Proterozoic Multistage (Ca. 1.1 and 0.8 Ga) Extension Recorded in The

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 .5000-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 (;2100 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. For permission to copy, contact Copyright Clearance Center at www.copyright.com q 2001 Geological Society of America 163 TIMMONS et al. 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 108±308NE toward normal faults that dip 608SW (Sears, 1990). Overlying the Unkar Group, the Nankow- eap Formation is a relatively thin (120 m) section of red sandstone, mudstone, and quartz arenite bounded by unconformities (Fig. 3). Elston (1993) also recognized a major unconformity 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 ;1600 m of tilted and gently folded unmetamorphosed sedimentary rocks exposed over ;50 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 exposures, 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 faulting and tilting event (Noble, 1914) during deposition 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 monocline 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 determinations 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 beneath Unkar Group strata yield relatively ¯at age spectra for ;90% 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.

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