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Pennsylvanian Sinistral Faults Along the Southwest Boundary of the Uncompahgre Uplift, Ancestral Rocky Mountains, Colorado

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Pennsylvanian Sinistral Faults Along the Southwest Boundary of the Uncompahgre Uplift, Ancestral Rocky Mountains, Colorado Pennsylvanian sinistral faults along the southwest boundary of the Uncompahgre uplift, Ancestral Rocky Mountains, Colorado William A. Thomas* Department of Geological Sciences, University of Kentucky, Lexington, Kentucky 40506-0053, USA ABSTRACT sylvanian-Permian Ancestral Rocky Mountains interpretations, and the conclusions provide have attracted a wide range of interpretations, testable implications for integration of regional Resolution of the large-scale kinematics of each of which ultimately depends on the time and kinematics. the Pennsylvanian-Permian Ancestral Rocky sense of slip on each of the specifi c faults within The Pennsylvanian-Permian Ancestral Rocky Mountains requires defi nition of sense of a large array of generally high-relief basement Mountains structures refl ect protracted tectonic slip and time of movement on specifi c faults fault blocks (Fig. 1) (e.g., Baars, 1966; Kluth and inheritance from Precambrian basement faults, within the system of fault-bounded basins Coney, 1981; Baars and Stevenson, 1982, 1984; as well as a history of episodic reactivation of and uplifts (e.g., Paradox basin, Uncompah- Kluth, 1986, 1998; Stevenson and Baars, 1986; Precambrian and Paleozoic faults (e.g., Baars gre uplift). Along an east-west–striking seg- Thomas and Baars, 1995; Ye et al., 1996; Mar- and See, 1968). In turn, the Ancestral Rocky ment of the boundary between the Paradox shak et al., 2000; Dickinson and Lawton, 2003). Mountains structures have been variously basin and Uncompahgre uplift, a system of Thick accumulations of locally derived, coarse overprinted by Cretaceous-Eocene Laramide steep faults defi nes a horst and graben sys- clastic sediment of Pennsylvanian-Permian age basement-rooted structures, and the relative tem, which includes the Grenadier fault provide the primary evidence for the large-scale magnitudes of Ancestral Rockies and Laramide block. Positive fl ower structures (anticlines) structure of the Ancestral Rocky Mountains. components of deformation remain in question along both the bounding (Coal Bank Pass Fault boundaries between large-scale basement along many structures. This article begins with a and Molas Creek faults) and internal (Snow- uplifts (e.g., Uncompahgre and Front Range) summary of pre-Pennsylvanian fault movements don fault) faults of the Grenadier fault block and basins (e.g., Paradox and Central Colorado) and concludes with an evaluation of Pennsylva- indicate strike-slip displacement, and oblique are mapped from scattered exposures of faults, nian-age fault movements in the regional con- subsidiary folds show sinistral slip. On both sedimentary thickness and facies distribution, text of the Ancestral Rocky Mountains and pos- sides of the Snowdon fault, stratigraphic and subsurface data (e.g., summaries in Baars, sible Laramide overprints. thinning of parts of the Pennsylvanian Her- 1966; Mallory, 1972; Rascoe and Baars, 1972; Parts of the boundary-fault system between mosa Group toward the fault documents Weimer, 1980; Baars et al., 1988). For many of the Uncompahgre uplift and the Paradox basin synsedimentary growth of the positive fl ower the faults, the dip is not well constrained, and a on the southwest are exposed in the Laramide- structure, suggesting Pennsylvanian sinistral clear defi nition of slip sense is rare. age San Juan dome in southwestern Colorado slip along this part of the Ancestral Rocky Resolution of the various large-scale tectonic (Fig. 1). In that area, a system of faults defi nes Mountains. Sinistral strike-slip along one models for the Ancestral Rocky Mountains has horst and graben blocks (the Grenadier and east-west–striking segment of the fault sys- been hampered by lack of data for the time and Sneffels fault blocks, Fig. 1) within the north- tem between the Paradox basin and Uncom- sense of slip on specifi c faults from which inter- eastern part of the Paradox basin (Baars, 1966; pahgre uplift is compatible with reverse slip pretations of an integrated regional stress fi eld Baars and See, 1968; Weimer, 1980). A Penn- on northwest-striking segments, demon- can be derived. This article reports on faults sylvanian-Permian (Hermosa Group and Cutler strating that a regional assembly of data to along the Grenadier fault block, which is within Group) succession fi lls the Paradox basin, over- defi ne sense of slip and time of movement on a system of faults at the boundary between the laps the Grenadier and Sneffels fault blocks, specifi c faults will better constrain regional- southwestern side of the Uncompahgre uplift and pinches out against the Ridgeway fault at scale models for kinematics and mechanics of and the Paradox basin in southwestern Colorado, the southwest boundary of the Uncompahgre Ancestral Rocky Mountains deformation. where the time and sense of slip can be inter- uplift. The Ridgeway fault strikes east-west for preted. The objective is similar to that of a study some distance, constituting a “kink” in the gen- Keywords: Ancestral Rocky Mountains, sinis- of faults along the boundary between the north- erally northwest-striking boundary between the tral fault, synsedimentary fault, Pennsylvanian, eastern side of the Uncompahgre uplift and the Uncompahgre uplift and Paradox basin (Fig. 1) Uncompahgre uplift. Central Colorado trough in southern Colorado, (Stevenson and Baars, 1986), and the bound- where sedimentary facies and folded angular aries of both the Sneffels and Grenadier fault INTRODUCTION unconformities document Pennsylvanian-Perm- blocks strike approximately east-west, parallel ian synsedimentary northeast-directed thrusting with the Ridgeway fault. West of the kink, the The large-scale tectonic setting and driving (Hoy and Ridgway, 2002). While these exam- boundary between the Uncompahgre uplift and mechanisms of the basement faults of the Penn- ples alone will not resolve the regional-scale Paradox basin curves to northwest strike. East of mechanics, they illustrate the kinds of data that the kink, the Sneffels and Grenadier fault blocks *[email protected]. are necessary for resolution of the alternative along with the uplift boundary bend abruptly to Geosphere; June 2007; v. 3; no. 3; p. 119–132; doi: 10.1130/GES00068.1; 10 fi gures. For permission to copy, contact [email protected] 119 © 2007 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/3/3/119/865160/i1553-040X-3-3-119.pdf by guest on 26 September 2021 Thomas Front Utah Central Colorado tro Colorado Range Uncompahgre Paradox basin u UNCOMP gh Figure 1. Index map of Ances- AHGRE UPLIFT tral Rocky Mountains, show- Gateway, CO ing locations of faults along the boundary between the Uncom- Ridgeway pahgre uplift and the Paradox fault basin (compiled from Larsen Area of and Cross, 1956; Baars and See, Figure 2 1968; Weimer, 1980; Frahme and Vaughn, 1983; Hoy and PA Ridgway, 2002). Dashed-line RADO Snef SC fels block -C circle shows location of the Laramide San Juan dome. SC- X BASI Gre C—Sand Creek–Crestone fault. nadier block Utah Colorado N New Mexico N 100 km os-Picuris fault Pec 36°N 110°W the southeast and south (Fig. 1). These struc- sedimentary cover (Figs. 1 and 2)1. The Coal granite, indicating a nonconformable sedi- tures are now exposed in Precambrian to Penn- Bank Pass and Molas Creek faults are part of a mentary cover over crystalline basement rocks sylvanian rocks in the structurally highest part roughly en echelon fault set, which includes the (Barker, 1969; Gibson, 1990); alternatively, a of the Laramide San Juan dome. The Laramide Snowdon, Andrews Lake, and Little Molas Lake zone of sheared, foliated rocks along the con- structure is framed by relatively low-angle dips faults within the Grenadier fault block (Fig. 2). tact suggests a thrust boundary (Tewksbury, in Permian and younger rocks around the west Excellent exposure permits detailed observation 1985). Cuspate infolding of the cover strata and south sides of the dome; however, Tertiary of structure and stratigraphy in the area of the into the basement provides a mechanism for rocks of the San Juan volcanic fi eld overlap Pre- east-striking fault system, supporting interpreta- thrusting along the contact between the par- cambrian rocks on the north and east sides of the tions of time and sense of fault slip. autochthonous Uncompahgre Group and the core of the dome. basement rocks (Harris et al., 1987, Fig. 3 Precambrian therein). A convex-upward geometry of the PRE-PENNSYLVANIAN TECTONIC limbs of cuspate folds is expressed in upwardly HISTORY OF THE GRENADIER FAULT The Precambrian Uncompahgre Group con- and outwardly decreasing dips, and the limbs BLOCK sists of alternate units of quartzite and phyl- are broken by strike-slip shear zones. Although lite, each with a range of thickness averaging a cuspate-infold geometry adequately explains The Grenadier fault block consists of Pre- ~200 m. The Uncompahgre Group physically the east-striking elongate outcrop area of the cambrian quartzite and phyllite (Uncompahgre overlies higher grade Precambrian gneiss and Uncompahgre Group between contacts with the Group) with a Paleozoic sedimentary cover. The older Precambrian gneiss and granite, the con- bounding faults of the Grenadier fault block, tact subsequently was displaced by steep faults the Coal Bank Pass and Molas Creek faults on 1If you are viewing the PDF of this paper, or if with substantial vertical separation, defi ning you are reading this offl ine, please visit http://dx.doi. the south and north, respectively, juxtapose the org/10.1130/GES00068.S1
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