaments lacogens developed at high angles to the Cor- 1989; Ross, 1991) and Grand Canyon (this and Laramide monoclines (Fig. 19) and north- dilleran miogeocline, the so-called ``Udder study) documents a major, continental-scale trending Laramide monoclines and uplifts Hypothesis.'' However, new U-Pb geochro- rifting event (thousands of kilometers in (Fig. 1). Laramide monoclines and uplifts nology indicates that the lower Belt Super- length and hundreds of kilometers in width), (e.g., Uncompahgre, Circle Cliffs, and perhaps group accumulated between 1470 and 1430 presumably related to the separation of west- Wind River), as well as some Phanerozoic lin- Ma and the upper by 1370 Ma (Link et al., ern continents from western North America. eaments like the Mojave-Sonora, Texas-Walk- 1993; Aleinikoff, 1996). The Unkar Group Australia may have been part of this west- er Lane, and Mogollon Rim, have similar (and parts of the Apache Group) are 1.2±1.1 ern continent (Karlstrom et al., 1999), and pa- trends and dips to known faults and dikes Ga (Elston and McKee, 1982; Elston, 1993; leomagnetic data for North America and Aus- whose age matches that of the Unkar Group. this study), and the Chuar Group (and Uinta tralia suggest that by 755 Ma, the two In fact, the strong northwest trend observed Mountain Supergroup?) are Neoproterozoic continents were separated (Wingate and Gid- over much of Laurentia (Grand Canyon, Cen- (800±742 Ma; Link et al., 1993; this study). dings, 2000). Alternative rift models that pro- tral Basin platform, Sudbury dikes, and Mac- Thus, it is clear that these different Mesopro- pose younger rifting, based on subsidence kenzie dikes; cf. Fahrig and West, 1986; Fig. terozoic and Neoproterozoic basins were not models for rifting (Bond and Kominz, 1984; 19 inset) apparently records regional northeast deposited synchronously in aulacogens (Els- Levy and Christie-Blick, 1991), rely heavily extension, which overlaps in time with Gren- ton, 1993). There is no reason to believe that on the Paleozoic record and may not be sen- villian northwest-directed contraction. We sedimentation spanned hundreds of millions sitive to a complex rift history. Complexities, suggest that the northwest-trending structural of years in each basin. Rather, sedimentary se- such as multiple rift episodes or an asymmet- grain was ®rst established in the late Meso- quences record local punctuated depositional ric rift geometry, could conceivably disturb proterozoic during the late stages of Grenville and extensional episodes. the thermal subsidence history of the Cordil- orogenesis and records a period of continen- Our interpretation of the mechanisms driv- leran margin. Thus, a single rift event along tal-scale northeast-directed extension. ing extensional deformation and basin for- the western margin of North America is too The other common structural grain on the mation are as follows. Incipient rifting of the simplistic, and emerging models for multiple Colorado Plateau and surrounding region is a continental interior was accompanied by the rift episodes seem to better describe the Neo- north trend, similar to that of the Neoproter- 1.1 Ga ma®c magmatic event, perhaps facili- proterozoic extensional history of the ozoic Butte fault (Fig. 1). It is interesting that, tated by mantle insulation by a stable crust Cordillera. like the East Kaibab monocline (Fig. 1), many (Hoffman, 1989), but also related to far-®eld Understanding the multiphase extensional monoclines step from northwest- to north- stresses and intracratonic rifting (1130±1080 history, and importance of inherited structural trending segments, suggesting a linked net- Ma in the Midcontinent). Rift basins and as- grain, may ultimately help to re®ne models for work of reactivated faults (e.g., De®ance uplift sociated magmatism seem to overlap in time the positioning of Laurentia's Proterozoic con- and Grand Hogback). Other north-trending with late stages of Grenville contraction and tinental neighbors by identifying structural structural features include the Front Range of may be kinematically linked to Grenville oro- grains that were preferred during rifting versus Colorado, the eastern and western edges of the genesis (Lambeck, 1983; DeRito et al., 1983; those that were transforms (Fig. 1). Brook®eld Colorado Plateau, and the Rio Grande rift. We Karlstrom and Humphreys, 1998; Fig 19). (1993) proposed that northwest- and north- to suggest that the north-trending structural grain Neoproterozoic (age of Chuar deposition) ex- northeast-trending segments (reconstructed observed on the Colorado Plateau and sur- tension apparently re¯ects east-west extension coordinates) on both Laurentia and Australia rounding region was established during rifting during breakup of the supercontinent Rodinia were once a continuous rift-transform system of western Laurentia in the Neoproterozoic. and initiation of the Cordilleran miogeocline during rifting of Rodinia. The northwest- Previous workers have also attributed the along the western margin of North America trending segments, such as the Koonenberry structural grains observed on the Colorado (Fig. 1). Thus, emerging models point toward fault zone in Australia (Burrett and Berry,

178 Geological Society of America Bulletin, February 2001 PROTEROZOIC MULTISTAGE EXTENSION RECORDED IN THE GRAND CANYON SUPERGROUP

2000) and the Mojave-Sonora and Texas- dicating that the syncline is Neoproterozoic views of the manuscript. Maya Elrick, Laurie Cros- Walker Lane lineaments in southern Laurentia, and unrelated to the reverse slip (300 m) along sey, Gary Smith, and Gene Humphreys helped us improve the manuscript. The following individuals may have been preexisting structures estab- the Butte fault during Laramide contraction. assisted us in the Grand Canyon: Adam Read, Mary lished during Grenvillian intracratonic rifting Thickness variations of the Chuar Group Simmons, Colin Shaw, Brad Ilg, Mike Doe, Jake and basin development. These were apparent- (especially Carbon Canyon through Walcott Armour, Casey Cook, Sarah Tindall, Arlo Weil, and ly rift segments during the initial accumula- Members) indicate synchronous deposition, Paul Bauer. tion of the Adelaidian sequences in Australia normal faulting, and syncline development. (Preiss, 1987), but may have evolved into Siliciclastic strata tend to be thicker in the REFERENCES CITED transforms at later stages of a complex pro- synclinal-axis region and thinner on the east gressive rifting (Brook®eld, 1993; Karlstrom limb proximal to the fault. Abrupt thickness Aleinikoff, J.N., 1996, SHRIMP U-Pb ages of felsic igne- et al., 1999; Burrett and Berry, 2000). Grand and facies changes are also observed across ous rocks, Belt Supergroup, western Montana: Geo- Canyon Supergroup data suggest that, by 740 subordinate normal faults within the Butte logical Society of America Abstracts with Programs, v. 28, no. 7, p. 376. Ma, extension was oriented east-west in west- fault system. Sedimentary and soft-sediment Bond, G.C., 1997, New constraints on Rodinia break-up ern Laurentia, with probable initiation of a deformational structures in the Chuar Group ages from revised tectonic subsidence curves: Geo- logical Society of America Abstracts with Programs, north-trending Cordilleran miogeocline by this are also consistent with synchronous deposi- v. 29, no. 6, p. 280. time. tion, faulting, and syncline development. Con- Bond, G.C., and Kominz, M.A., 1984, Construction of tec- volute bedding, bedding-parallel slip surfaces, tonic subsidence curves for the early Paleozoic mio- geocline, southern Canadian Rocky Mountains: Im- SUMMARY pinch-out of units toward the fault, and intra- plications for subsidence mechanisms, age of formational breccia collectively indicate that break-up, and crustal thinning: Geological Society of Strata of the Grand Canyon Supergroup re- Butte fault movement and syncline develop- America Bulletin, v. 95, p. 155±173. Brook®eld, M.E., 1993, Neoproterozoic Laurentia-Australia cord at least two distinct episodes of intracra- ment continued during deposition of the upper ®t: Geology, v. 21, p. 683±686. tonic rifting and basin formation in south- Chuar and Sixtymile strata. Thus, rather than Burch®el, B.C., Cowan, D.S., and Davis, G.A., 1992, Tec- tonic overview of the Cordilleran orogen in the West- western North America, one related to the 1.1 a Grand Canyon ``disturbance'' after Chuar ern United States, in Burch®el, B.C., Lipman, P.W., Ga Grenville collision and the second to the Group deposition, we interpret extension to and Zoback, M.L., eds., The Cordilleran orogen: Con- ca. 850±742 Ma incipient rifting of Rodinia. also have been concurrent with Chuar Group terminous U.S.: Boulder, Colorado, Geological Soci- ety of America, Geology of North America, v. G-3, The ®rst event is recorded by sedimentary and deposition (ca. 800±742 Ma). p. 407±479. igneous rocks of the Mesoproterozoic Unkar Laramide contractional faulting locally re- Burrett, C., and Berry, R., 2000, Proterozoic Australia± Group (1200±1100 Ma). These rocks are ex- activated both Mesoproterozoic and Neopro- Western United States (AUSWUS) ®t between Lau- rentia and Australia: Geology, v. 28, p. 103±106. posed in tilt blocks and grabens that record terozoic structures. The East Kaibab mono- Christie-Blick, N., 1997, Neoproterozoic sedimentation and northeast-directed extension across northwest- cline in the Grand Canyon mainly follows the tectonics in west-central Utah: Brigham Young Uni- versity Geology Studies, v. 42, part 1, 30 p. striking normal faults prior to deposition of north-trending Neoproterozoic Butte fault, ex- Cook, D.A., 1991, Sedimentology and shale petrology of the unconformity-bounded Nankoweap For- cept where it follows older Mesoproterozoic the upper Proterozoic Walcott Member, Kwagunt For- mation with an inferred age of 900 Ma. This northwest-trending structures such as the Pal- mation, Chuar Group, Grand Canyon, Arizona [M.S. thesis]: Flagstaff, Northern Arizona University, 128 p. Mesoproterozoic extension is marked by a isades fault. Many monoclines have north- DeRito, R.F., Cozzarelli, F.A., and Hodge, D.S., 1983, Ͻ5Њ angular unconformity between the Unkar and northwest-trending segments, and we Mechanism of subsidence of ancient cratonic rift ba- Group and overlying Nankoweap Formation speculate that many of the monoclines of the sins: Tectonophysics, v. 94, p. 141±168. Elston, D.P., 1979, Late Precambrian Sixtymile Formation and Chuar Group and by intraformational Colorado Plateau and Rocky Mountains also and the orogeny at the top of the Grand Canyon Su- faults in Nankoweap strata. Extension was reactivated faults of Neoproterozoic age. Like- pergroup, northern Arizona: U.S. Geological Survey possibly active during, but de®nitely after, 1.1 wise, many northwest-trending Phanerozoic Professional Paper 1092, 20 p. Elston, D.P., 1989, Middle and Late Proterozoic Grand Can- Ga ma®c magmatism, with renewed or contin- structures, like the Mogollon Rim and Un- yon Supergroup, Arizona, in Elston, D.P., Billingsley, ued extension during Nankoweap deposition compahgre uplift, may re¯ect inheritance from G.H., and Young, R.A., eds., Geology of the Grand Canyon, northern Arizona (with Colorado River (ca. 900 Ma). Mesoproterozoic trends. Thus, new data from guides): Washington, D.C., American Geophysical The second event is recorded by the Neo- the Grand Canyon may provide a critical clue Union, p. 94±105. proterozoic Chuar Group (ca. 800±742 Ma). for piecing together two regional extensional Elston, D.P., 1993, Middle and Early-late Proterozoic Grand Canyon Supergroup, northern Arizona, in Reed, J.C., The Chuar Group records east-west extension events, at ca. 1.1 Ga and ca. 800±700 Ma, that Bickford, M.E., Houston, R.S., Link, P.K., Rankin, and formation of the north-striking, west-dip- were responsible for forming two major tec- D.W., Sims, P.K., and Van Schumus, W.R., eds., Pre- ping Butte fault that both truncates and locally tonic ``grains'' in the crust of the Western cambrian, Conterminous U.S.: Boulder, Colorado, Geological Society of America, Geology of North reactivates northwest-striking Mesoprotero- United States. America, v. C-2, p. 521±529. zoic faults. The Butte fault is the eastern limit Elston, D.P., and Gromme, C.S., 1974, Precambrian polar of Chuar Group exposures and accommodates ACKNOWLEDGMENTS wandering from Unkar Group and Nankoweap For- mation, eastern Grand Canyon, Arizona, in Karlstrom, ϳ1800 m of Proterozoic stratigraphic separa- T.N.V., Karlstrom, K.E., Swann, G.A., and Eastwood, tion. The Chuar syncline is just west of and This work was made possible by National Sci- R.L., eds., Geology of northern Arizona with notes on parallel to the trace of the Butte fault. This ence Foundation (NSF) grant EAR 9706541 to Karl archeology and paleoclimate, Part I. Regional studies: Karlstrom, John Geissman, and Maya Elrick. Fur- Geological Society of America, Rocky Mountain Sec- geometry, plus tightening of the syncline at ther thanks go to NSF for the ED-MAP grant to tion Guidebook, p. 97±117. depth and sedimentary evidence, suggests that Karlstrom for mapping in the Grand Canyon Su- Elston, D.P., and Mckee, E.H., 1982, Age and correlation pergroup. We also thank Grand Canyon National of the Late Proterozoic Grand Canyon disturbance, the syncline is a growth structure. The syn- northern Arizona: Geological Society of America Bul- cline fold axis is doubly plunging along the Park for our research agreement and sampling per- letin, v. 93, p. 681±699. mit. Further support came from Ben Donegan (con- strike of the fault, suggesting along-strike var- Elston D.P., and Scott, G.R., 1973, Paleomagnetism of sulting geologist), Conoco Inc., and Schlumberger some Precambrian basalt ¯ows and red beds, eastern iations in normal displacement across the Inc. We thank Michael Wells, Paul Link, and as- Grand Canyon, Arizona: Earth and Planetary Science fault. The Paleozoic cover is not folded, in- sociate editor John Bartley for their thoughtful re- Letters, v. 18, p. 253±265.

Geological Society of America Bulletin, February 2001 179 TIMMONS et al.

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180 Geological Society of America Bulletin, February 2001