The Laramide belt of the North American Cordillera is a thick-skinned orogen that continues to garner attention due to many unresolved ambiguities, particularly in the subsurface. Recent seismic studies provide a better understanding of Laramide tectonism at deep crustal levels. However, mechanisms for deformation accommodation in the upper crust remain unclear.
A structural/tectonic analysis of Precambrian fabrics and structural grain of basement-cored Laramide arches and uplifts in Wyoming using only previously collected data, along with a hypothesis on the potential role of these features in Laramide orogenesis, is presented. This work provides evidence for the presence of Neoarchean convergence zones dominantly directed from the SW-NE toward the Wyoming Province forming NNW anisotropies. In addition, regional compressional forces from convergence formed WNW- and NE-striking conjugate shears. Precambrian basement fabrics characterize all three directions of major anisotropy, and they likely have a complex history of deformation since the Precambrian, most recently, during the Laramide orogeny. This Precambrian deformation system was likely a fundamental tectonic control in Laramide arch/uplift formation in Wyoming.
During the Laramide orogeny, reactivation of anisotropies occurred throughout Laramide contraction, forming somewhat symmetrical, but discrete zones of transpression, displaced along a SW–NE-directed Laramide deformational front. Reverse-left oblique-slip faults developed from reactivation of WNW fabrics and, where connected, acted as relay zones, facilitating major arch development along NNW-striking faults. Internal controls for Laramide orogenesis in the upper crust are likely related to these basement anisotropies, which may link the evolution of foreland arches at deeper crustal levels to surface structures.
The archetypical Laramide belt of the North American Cordillera is a basement-involved orogen that has been heavily studied over the past 50 yr (e.g., Prucha et al., 1965; Stone, 1969; Matthews and Work, 1978; Stearns, 1978; Smithson et al., 1979; Brewer et al., 1982; Jordan et al., 1983; Blackstone, 1990a; Erslev, 2005; Yonkee and Weil, 2015). Laramide basement and structures are well exposed in Wyoming, and work has yielded extensive data related to the origin of the Laramide belt across the Wyoming Province. However, likely due to the complex polyphase deformation history of the Wyoming Province, the significance and magnitude of structural inheritance in Laramide orogenesis remain enigmatic.
Using available geologic and structural data, this study presents a structural analysis of the Wyoming portion of the Wyoming Province (study area; Fig. 1) that shows evidence for structural inheritance during Laramide contraction (Hoppin, 1961; Hoppin et al., 1965; Allison, 1986; Paylor and Yin, 1993; Ver Ploeg and Greer, 1997; Frost et al., 2000, 2006a; Stone, 2002; Erslev and Koenig, 2009; Yonkee and Weil, 2015, 2017). This work is unique in that the data set for the study consists of assimilation and synthesis of voluminous amounts of previous and pertinent research conducted by others on Laramide orogenesis, including both published and unpublished disparate data sets. I performed the work in order to more clearly understand how all these data may integrate, and herein I present the results and interpretations from synthesis and review of the integrated material.
Based on the data, a hypothesis is presented for the development of a set of basement fabrics that formed from pure shear in an Archean convergent-deformation system, resulting in the formation of regional structural grains oriented to the NNW, WNW, and NE. I also propose that reactivation of these fabrics occurred during Laramide transpression (simple shear), resulting in the major arches and uplifts of Wyoming (Fig. 1). This zone of Laramide deformation is therefore referred to as the Wyoming transpressive zone in this paper. The hypothesis is tested/supported by: (1) analyzing ages, spatial distributions, orientations, and kinematics of specific Precambrian structures (faults, shears, suture zones, supracrustal belts, and dike swarms) present in exposed rocks in Wyoming; (2) analyzing fabrics and structural grains developed from said structures; (3) comparing the results to Laramide deformational patterns; and (4) reviewing and integrating current data related to Laramide structures of Wyoming in order to develop a structural inheritance model.
The relatively small Wyoming Province is the southernmost Archean province in North America. The craton consists of a 3.6–3.0 Ga gneissic core and sparse supracrustal rocks that are intruded by 2.90–2.50 Ga potassium-rich granites, as well low-potassium tonalites and granodiorites (Frost et al., 2006a; Mueller and Frost, 2006). The gneissic and granitic rocks occur as alternating belts that are roughly semicircular in shape around the older central core, as seen on aeromagnetic maps of Sims et al. (2001) and shown here in Figure 2. These belts represent several younger magmatic and/or tectonic belts that developed as a result of the reworking of the older gneissic core (Houston, 1993). The magmatic belts and supracrustal rocks tend to be younger to the south-southwest, away from the older central core in north-central Wyoming (Fig. 2; Chamberlain et al., 2003).
The Laramide belt consists of an anastomosing network of structurally “positive” zones separated by broad elongate, sigmoidal basins (Fig. 1; Erslev, 1993). Structurally “positive” areas have historically been referred to as arches and/or uplifts. This distinction has been problematic and very inconsistent. Therefore, across central and northern Wyoming, structures with lesser topographic relief that generally strike WNW and NE are referred to as “uplifts” in this paper. Uplifts interconnect with the more topographically significant structural features that I refer to as “arches,” striking NNW.
The Laramide belt of Wyoming involves dominantly basement rock of the Wyoming Province (Fig. 1). It consists of numerous thick-skinned foreland deformations that developed within cratonic lithosphere well inland (1500 km) from the Late Cretaceous–Paleogene active continental margin, resulting from low-angle subduction of the Farallon plate ca. 75–50 Ma (Figs. 1 and 3; Dickinson and Snyder, 1978; Erslev, 1993; Yonkee and Weil, 2015). The belt shows a general NW-SE structural grain, but individual structural trends vary from WNW to NNE and may merge along strike, creating a somewhat symmetrical, curvilinear/rhombic-shaped network of deformation (Figs. 1–4). Folding of sedimentary cover ranges in trend from N-S to E-W, exhibiting varying degrees of vergence, with both right- and left-stepping en échelon zones formed locally (Fig. 3; Weil et al., 2016). Reference to “Laramide” in this paper refers to this structural style only.
The tectonic nature of Laramide arches and uplifts has been debated since the 1960s, including vertical tectonics (e.g., Prucha et al., 1965), wrench faulting (Sales, 1968; Stone, 1969), and then regional horizontal shortening/contraction with associated thrusting/reverse faulting (e.g., Smithson et al., 1979). The present general consensus is that these structures formed from WSW-ENE horizontal contraction of the foreland, resulting in significant basement arches bounded by thrust/reverse faults adjacent to the surrounding basins (e.g., Erslev and Koenig, 2009). Some major Laramide faults, such as the Piney Creek and Wind River thrusts, have dips of 30°–45° (i.e., moderate-angle reverse faults; Smithson et al., 1979; Stone, 2003), and movement on these faults has created kilometers of structural relief relative to surrounding basins (Snoke, 1993). Based on the observed configuration (attitude and slip) of major Laramide faults on the surface and in basement rocks of Wyoming (Paylor and Yin, 1993; Ver Ploeg and Greer, 1997; Molzer and Erslev, 1995; Stone, 2002; Otteman and Snoke, 2005; Weil et al., 2014, 2016), numerous oblique-slip faults (uplifts striking WNW and NE) appear to be present and are interconnected with large thrust and/or moderate-angle reverse faults (“prototypical” arches striking NNW). This interconnected pattern of deformation is present across Wyoming (Fig. 2; Erslev, 1993), and it suggests that the Laramide-style arches of Wyoming may be the product of a fundamental, multicomponent structural/tectonic system that I postulate had beginnings with initial formation during the Archean and culminated in the Laramide orogeny, as suggested by Weil et al. (2014, 2016). The late Archean (2.9–2.5 Ga) convergent history and continental growth of the Wyoming Province are discussed with specific emphasis on the relation of pre-Laramide cratonic deformation to Laramide orogenesis.
Existing Models for Structural Style—What Drove the System?
Three general kinematic models have been proposed to explain the structural style and various orientations of Laramide arches and uplifts across Wyoming (Weil et al., 2016).
Model 1: Bulk shortening with transpression and wrench faulting. The diverse trends in structures of the Laramide belt of Wyoming were initially attributed to wrench-shear along differently oriented zones (Sales, 1968; Stone, 1969). Sales (1968) envisioned a large transpressive megashear system that affected the Wyoming foreland, and Stone (1969) explained WNW and NE structures as major high-angle wrench faults forming in conjunction with NE-SW compression.
Model 2: Temporal changes in shortening directions.Gries (1983) proposed that the varied Laramide trends observed in Wyoming relate to temporal changes in the principal horizontal stress, proposing more E-W compression in the early Laramide orogeny, followed by more N-S compression later in the orogeny. Gries (1983) noted the formation of thrust faults along the E–W-striking mountain ranges as evidence for N-S compression. This model predicts that arches will form transverse to the principal horizontal stress (Weil et al., 2014).
Model 3: Bulk shortening and basement anisotropies. Models for bulk shortening from the WSW to ENE, with some basement control on orientation, were first presented by Erslev (1993) and supported by Erslev and Koenig (2009). This model indicates only a single, regional shortening direction with local deviations in strain related partly to basement fabric orientation. For Wyoming, this model predicts that NNW arches will form transverse to the principal horizontal stress, and uplifts will form at oblique angles to this stress, with sinistral and dextral slip on more WNW- and NE-striking reverse faults (Weil et al., 2014).
Studies of structural and anisotropy of magnetic susceptibility data for the southern Bighorn arch and the Sweetwater uplift area (Weil et al., 2014, 2016) provided definitive evidence supporting model 3. Weil et al. (2016) suggested that reactivation of preexisting (Precambrian) basement weakness and heterogeneities, and flat-slab subduction during regional WSW-ENE shortening were the major influences on regional Laramide stress/strain, along with lesser amounts of transpression and creation of localized wrench-related structures in sedimentary cover across Wyoming.
PRECAMBRIAN DEFORMATIONS—PREVIOUS WORK
Basement rocks of the Cordilleran foreland have many zones of potential structural weakness. Because these zones exhibit varying degrees of orientation, their role in Laramide tectonism is not always clear, especially in the Wyoming Province, where Archean basement rocks have a complex polyphase deformation history (Fig. 2; Sims et al., 2001, 2004), as compared to the Yavapai Province to the south, where Laramide tectonism involved younger (Proterozoic) basement. Because the basement architecture of the Wyoming and Yavapai Provinces is significantly different, an assessment on the role of basement control on Laramide tectonism requires independent assessment for each of these domains, as explained by Chamberlain et al. (2003). Exposures of Precambrian rocks in the cores of several Laramide arches and uplifts in Wyoming allow for comparison of basement fabric orientations and structural grains with those of major Laramide structures. Note, areas to the SSW of the Laramie Peak shear system, including the Medicine Bow Mountains and Sierra Madre, were not considered in this study because of likely overprinting of Archean fabrics from 1.78 to 1.74 Ga Cheyenne belt deformation events.
Sims et al. (2001) and Sims (2009) used several lines of geological and geophysical evidence to show that potential zones of basement weakness (faults, shears, suture zones) in Wyoming are generally oriented to the NNW, WNW, and NE (Fig. 2). Sims (2009) interpreted NNW- and WNW-striking features to be conjugate shears that formed due to continental-scale transpression in the Precambrian. However, fault-bounded (e.g., Wind River and Piney Creek thrusts) domains related to a convergent tectonic regime for the NNW structures (e.g., Wind River and Bighorn arches) are more likely and have been identified by Frost et al. (2000, 2006a) and described by Palmquist (1967) and Chamberlain et al. (2003). Frost et al. (2000) indicated that the Precambrian core of the Wind River Mountains records a long-lived (300+ m.y.) active continental margin along the southwest edge of the Archean Wyoming Province boundary.
WNW- and NE-striking zones of potential basement weakness are also present across Wyoming (Fig. 2; Sims et al., 2001), with the most significant being the North Owl Creek, Tensleep, North Granite Mountains, and Big Trails faults, all likely having Precambrian ancestries (Hoppin et al., 1965; Allison, 1986; Paylor and Yin, 1993; Ver Ploeg and Greer, 1997; Grace et al., 2006).
Other significant structural/tectonic features that likely developed structural grains in the Precambrian include Archean supracrustal belts (convergent tectonism and magmatism) with northwesterly trends and Proterozoic dike swarms (extensional rifting) with multiple orientations (Fig. 2).
Detailed studies of these basement trends show that many are regionally parallel to Laramide arch or uplift trends (Figs. 1–4; Erslev, 1993; Erslev and Koenig, 2009; Neely and Erslev, 2009), which suggests that basement anisotropies properly aligned with the Laramide bulk shortening direction may have played a major role in the orientation and location of Laramide arches and uplifts in Wyoming. I tested this hypothesis by comparing the orientations of Precambrian basement structural grains and fabrics to Laramide structural trends.
Wyoming Province Boundary and the Laramide Belt
Craton boundaries or sutures are obvious zones of basement weakness (Bader, 2008). Terranes of the Trans-Hudson orogen bound the Wyoming Province on the east (Brewer et al., 1982; Whitmeyer and Karlstrom, 2007; McCormick, 2010). The Cedar Creek fault defines this boundary, which extends from the northeastern corner of Montana southwards to southern South Dakota, where it connects with the southern boundary of the Wyoming Province, the Cheyenne belt suture (Fig. 1). The Wyoming Province is bounded on the NNW by the Great Falls tectonic zone, a major shear (?) or suture (?) corridor that extends across central Montana from the SW to NE. The Laramide belt of Wyoming is therefore bounded to the east and south by Proterozoic terranes of the Trans-Hudson orogen and Yavapai Province, respectively. To the north, the Laramide belt does not extend to the edge of the Wyoming Province and is bounded by the Nye-Bowler fault zone, a major structural transition zone between Wyoming and Montana. To the west, the Laramide belt extends to the Sevier overthrust belt (Figs. 1 and 3; Yonkee and Weil, 2015).
Numerous, relatively small, Neoarchean supracrustal sequences outcrop in Precambrian basement of south-central Wyoming (Fig. 2; Frost et al., 2006b). The major belts are generally exposed as slivers or blocks brought to the surface along Laramide faults (Love and Christiansen, 1985). Major exposures are in the Bridger Mountains (Copper Mountain), Granite Mountains (Rattlesnake Hills), Wind River Mountains (Medina Mountain and South Pass), Seminoe Mountains (Bradley Peak), Laramie Mountains (Elmers Rock), and the Sierra Madre (Phantom Lake). These supracrustal belts define zones of likely subduction-related accretion, along with associated synchronous magmatism, and thus are likely zones of basement weakness (Fig. 2). The ages and origin (continental vs. oceanic crust) of the belts form roughly northwesterly trends in south-central Wyoming, and thus they may define a northwesterly structural grain where paleo-subduction zones formed in the Neoarchean (Chamberlain et al., 2003; Souders and Frost, 2006).
Numerous Precambrian dike swarm exposures occur in basement rocks of Wyoming (Condie et al., 1969). Major swarms outcrop in the Beartooth, Bighorn, Owl Creek, Wind River, and Laramie Mountains (Fig. 2) and are predominantly of Proterozoic age, indicating several periods of intrusion at ca. 2500, 2200–1900, 1800–1400, and 1000–700 Ga (Condie et al., 1969; Cox et al., 2000). Trends are dominantly to the WNW, NW, ENE, and NE (Fig. 2). These zones represent periods of extensional tectonics that can form preferred zones of basement anisotropy along dominant trends.
Limited Precambrian basement of the Wyoming Province is available for study and is only exposed in the cores of Laramide arches and uplifts; however, Precambrian fabric data from the Bighorn (Hoppin et al., 1965; Palmquist, 1967; Hudson, 1969), Wind River (Frost et al., 2000; Chamberlain et al., 2003), and Owl Creek (Paylor and Yin, 1993) Mountains allow for comparison of Precambrian fabrics in proximity to major Laramide structures of Wyoming along the three hypothesized directions of potential basement weakness. These include the Piney Creek and Wind River thrusts (NNW strike), the Tensleep and North Owl Creek faults (WNW strike), and the Big Trails fault (NE strike). Data available for Precambrian fabrics of Wyoming were manually measured from preexisting maps, compiled, and synthesized, and new plots were created on equal-area, lower-hemisphere projections, from which new rose diagrams were constructed for each area using Stereonet 10 (Figs. 5–9). These plots were used for comparison to data from Laramide structures (Fig. 4).
Tensleep fault zone. Fabric data from Hoppin et al. (1965) for Precambrian gneiss exposed adjacent to the Tensleep fault in the central Bighorn Mountains were used for Figures 5A and 5B. Foliations (n = 55) have an average strike of N81W (99°) and are subparallel with the strike of the Tensleep fault at N75W (105°; Fig. 5B).
Horn fault.Figures 6A and 6B were prepared using Precambrian fabric data from Palmquist (1967) associated with the Horn fault, a major reverse fault that is subparallel to the Piney Creek thrust on the eastern flank of the central Bighorn Mountains (Fig. 1). Archean granitic gneiss is exposed east of the Horn fault over an area of ∼6.5 km2. Foliations (n = 41) have an average strike of N38W (142°) and are subparallel with the strike of the Horn fault at N32W (148°; Fig. 6B) and the Piney Creek thrust at N24W (156°).
Big Trails fault zone. Fabric data from Hudson (1969) for Precambrian gneiss exposed near the northern extension of the Big Trails/Crazy Woman fault in the central Bighorn Mountains were used for Figures 7A and 7B. Foliations (n = 26) have an average strike of N26E (26°) and are subparallel with the strike of the Big Trails fault extension at N21E (21°; Fig. 7B).
Wind River Mountains
Mount Helen structural belt. Precambrian basement fabric data for the area near the Mount Helen structural belt are presented in Figures 8A, 8B, and 8C, based on data from Chamberlain et al. (2003) and Frost et al. (2000). The zone includes the Bridger batholith to the southwest of the northwest-striking shear zone within the belt, which is composed of intensely foliated gneiss (Fig. 2; Frost et al., 2000). Foliations from Chamberlain (2003; n = 16) have an average strike of N57W (123°), and data from Frost et al. (2000; n = 194) have similar trends with an average strike at N67W (133°). Both sets of data indicate that the Mount Helen structural belt is subparallel to the strike of the Wind River thrust at N56W (124°; Fig. 8).
Lake Surprise shear zone. The Lake Surprise shear zone consists of an en échelon corridor of steeply dipping mylonites of definitive Precambrian age exposed in the central-western portion of the Wind River Mountains. Fabric data for this area were unavailable; however, mylonites are present ∼15–20 km east of the Wind River thrust and are subparallel to the fault (Fig. 2; Dvjoracek, 1988).
Owl Creek Mountains
LARAMIDE DEFORMATIONS—PREVIOUS WORK
Recent studies indicate that the principal Laramide shortening directions were oriented WSW-ENE throughout the orogeny (Weil et al., 2016). Major Laramide structural trends in Wyoming are predominantly in three directions: NNW, WNW, and NE (Fig. 4; Erslev and Koenig, 2009). The NNW trends are likely related to the WSW-ENE bulk shortening direction, roughly transverse to major arch trends (Varga, 1993; Erslev and Koenig, 2009). The WNW and NE trends have been attributed to more localized strain partitioning along zones of preexisting basement weakness that were oblique to the regional bulk shortening direction (Varga, 1993; Erslev and Koenig, 2009; Weil et al., 2014, 2016). All three of these trends define several deformation zones that are present across north-central Wyoming, and, collectively, they define the Wyoming transpressive zone (Fig. 1). I also compiled geologic and structural mapping data for this section from the literature and include interpretations from previous workers. This previously published material is critical to the hypothesis and interpretation, and, therefore, detailed reviews are presented in the following narrative for each zone and on Figures 1–4, 10, and 11.
NNW Arcuate Zones
Prominent bow-shaped physiographic features at the surface, including the Wind River (Wind River Mountains) and Piney Creek (Bighorn Mountains) thrusts, define NNW-striking major zones of deformation in Wyoming (Fig. 1). Other less significant thrusts with a NNW orientation include the Rio, Emigrant Trail, Casper Arch, and Seminoe thrusts. The major thrusts are the thick-skinned, basement-involved, Laramide structures, with dips of 30°–45° at the surface (Smithson et al., 1979; Stone, 2003; Weil et al., 2016), and movement on these faults formed the prototypical Laramide arches of Wyoming. Subsurface studies have suggested that these structures may sole out into a midcrustal detachment zone at depths of ∼25 km (Blackstone, 1990a; Erslev, 1993; Yeck et al., 2014; Worthington et al., 2016). However, the Consortium for Continental Reflection Profiling (COCORP) line across the southern Wind River thrust shows a relatively consistent apparent fault dip (38°) to ∼25 km, below which the reflector disappears (Smithson et al., 1979). Thus, there is no direct evidence of a listric geometry with the fault soling into a crustal detachment. In addition, recent seismic studies have not imaged a planar detachment surface at midcrustal depths for the Bighorn arch or Piney Creek thrust (Worthington et al., 2016).
WNW Rectilinear Zones
Intracratonic strike-slip deformation has been the subject of numerous studies throughout the world (e.g., Storti et al., 2003). Several studies in the Wyoming Province have dealt either directly or indirectly with strike-slip or wrenching deformation in basement rocks with structural zones that are distinctly rectangular in plan view (Stone, 1969, 1985, 2002; Sims, 2009). To this point, rectilinear zones of deformation have been identified in northern and into south-central Wyoming (Figs. 1 and 3; Stone, 1969). They include several WNW-striking faults or fault zones, many of which have documented oblique slip, with major reverse and minor sinistral-slip components (Molzer and Erslev, 1995; Stone, 2002; Otteman and Snoke, 2005; Weil et al., 2014, 2016). Left-slip magnitudes are difficult to quantify, but structural patterns in sedimentary cover rocks represent transcurrent components of these oblique-slip faults (Fig. 3; Stone, 1969; Wilcox et al., 1973; Christie-Blick and Biddle, 1985; McClay and Dooley, 1995; McClay and Bonora, 2001; Bader, 2008, 2009).
Tensleep fault. The Tensleep fault is an E–W-striking, high-angle fault that extends ∼50 km across the Bighorn Mountains and into the southern part of the Bighorn Basin (Finn et al., 2010). The fault is interpreted as a Precambrian structure that was reactivated in the Phanerozoic (Hoppin et al., 1965; Allison, 1986). Allison (1986) indicated that the fault is dominantly a reverse fault with only minor sinistral slip, an interpretation supported by the work of Weil et al. (2014), who found no widespread evidence for vertical-axis rotations on the fault, thus suggesting limited sinistral slip. Hoppin et al. (1965) and the work of Stone (1985, 1993) also suggested that the fault has a minor component of left-lateral displacement. Curvilinear (reverse-S shape), en échelon, and left-stepping folds trending to the NW-SE across the buried portion of the fault in the Bighorn Basin may support this argument (Fig. 3; Finn et al., 2010). The fault appears to connect with the Rio thrust fault in the basin interior, but Stone (1969, 1993) indicated that the Tensleep fault may extend across the entire Bighorn Basin, based on seismic evidence.
Owl Creek fault zone. The Owl Creek and Bridger Mountains separate the Bighorn Basin from the Wind River Basin to the south (Figs. 1 and 10A; Keefer, 1970). These mountains are the product of SW-directed oblique slip along the complex, WNW-striking Owl Creek fault zone, based on detailed studies using minor fault and structural analysis (Paylor and Yin, 1993; Stone, 1993; Molzer and Erslev, 1995).
The Owl Creek Mountains define the western Owl Creek block and are bounded on the north by the E–W-striking North Owl Creek fault, a basement-rooted (master fault) structure that extends ∼30 km at the surface. The fault is nearly vertical, with dips generally to the north, and locally to the south (Paylor and Yin, 1993). On the east, the fault appears to abruptly change attitude, merging with the Mud Creek thrust, which strikes to the SE and dips ∼40°–60° to the SW. However, the North Owl Creek fault may extend into the subsurface to the east of this junction based on an en échelon, left-stepping, rectilinear zone of curvilinear folds and reverse faults mapped on top of the Tensleep Sandstone along the southern margin of the Bighorn Basin (Figs. 3 and 10A; Ver Ploeg, 1985). The likely presence of an E–W-striking basement-rooted master fault (North Owl Creek fault) therefore suggests that the Mud Creek thrust is a splay off of this master fault (Paylor and Yin, 1993).
The South Owl Creek fault is a north-dipping, oblique-slip fault (reverse/sinistral) that is buried by the Eocene Wind River Formation (Molzer and Erslev, 1995). It extends ∼70 km along the northern margin of the Wind River Basin, from near the surface terminus of the Mud Creek thrust to the SW end of the Bighorn Mountains. It, too, changes attitude to the east and likely merges with the Casper arch thrust, which strikes to the SE and dips 20°–45° to the NE (Stone, 2002). The South Owl Creek fault, the eastern extension of the Shotgun Butte thrust, and the Madden thrust contribute to the uplift of the Bridger Mountains complex (eastern Owl Creek block), an asymmetric anticline in the hanging wall on the eastern side of this major oblique-slip fault system (Molzer and Erslev, 1995).
The western and eastern Owl Creek fault blocks define the axis of an oblique-slip deformation zone that shows regional evidence for left-slip movement (Fig. 10A; Paylor and Yin, 1993; Molzer and Erslev, 1995), associated with nearly 8000 m of vertical reverse separation (Blackstone, 1990a). Curvilinear thrust/reverse splays strike to the SE off the North Owl Creek fault, including the aforementioned Shotgun Butte and Madden thrusts, and the South Owl Creek fault. To the east, these faults become subparallel with the master North Owl Creek fault. A complicated configuration of faults and folds, including the Boysen–Iron Dike and Cedar Ridge faults, and the McComb anticline, a curvilinear fold trending across the Cedar Ridge fault (Thaden, 1980a, 1980b), characterizes the eastern Owl Creek block. Faulting in the eastern Owl Creek block is further complicated by a zone of normal faults at the southern margin of the system (Fig. 10A). These normal faults likely represent post-Laramide Paleogene collapse of the hanging-wall block, which was thrust over the northern margin of the Wind River Basin during Laramide orogenesis (Sales, 1983; Livaccari, 1991; Snoke, 1993). Such extensional collapse likely affected older, high-angle, E–W-striking faults (e.g., Boysen–Iron Dike fault) further to the north along the eastern Owl Creek block.
Cross-section a-a′ (Fig. 10B) is a schematic cross section across the eastern end of the North Owl Creek fault generated by Stone (1993; see also Stone and Hollberg, 2007; and Figs. 1 and 10A herein). The left-slip nature of the high-angle North Owl Creek fault, and the thrust and synthetic splays off the master fault are visible from Stone’s interpretation of surface (field mapping) and subsurface (seismic and well logs) data, and they are consistent with the model of McClay and Bonora (2001, their fig. 4) for an underlapping 30° restraining stepover.
Paylor and Yin (1993) and Sundell (1990) showed evidence for 4–10 km of left-slip movement on the North Owl Creek fault system during Laramide shortening based on three-dimensional modeling and minor fault analysis of slickenlines.
North Granite Mountains fault zone. The Granite Mountains define the southern edge of the Wind River Basin, where Precambrian rocks are uplifted along the North Granite Mountains fault (Fig. 1). The fault zone forms the northern margin of the Sweetwater uplift (Fig. 3; Love, 1970) and defines the northern limit of the Oregon Trail structural belt, where several WNW and WSW shear zones are present (Grace, et al., 2006). The North Granite Mountains fault is a southerly dipping, nearly vertical fault that generally juxtaposes Precambrian rocks on the south against Phanerozoic sedimentary rocks on the north. The fault extends for ∼100 km from the southern end of the Casper arch thrust to the west, where it terminates against the Emigrant Trail thrust in the central part of the Wind River Basin. Direct evidence of oblique slip along this fault has not been documented. The lack of such evidence is probably related to reactivation of the North Granite Mountains fault as a normal fault during Paleogene extension after the Laramide orogeny, which caused downdropping of the previously uplifted Sweetwater structure along the hanging wall of the fault and likely obscured and/or destroyed evidence of earlier deformation (Fig. 3; Love, 1970; Sales, 1983). However, Weil et al. (2016) provided indirect evidence of some oblique slip along this fault based on structural and anisotropy of magnetic susceptibility studies performed across the Sweetwater uplift. There is also evidence of a left-slip component based on well-developed NW-trending en échelon folds to the north of the fault (Weil et al., 2016), along with a linear trend (NE-SW) of Eocene volcanic rocks that may reflect NW–SE-directed crustal extension and thinning related to left-slip motion (Fig. 3).
Casper Mountain fault zone. Due east of the North Granite Mountains fault, there is the Casper Mountain fault zone, which has been studied in detail by Molzer and Erslev (1995) and Stone (2002). The Casper Mountain fault zone, as described by Stone (2002), consists of an E–W-striking corridor of moderate-angle (40°–60°), southerly dipping faults along which Casper Mountain has been uplifted (Fig. 1). The fault system strikes E-W for ∼40 km just south of the town of Casper, Wyoming. Molzer and Erslev (1995), using minor fault and structural analysis, and Stone (2002), using structural analysis, provided conclusive evidence that slip on the Casper Mountain fault zone is reverse-left oblique in nature. Stone (2002) also indicated that the Casper Mountain structure represents a zone of reactivated Precambrian fabrics based on ENE-striking bands of pervasive deformation in the basement rocks of Casper Mountain.
South Granite Mountains fault zone. The South Granite Mountains fault system trends WNW along the southern margin of the Sweetwater uplift (Figs. 1 and 3; Love and Christiansen, 1985). The SE-striking South Granite Mountains fault dips to the north and extends for ∼80 km from just north of the Seminoe Reservoir to where it merges with the southern terminus of the Emigrant Trail thrust in the south-central part of the Wind River Basin (Bowers and Chamberlain, 2006). The fault ties into the Shirley Mountains/Seminoe thrust (Love and Christiansen, 1985) to the SE. Strike-slip and reverse faulting along the Shirley Mountains thrust occurred during Late Cretaceous and early Paleocene time according to Wilson et al. (2001). This evidence of oblique slip may indicate that the South Granite Mountains fault zone, as a whole, may have a small component of reverse-left, oblique slip, as suggested by Weil et al. (2016). The trend of the Sweetwater uplift and bounding faults has been postulated to have been due to reactivation of Precambrian basement structures during the Laramide orogeny (Bowers and Chamberlain, 2006; Frost et al., 2006b; Weil et al., 2016). Two WNW-striking Precambrian shear zones (Miners Canyon and Kortes) along the South Granite Mountains fault zone trend support this interpretation (Fig. 2).
Wind River fault. On the southeast flank of the Wind River Mountains, the strike of the Wind River thrust gradually changes from NW to WNW, where it terminates and appears to align with a reverse/thrust fault (Wind River fault) mapped by Love and Christiansen (1985) and Blackstone (1990b) (Figs. 1 and 3). The dip on the Wind River thrust near the southern terminus is ≥38° (Smithson et al., 1979), indicating that as fault strike changes to WNW, dip may be increasing. The reverse/thrust fault has been mapped at the surface (Love and Christiansen, 1985), and for 80 km in the subsurface (Blackstone, 1990b; Sims et al., 2001). The fault strikes to the SE, where it terminates near a probable blind thrust extending north from the Rawlins arch (Sims et al., 2001; Weil et al., 2016).
Unnamed fault.Perry and Flores (1997) showed a sinistral fault connecting the Rawlins arch thrust and a NW-striking, west-directed thrust to the south of Sheephead Mountain in south-central Wyoming (Figs. 1 and 3; Kraatz, 2002). This structure extends for ∼40 km from just south of Rawlins, Wyoming, SE to the south of Sinclair and the Grenville Dome to just NW of Sheephead Mountain. Otteman and Snoke (2005) interpreted the E-W strike oblique to the principal horizontal stress as evidence of an important component of sinistral, oblique-slip displacement along this fault. The prominent bend in the structural trend from N-S to E-W is likely related to a preexisting crustal weakness zone in Precambrian basement rocks (Otteman and Snoke, 2005).
NE Rectilinear Zone
Big Trails fault zone. The Big Trails fault is the only major NE-striking zone of deformation in the study area (Fig. 1). The fault is present in north-central Wyoming and is a basement-rooted, nearly vertical, right-oblique slip fault, as determined from slickenline and structural analysis (Fig. 3; Ver Ploeg and Greer, 1997; Weil et al., 2014). It starts at the intersection of the Owl Creek fault zone and Casper arch thrust to the south and terminates at the Tensleep fault to the north, extending over 80 km along the crest of the southern Bighorn Mountains. The fault likely extends north of the Tensleep fault, where it is mapped as the Crazy Woman fault (Weil et al., 2014). The main fault trace is up to 0.8 km wide, and it is characterized by several braided fault strands. Vertical offset ranges from almost zero at the northern fault terminus to a maximum of more than 1400 m near the middle portion of the fault. Right slip is estimated at 3200–4800 m. Ver Ploeg and Greer (1997) and Weil et al. (2014) indicated that the Big Trails fault appears to be basement controlled and is coincident with zones of northeast-striking dikes of Precambrian age.
Relation of Precambrian Fabrics to Laramide Deformations
Hoppin et al. (1965) interpreted the pervasive zone of foliations within the deformation zone of the Tensleep fault as a deep, recrystallized shear zone of Precambrian age (Fig. 2). The foliation data support the conclusion of a Precambrian ancestry for the Tensleep fault (Figs. 5A and 5B) and subsequent reactivation in the Laramide orogeny as an oblique-slip fault with a component of sinistral slip.
Foliations in granitic gneiss of the Horn fault are subparallel to the main segment of the Horn fault, indicating a preferred orientation of weakness to the NNW (Fig. 2). These foliations indicate that Laramide development of NNW-striking structures (Horn fault and Piney Creek thrust) in this area may have been guided by Precambrian-age fabrics, as interpreted by Palmquist (1967) and supported by plots of Precambrian foliations (Figs. 6A and 6B). Hudson (1969) indicated that there was no evidence that fabrics in basement rocks influenced Laramide deformation along the NNW-striking faults. However, a few (3), and only foliations not associated with other structures discussed below, are subparallel to the NNW structures. Again, this suggests that, along with the data from the Horn area, the NNW-striking faults may have been guided by basement anisotropies during the Laramide orogeny. The NNW-trending Bighorn magmatic domain and NNW-trending contacts between gneissic and magmatic domains in the Bighorn area (Fig. 2; Sims et al., 2001) support this conclusion.
Hudson (1969) also interpreted foliations present lateral to the NE end of the Big Trails fault zone to be a Precambrian shear zone reactivated as a dextral strike-slip fault during the Laramide orogeny, as was also similarly interpreted by Ver Ploeg and Greer (1997) for the Big Trails fault. Plotted foliations support these interpretations (Figs. 7A and 7B).
Wind River Mountains
The Mount Helen structural zone has been interpreted as a major high-temperature shear zone (Frost et al., 2000) that formed during convergence adjacent to a late Archean magmatic arc oriented to the NNW (Fig. 2; Chamberlain et al., 2003). Data from foliations within and adjacent to the Mount Helen structural zone provide possible evidence supporting this conclusion (Fig. 8). Precambrian mylonites from the Lake Surprise shear zone are also consistent with this interpretation (Dvjoracek, 1988) and again indicate that these regional Precambrian fabrics may have influenced localization of the Wind River thrust during the Laramide orogeny.
The Medina Mountain and South Pass supracrustal belts strike WNW and WSW, i.e., more in alignment with the Oregon Trail structural belt and North Granite Mountains fault zone, and may have contributed to this westerly zone of weakness, identified as probable Archean shear zones (Grace et al., 2006). More likely, due to the age differences of supracrustal belts along the Oregon Trail structural belt and the distribution lateral to Laramide faults, they are fault slices brought to the surface during the Laramide orogeny (Fig. 2). Dike swarms in the Wind River Mountains are also oriented WSW, suggesting that Proterozoic extension may have occurred locally along the earlier-formed WSW-striking Archean shear zones.
Owl Creek Mountains
Mafic dikes just south and subparallel to the trend of the North Owl Creek fault at two locations are Precambrian in age (Condie et al., 1969; Paylor and Yin, 1993), suggesting that Precambrian structural trends controlled development of the structure during Laramide orogenesis (Figs. 9A and 9B). However, Paylor and Yin (1993) mapped regional foliations with a NW strike south of the fault and interpreted WNW-striking foliations near the fault to represent rotation along the interpreted sinistral fault and, therefore, inferred that basement weaknesses did not control Laramide deformation. Because the mafic dikes are present at two locations along and where the fault strike changes, a Precambrian ancestry is more likely, but not definitive.
Metasedimentary rock fabrics of the Copper Mountain supracrustal belt in the eastern Owl Creek block (Bridger Mountains) are subparallel to slightly oblique to the Boysen-Iron Dike fault, indicating basement control on this fault zone during Laramide orogenesis along a very similar westerly trend, as seen for the North Owl Creek fault in the western Owl Creek Mountains.
The North Granite Mountains fault zone parallels the Oregon Trail structural belt, which contains several WNW–WSW-striking Precambrian shear zones and supracrustal belts (Grace et al., 2006), indicating that this significant zone of Precambrian deformation has influenced Laramide tectonism. Similarly, shear zones and/or supracrustal belts along the southern margin of the Granite Mountains likely indicate Precambrian influence oriented WNW.
Hudson (1969) used outcrop fracture data from Precambrian gneiss and Paleozoic sedimentary rocks associated with the Piney Creek thrust to elucidate Precambrian/Laramide paleostress directions in the Bighorn Mountains (Fig. 12). These patterns indicate very similar principal horizontal stress orientations for both sets of fractures. Hudson (1969) interpreted this to indicate that the fractures were formed at the same time and must be Laramide in age. However, they may also indicate that principal horizontal stress directions were from the same direction in both the Precambrian and the Laramide deformational events. Precambrian basement fabrics indicate that the latter is more likely, and that zones of weakness formed in the Precambrian were reactivated in the Laramide under similar NE-SW principal horizontal stress.
Data from previous work discussed here indicate that three directions of potential basement weakness dominate in the central and northern Wyoming Laramide belt, NNW, WNW, and NE. The NNW trends, associated with arches, have relatively simple geometries with master reverse faults. Evidence that basement weaknesses control formation of major fault zones associated with arches is not conclusive, based on Precambrian fabrics and regional structural grain. The WNW and NE trends associated with uplifts have more complicated geometries with rectilinear zones of deformation including near-vertical faults, reverse-oblique–slip faults, relay zones, en échelon folds, etc., and basement weaknesses appear to have controlled their development. Hoppin et al. (1965) and Grace et al. (2006) provided conclusive evidence that WNW-striking features (Tensleep fault and Oregon Trail structural belt) are Precambrian shear zones. Ver Ploeg and Greer (1997) concluded that the NE-striking Big Trails fault follows Precambrian fabrics as well. In addition, these rectilinear deformation zones define an apparent conjugate shear pattern that is regional in extent, but best displayed in north-central Wyoming along the North Owl Creek, Big Trails, and Tensleep faults. Assuming these faults are conjugate shears developed in the Precambrian, a significant principal horizontal stress oriented ∼N55E (55°) is necessary. This bearing is transverse to the orientation of the major NNW-striking Wind River and Piney Creek thrusts, and the Wind River thrust is subparallel to an interpreted NW-trending late Archean magmatic arc, described by Frost et al. (2000) and Chamberlain et al. (2003). Therefore, the orientation of the Wind River thrust and proposed conjugate shears is consistent with a convergence model and is further evidence that the Wind River thrust may be a reactivated structure related to late Archean subduction (Chamberlain et al., 2003). This relationship may therefore apply to the Piney Creek thrust as well, and I describe this system kinematically as a conjugate shear set with paleosubduction pattern. Finally, convergent deformation at active continental margins is likely to develop conjugate shears in-board of the subduction zone (Sylvester, 1988). The observed paleosubduction pattern indicates that all three proposed directions of potential Precambrian zones of weakness presented in this paper are present, and I propose a relation to long-lived Precambrian convergence across Wyoming and the development of a convergent-deformation system, where NNW structures (e.g., Wind River and Piney Creek thrusts) represent paleosubduction/convergence zones, and WNW (e.g., Owl Creek and Tensleep fault zones) and NE-striking structures (e.g., Big Trails fault) are conjugate shears formed during convergence events. Deformation likely took place intermittently across a 300 m.y. time span (2.9 Ga to 2.6 Ga) dominated by convergent tectonics (Fig. 2; Chamberlain et al., 2003; Frost et al., 2000).
Supracrustal belts of similar 2.9–2.6 Ga age are present across central Wyoming and are generally exposed along the previously discussed fault zones. The distribution of these belts relative to age defines a NW trend (Chamberlain et al., 2003) that is consistent with convergence from the SW. Therefore, development of the aforementioned Precambrian subduction complex likely led to accretion of the supracrustal belts during this time frame.
Proterozoic dikes oriented WNW, NW, and NE probably developed along preexisting weakness formed in the Archean, and thus these zones also are in an alignment conducive for Laramide reactivation. However, dike swarms oriented to the ENE are subparallel to the Laramide bulk shortening direction and therefore were not likely reactivated during Laramide orogenesis.
Subsurface Relations and Two-Dimensional Interpretations
Cross-section A-A’ (Fig. 1; Stone, 1993; Stone and Hollberg, 2007) trends across the Owl Creek fault zone and presents Stone’s (1993) near-surface interpretation of the subsurface relations to a depth of ∼18,300 m (60,000 ft; Fig. 11), which is consistent with current data. Below 9150 m (30,000 ft), the figure shows my interpretation of Stone’s (1993) near-surface work, again based on data from this paper. The near-surface portion of the cross section is based on a robust set of seismic and well data from petroleum exploration in northern Wyoming and trends across several significant structures described earlier. The major faults shown here are the Wind River and Piney Creek thrusts, which, when reconstructed, create the “classic” Laramide arches described by various workers (Blackstone, 1990b; Erslev, 1993). In contrast, slip across the E–W-striking North Owl Creek fault has created a “rumpled uplift,” which is different from the “prototypical” arch form. This difference in surface expression is likely due to the nature of the slip, with NNW-striking faults (Wind River and Piney Creek thrusts) being lower-angle, pure-dip-slip thrust/reverse faults.
Conversely, I interpret the WNW-striking North Owl Creek fault zone to be reverse-sinistral slip in nature, with a steep master fault and numerous splays that create a positive flower structure and the resulting “rumpled” expression seen at the surface (Figs. 10B and 11; Paylor and Yin, 1993; Stone, 1993). The zone of subsurface folds to the east of the North Owl Creek fault–Mud Creek thrust junction (Fig. 10A) suggests a broad zone of Laramide transpression for the Owl Creek fault zone (Paylor and Yin, 1993). Near the junction, most of the lateral slip on the North Owl Creek fault was transferred to the SE along the Mud Creek thrust and the South Owl Creek fault (Molzer and Erslev, 1995). The South Owl Creek fault, the eastern extension of the Shotgun Butte thrust, and the Madden thrust, like the Mud Creek thrust to the west, are likely splays off of the master North Owl Creek fault, along which the eastern Owl Creek block was thrust over the deepest part of the Wind River Basin (Fig. 10A), accommodating much of the oblique slip within the system (Paylor and Yin, 1993; Molzer and Erslev, 1995). Thus, I argue that the entire Owl Creek fault zone represents a relay zone connecting major NNW-striking structures (e.g., Casper Arch thrust). Subsidiary features consistent with left slip include: NW-trending, curvilinear, en échelon folds that straddle both the north and south sides of the North Owl Creek fault, and a complex zone of deformation in the Bridger Mountains that is consistent with oblique-sinistral slip on the South Owl Creek fault (Molzer and Erslev, 1995). The right-stepping nature of the North and South Owl Creek faults creates a right-stepover restraining geometry, facilitating uplift of the Owl Creek Mountains along the Mud Creek and Shotgun Butte thrusts (underlapping 30° restraining stepover of McClay and Bonora, 2001, their fig. 3).
At depth, both the Wind River and Piney Creek thrusts must terminate at ∼25 km (Fig. 11), as observed on seismic images from Smithson et al. (1979) for the Wind River thrust, and depicted by Stone (1993, 2002), but the nature of the termination has not been determined. The North Owl Creek fault may extend into the middle crust, and faults in the northeast Wind River Basin are shown as splays off the master North Owl Creek fault, thus better defining the positive flower structure of Stone and Hollberg (2007), and supporting the interpretation of an oblique-slip (reverse-sinistral) fault (Fig. 11). Similarly, the Tensleep fault is present in the subsurface along section A-A′ (Stone, 1993) and is likely an oblique-slip (reverse-sinistral) fault with associated splays into the Bighorn Basin. The nature of these faults at greater depths is even more uncertain. Previous studies in the region have identified vertical seismic velocity gradient increases at ∼25 km depth, thus defining a midcrustal transition zone (Gorman et al., 2002; Worthington, et al., 2016). This midcrustal transition has been identified at ∼6.5 km/s and has been interpreted as either a crustal detachment surface (Erslev, 1993) or, where velocities exceed 7 km/s, a mafic underplate (Snelson et al., 1998; Gorman et al., 2002; see also Fig. 11). I propose that the thrusts and perhaps the high-angle faults terminate at depth into the mafic underplate that has been positively identified in this region through seismic studies (Gorman et al., 2002; Worthington et al., 2016).
An alternative, less-preferred interpretation is that the North Owl Creek fault is a back thrust off of the Shotgun Butte thrust, and the smaller thrusts sole out at depth, more consistent with crustal detachment models (Fig. 11, inset; Erslev, 1993). Although viable, this interpretation does not explain the near verticality, reversal of dip, reversal of sense of displacement, and subsidiary structures (en échelon, left-stepping, curvilinear folds and faults) exhibited by the North Owl Creek and the Big Trails faults (Paylor and Yin, 1993; Ver Ploeg, 1985).
The presented data and my interpretations indicate that major structures of the Wyoming transpressive zone oriented NNW, WNW, and NE may have originally formed in a convergent active margin setting in the Archean. In addition, the similarities between Precambrian fabrics and structural grains and Laramide structures suggest that relatively weak and favorably oriented zones of Precambrian weakness controlled Laramide reactivation. These similarities include: (1) likely Precambrian, basement-rooted, structures consistent with origins in a convergence zone deformational setting (NNW-striking reverse/thrust faults and associated WNW- and NE-striking conjugate shears); and (2) WNW- and NE-striking faults of moderate to steep dip that are consistent with Laramide sinistral/dextral displacement based on the existence of numerous lateral-slip–related structures in sedimentary cover rocks (Fig. 3).
During the Laramide orogeny, WNW uplifts developed during reverse/left-lateral, oblique slip along ramps (relay zones) that connected NNW-trending arch culminations (Figs. 1 and 13; Erslev, 1993; Molzer and Erslev, 1995). Numerous authors have documented this deformation pattern across several fault zones in Wyoming (Hoppin et al., 1965; Stone, 1985, 1993, 2002; Sundell, 1990; Paylor and Yin, 1993; Molzer and Erslev, 1995; Wilson et al., 2001; Otteman and Snoke, 2005), indicating a causal and fundamental relationship for the Wyoming transpressive zone.
The Wyoming transpressive zone represents a significant change in structural style within the Wyoming Province that occurs southward from southern Montana into Wyoming south of the Nye-Bowler fault zone (Fig. 1). Basement-cored Laramide-style arches, including the Bighorn and Beartooth Mountains, are present in southern Montana and northern Wyoming, and this style continues into south-central Wyoming. However, they are conspicuously absent in the Montana portion of the Wyoming Province, and generally in southern Wyoming, except along the Cheyenne belt and into the Yavapai Province (Ye et al., 1996; Soreghan et al., 2012). This change in the structural style across the Wyoming Province from N to S has been related to varying paleostress conditions related to flat-slab subduction of the Farallon plate during the Laramide orogeny (Erslev and Koenig, 2009; Yonkee and Weil, 2015; Weil et al., 2016). I propose that the presence of the Wyoming transpressive zone (Fig. 13), along with NNW-striking zones of weakness, also relates to this change in style.
Possible Precambrian Origins
The tectonic map of the area, showing locations, kinematics, and ages of Precambrian structural and igneous features discussed herein, along with strain ellipses for proposed intermittent convergent episodes in the late Archean, is presented on Figure 13. Based on the available data, I propose that the major Laramide structural features of Wyoming can be best explained by contractional episodes from 2.9 to 2.7 Ga with NE–SW-directed convergence forming the Wind River plutons along the SSW margin of the Wyoming Province, as depicted on Figures 2 and 13 here and described by Frost et al. (2000) and Chamberlain et al. (2003). Older, northerly trending magmatic zones of Archean age in the Beartooth, Bighorn, and northern Wind River Mountains are likely related to this period of Archean convergence (Sims et al., 2001; Chamberlain et al., 2003; Frost et al., 2006a; Mueller and Frost, 2006). Younger magmatic zones likely reworked and overprinted the older northerly trends along the western and southern margins of the Wyoming Province from 2.7 to 2.5 Ga (Chamberlain et al., 2003), with significant accretion of supracrustal belts. Overprinting by the younger magmatic belts would explain the E-W structural grain and magmatic domain trends of Sims et al. (2001) for the 2.64 Ga plutons exposed in the Granite and Owl Creek Mountains. Data suggest that NNW-trending anisotropies may be the roots of Archean convergence zones that generally young from the Bighorns W and SW toward the Beartooth and Wind River Mountains (Figs. 2 and 13; Chamberlain et al., 2003). I therefore propose that this long-lived (∼300 m.y.; Frost et al., 2000) convergence-dominated region developed conjugate shears that formed due to pure shear stress across the convergent orogenic belt (Sylvester, 1988).
Development of a Precambrian convergent-deformation system, with fabric reactivation in the Laramide orogeny, explains the major deformation of the study area, and, more importantly, it explains the underlying subtle symmetry of Laramide deformation seen across Wyoming (e.g., Wind River/Shirley basin rhomb-shaped geometry). Because the principal horizontal stress directions during the proposed Archean convergence events were roughly the same as those during the Laramide orogeny (∼N60E; Bird, 1998; Erslev and Koenig, 2009; Weil and Yonkee, 2012), ideal conditions were present for reactivation of the Archean fabrics during Laramide tectonism.
The Laramide structural style presented in this paper is consistent with model 3 presented earlier and supported by Weil et al. (2014, 2016); however, I postulate that structural inheritance had a more significant role in the development of most major Laramide arches and uplifts, with bulk shortening acting only as the driving mechanism (i.e., Laramide NE-SW shortening reactivated zones of Archean basement weakness that originally formed under similar NE-SW contraction). Transpression, along with limited and localized wrenching, also occurred in the Laramide orogeny, as documented earlier herein and by Weil et al. (2014, 2016). Strike-slip/shear motion was dominantly taken up on subsidiary faults (e.g., synthetic strike-slip faults and reverse/thrust faults) of the WNW- and NE-striking system(s).
Matching Archean Convergence Deformation and Laramide Contractional Structures
Laramide structures across Wyoming oriented NNW, WNW, and NE (Figs. 1, 2, and 4; Erslev and Koenig, 2009) compare very favorably with the orientations of postulated Archean convergence deformation zones shown on Figure 13 and therefore likely indicate a causal relationship. In the Wyoming transpressive zone, the NNW arches are significant, whereas the WNW-striking uplifts have less relief. The Big Trails fault defines the NE-trending uplift in the Wyoming transpressive zone.
A simplified convergent-deformation system model of Wyoming based on previously discussed data (Fig. 13) is displayed on Figure 14. The zones of weakness define discrete basement blocks along which Laramide reactivation occurred, creating major Laramide structures. The model presents interpreted relative slip on faults between blocks that had different temporal movements (Perry and Flores, 1997), depending on the orientation and “linkage” of faults and the Laramide stress field at any given time. Some of these structures were again reactivated during post-Laramide deformation due to arch collapse and/or Paleogene extension (e.g., North and South Granite Mountains faults).
Laramide basins and structural highs have sigmoidal (flattened reverse S/lazy Z) shapes, which may indicate some sinistral displacement on WNW-striking faults throughout the Laramide orogeny (Figs. 1, 4, 13, and 14). Such movement gave the arches and uplifts the “connected” and “anastomosing” geometry that is pervasive across Wyoming, as described by Erslev (1993).
The proposed convergent-deformation system that developed in the Precambrian was probably a smaller configuration of basement fault blocks. I propose that these zones grew somewhat during the Laramide orogeny as intermittent sinistral deformation took place along WNW-striking, oblique-slip faults, and pull-apart zones, restraining bends, and stepover uplifts developed lateral to, or along, these deformation zones. The rhomboid shapes of the Wind River/Shirley basins, the Owl Creek Mountains, and the Pryor Mountains are classic examples of these phenomena (Figs. 1, 10, 13, and 14).
Weight of Basement Control on Laramide Orogenesis
There are four predominant lithospheric-scale tectonic models for crustal shortening and the development of foreland arches (Erslev, 2005). These include:
tilted lithospheric blocks;
pure shear thickening;
lower-crustal detachment and upper-crustal buckling/fault propagation folding; and
All of these models involve basement deformation at some level; however, the significance of basement influence is uncertain.
EarthScope Bighorn Arch Seismic Experiment (BASE) investigations (Yeck et al., 2014; Worthington et al., 2016) indicate that the lithospheric block and pure shear thickening models are not likely scenarios for observed crustal geometries and that lithospheric buckling may have occurred, but it is not the prominent or sole shortening mechanism. Yeck et al. (2014) and Worthington et al. (2016) argued that midcrustal detachment with some lithospheric buckling controlled Laramide deformation. Major arches were the result of splays off of a Laramide midcrustal detachment surface in areas of random basement anisotropy, such as beneath the Bighorn Mountains. The convergent-deformation system model presented in this study would fit with tectonic models involving crustal detachment at depth with master thrust/reverse faults joining along a low-angle detachment surface in the middle crust along with some lithospheric buckling (Yonkee and Weil, 2015, their fig. 36), as has been postulated in the EarthScope BASE studies (Yeck et al., 2014; Worthington et al., 2016). However, the nature of the detachment surface is uncertain and was not identified in the BASE investigation (Worthington et al., 2016). In addition, preexisting zones of weakness along major, nonlistric reverse faults oriented to the NNW, identified in this study, indicate that Laramide structures relate to reactivation of the proposed Precambrian convergent-deformation system, rather than solely Laramide deformation associated with Cordilleran-derived midcrustal decoupling interacting with localized lower-crustal/upper-mantle anisotropies, as suggested by Erslev (1993), Yeck et al. (2014), and Worthington et al. (2016) (Fig. 11, inset).
The results of the current study suggest that Archean convergence zones and associated cratonwide sets of conjugate shears played a significant role in controlling Laramide deformation, thus adding a fifth possible scenario for development of foreland arches/uplifts, wherein preexisting (Archean) basement weaknesses in the mid- to upper crust were reactivated associated with flat-slab subduction, as suggested by Weil et al. (2016). This model does not necessarily require a Cretaceous–Paleogene zone of midcrustal detachment or lithospheric buckling, because the fractures in the upper crust may have already formed and been detached from the Moho in the Precambrian (e.g., mafic underplating), and any observed arching of the Moho may also have been solely Precambrian in origin (Worthington et al., 2016). Therefore, NE-to-SW contraction related to shallow-angle subduction may have been the only process necessary to reactivate the convergent-deformation system in the Laramide orogeny, at least in Wyoming (Fig. 11). In addition, the presence of mid- to upper-crustal basement weaknesses formed in the Precambrian helps to explain the presence of Laramide deformation so far inboard from the active continental margin.
Future deep crustal and lithospheric mantle studies, such as those conducted by Snelson et al. (1998), Schmandt and Humphreys (2010), and Shen et al. (2013), should continue to clarify the lithospheric nature of this deformation, with particular emphasis on the proposed conjugate shears as well as the nature of the midcrustal transition zone (midcrustal detachment surface or mafic underplate) identified and described by other workers (Fig. 11; Erslev, 1993; Gorman et al., 2002; Snelson et al., 1998; Yeck et al., 2014; Worthington et al., 2016). Specifically, they should investigate if there are any signs of convergence zone structures (subduction zones, conjugate shears) at the midcrustal transition, or below, in the lithospheric mantle. Such lithospheric mantle features would be similar to the relict subduction zones observed and interpreted from Deep Probe images (Gorman et al., 2002) and shown hypothetically on Figure 11. Higher-resolution seismic tomography and/or other technologies will likely be necessary to identify these features.
The consistent orientations of Precambrian-cored arches and uplifts within the Wyoming Province may be fundamentally related to the intracratonic nature of Archean basement weakness zones (i.e., proposed convergent-deformation system). Erslev and Koenig (2009) stated that a symmetric zone of regionally extensive Precambrian fault sets was not a major control on Laramide deformation across the orogen. This may be true when considering the entire orogen; however, the entire Laramide orogen consists of differing basement rocks that have vastly different lithology and tectonic histories. Therefore, the Wyoming Province should be considered separately from the Yavapai Province to the south. Such consideration presents a distinct pattern of Precambrian faults across north-central Wyoming. Such a pattern is very difficult to explain using other Laramide models without introducing numerous caveats or overlooking obvious discrepancies, including (1) shifting principal horizontal stress directions; (2) lack of explanation for high-angle fault zones across the entire Wyoming Province, where crustal shortening is dominant; (3) the presence of both extensional and compressional features; and, most importantly, (4) the hypothesis of basement-controlled random development of arches and uplifts related to midcrustal detachment, which is not consistent across a broad region that appears to have very fundamental and ordered origins in the Precambrian. The updated model presented in this study helps to resolve those problems.
The concept of a Wyoming transpressive zone is presented as a product of a long-lived, late Archean convergent-deformation system within the Cordilleran foreland. The system is bounded on the southwest by the Wind River thrust and on the northeast by the Piney Creek thrust. Laramide arches and uplifts in this area should be included within this system. I propose that fabrics of the convergent-deformation system underwent reactivation during subsequent tectonic events, including Laramide orogenesis. In this scenario, WSW–ENE-directed contractional stress (regional shortening) resulted in major thick-skinned Precambrian-cored arches developing in locations where NNW Precambrian convergence zone deformations and WNW sinistral shears connected. Classic Laramide arches formed, facilitated by left oblique-slip faulting, in the central part of the Wyoming Province. Rhomboid/sigmoidal basins developed and grew adjacent to sinistral deformation zones as the system evolved during the Laramide orogeny. Restraining bends and/or stepover uplifts such as the Owl Creek and Pryor Mountains also formed locally where wrench geometry facilitated formation of compressive structures.
A hypothesis and model have been presented that may contribute to a general understanding of Laramide structural geometries in central Wyoming and the important role that fundamental, preexisting heterogeneities played in Laramide orogenesis. The model incorporates nearly all available data regarding Laramide orogenesis and therefore provides an updated framework for contextualizing and motivating future studies that may test the model using new data.
I would like to thank those that contributed to this paper in many different ways. Thanks go to Fred Anderson, Lorraine Manz, and Tim Nesheim, my colleagues at the North Dakota Geological Survey, for early reviews and encouragement to move forward. Special thanks to Adolph Yonkee, Lindsay Worthington, Joe English, Bob Krantz, and Damian Nance for their detailed reviews and recommendations for publication. The Rocky Mountain Association of Geologists granted permission to use a portion of the Wyoming Transect of Donald Stone and John Hollberg, which again was an important contribution to this study. Finally, I would like to thank Daniel Schelling and Gary Stewart for their time and thoughtful discussions of unpublished work in north-central Wyoming. All of these people contributed to this paper and enhanced it greatly; however, interpretations and conclusions are solely those of the author. I dedicate this paper to my mom and dad.