Structural Evolution of an En Echelon Fold System Within the Laramide Foreland, Central Wyoming

Home , Fault block, Saddle (landform), Stylolite, Vein (geology)

THEMED ISSUE: Tectonics at the Micro- to Macroscale: Contributions in Honor of the University of Michigan Structure-Tectonics Research Group of Ben van der Pluijm and Rob Van der Voo

Structural evolution of an en echelon fold system within the Laramide foreland, central Wyoming: From early layer- parallel shortening to fault propagation and fold linkage

Adolph Yonkee1 and Arlo Brandon Weil2 1DEPARTMENT OF GEOSCIENCES, WEBER STATE UNIVERSITY, 1415 EDVALSON STREET, DEPARTMENT 2507, OGDEN, UTAH 84408, USA 2DEPARTMENT OF GEOLOGY, BRYN MAWR COLLEGE, 101 NORTH MERION AVENUE, BRYN MAWR, PENNSYLVANIA 19010, USA

ABSTRACT

Minor fault kinematics, fracture sets, anisotropy of magnetic susceptibility (AMS) fabrics, and remanent paleomagnetism within Triassic to Jurassic red beds and limestone along the Dallas–Derby–Sheep Mountain fold system and northeast flank of the Laramide Wind River arch record patterns of progressive deformation from early layer-parallel shortening (LPS) to large-scale fault propagation and fold linkage. The fold system comprises a series of doubly plunging, left-stepping anticlines with segmented southwest-vergent forelimbs cut by reverse faults and a gently northeast-dipping backlimb. Anticlines are linked across structural saddles (relay zones) that are locally cut by steep east- striking oblique-slip faults and thrust faults that accommodated shortening transfer. Minor wedge faults within limestone show consistent relationships with respect to bedding around anticline limbs, recording early LPS prior to and synchronous with initial fold growth. Within

red beds, LPS produced microkinked phyllosilicate grains that define AMS Kmax (axes of maximum susceptibility) lineations perpendicular to shortening directions. LPS directions estimated from minor faults and AMS trend approximately west-southwest–east-northeast within the backlimb, subparallel to the regional Laramide shortening direction, and partly refract across arcuate forelimbs. Paleomagnetic data record local counterclockwise rotations in forelimbs associated with map-view curvature and eastward-striking faults. Fracture sets and previously published calcite-twin strain data show more complex patterns related to evolving stress-strain fields, from pre-Laramide (intraplate) west- northwest–east-southeast compression, to early Laramide west-southwest–east-northeast LPS, to later Laramide fault propagation and local stress refraction. Reverse faults in basement rocks propagated upward into anticline forelimbs, and laterally with partial linkage by eastward- striking faults that formed along basement weaknesses.

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INTRODUCTION 2017). In this paper structural and paleomagnetic data sets for a flanking fold system of the basement-cored Wind River arch are examined to bet- The Laramide belt, spanning a region from southern Montana to New ter understand Laramide tectonics. Note that the term Laramide is used Mexico (USA), is an archetypical example of thick-skin foreland defor- for thick-skin structural style (compared to thin-skin style of Sevier belt), mation that developed far inboard from a plate margin (Fig. 1; Dickinson and not for a specific time interval. and Snyder, 1978; Bird, 1984; Saleeby, 2003). The belt is characterized Growth of variably oriented basement-cored arches and flanking fold by fault-bound, basement-cored arches that are bordered by flanking fold systems within the Laramide belt have generated multiple tectonic mod- systems and separated by broad basins (Berg, 1962; Brown, 1988; Erslev, els, including (1) temporal changes in shortening directions (Gries, 1983; 1993). The belt displays an overall northwest-southeast structural grain, Bergh and Snoke, 1992), (2) transpression with varying components of but individual basement faults have a range of strikes and cover folds form horizontal shear along differently oriented zones (Sales, 1968; Stone, both north-trending, right-stepping, and west-trending, left-stepping en 1969), and (3) regional shortening with spatial refraction of stress-strain echelon systems. General timing of large-scale Laramide deformation directions (Fig. 2A; Erslev and Koenig, 2009). Model 1 predicts early east- spanned ca. 70–50 Ma in the Wyoming region, constrained by sedimen- west shortening that formed north-trending arches, followed by late north- tologic patterns in the basins (Beck et al., 1988; Dickinson et al., 1988; south shortening that formed east-trending arches, with overprinting and DeCelles et al., 1991; Steidtmann and Middleton, 1991; Hoy and Ridg- possible linkage of early structures by later structures. Model 2 predicts way, 1997) and by thermochronologic data in the arches (Crowley et al., widespread development of en echelon folds, strike-slip to oblique-slip 2002; Peyton et al., 2012; Stevens et al., 2016). However, patterns vary faulting, and significant vertical-axis rotations, with sinistral shear and regionally and questions remain on timing of Laramide initiation, varying left-stepping folds along more west-trending zones, compared to dextral models of southwest to northeast, west to east, or northeast to southwest shear and right-stepping folds along more north-trending zones. Model propagation of deformation, and changes in shortening directions over 3 predicts smoothly varying stress-strain directions during temporally time (Chapin and Cather, 1983; Gries, 1983; Varga, 1993; Heller et al., overlapping development of variably trending arches, partly related to 2013; Fan and Carrapa, 2014; Lopez and Steel, 2015; Copeland et al., preexisting basement heterogeneities and weaknesses (Huntoon, 1993;

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A Paleogene ~ 60-50 Ma Current

o o o 111 110 109o 108o 107o 106o 105 45o Beartooth Arch Canada Black Hills U.S. Neogene Arch Volcanics Absaroka Bighorn Powder Volcanics Arch River North Basin American Bighorn

Basin o gmat Ma plate 44

Salitzia ic ar ic

c X2 Owl Creek Arch

s Wind

accreted River Fig. 3A o terrane Basin 43 Wind River Arch vier ftb Sweetwater Si erra Nevada arc (ina s Arch Se amide belt Green Lar River Laramie Basin Arch

X1 F hinterland r a Great o n 42 c core complexe t i s Rock c Moxa Hanna Basin a Valley n Arch Springs ct x S Arch u ive) xenoliths Colo Min Belt b Fo d x N u rearc x c t C x o m Wyoming Salien Medicine p M le ojave se Bow x Arch WYOMING 41o ct a or segmet-slab disrupted 050 100 km Sevier thrust Eocene volcanics Farallon n for t Laramide basins plate ear Laramide reverse fault c (dashed - concealed) with axis Anticlinal fold trace Triassic red beds normal fault Precambrian basement

Paleogene ~ 60-50 Ma B hinterland plateau X1 Sevier Laramide X2 Subduct Inactive arc core complexes fold-thrust belt foreland belt complex Forearc Accreted sed cover terranes basement Farallon oceanic mantle plate lithosphere

end loading n lithosphericric keelkeeel increased relative plate motio [NE-underthrust at slab] basal traction basal drag plateauteau crcrusc t ~100 km asthenosphere ow

Figure 1. (A) Generalized tectonic map of the Cordilleran system (left) illustrates setting of Laramide foreland belt and Sevier fold-thrust belt during the Paleogene; interpreted extent of flat-slab segment is indicated. Generalized geologic map of Wyoming (right) shows Laramide major fault traces, fold traces, basin axes, and outcrop distributions of Precambrian basement and Triassic red beds, and major thrust traces of the Sevier belt. Box out- lines general study region shown in Figure 3A. (B) Schematic tectonic cross-section X1–X2 across the Cordilleran system shows development of the Laramide foreland belt during flat-slab subduction related to possible increases in end loading, basal traction, and asthenospheric flow. Modified from Saleeby (2003) and Yonkee and Weil (2015).

Molzer and Erslev, 1995; Marshak et al., 2000; Stone, 2002; Neely and Relations of flanking fold system formation to basement deformation Erslev, 2009; Weil et al., 2014). For this model, along-strike fault propa- have also generated multiple models, including (i) detachment and buckle gation and fold linkage may also give rise to either left- or right-stepping folding of the sedimentary cover related to crowding (Petersen, 1983; Nel- en echelon fold systems, along with local partitioning of shear along son, 1993); (ii) fault-propagation folding associated with basement reverse obliquely oriented faults. It is important that these models predict differ- faults, as in fold-thrust and trishear models (Berg, 1962; Erslev, 1991; ent spatial and temporal patterns of large-scale fault and fold geometry, Stone, 1993); and (iii) drape folding above subvertical basement faults internal strain, and vertical-axis rotations, which can be evaluated from (Fig. 2B; Matthews and Work, 1978). For detachment models (i), folds integrated structural and paleomagnetic studies. Strike tests, in which should be broadly symmetric and associated with listric thrust faults. For changes in shortening directions and paleomagnetic declinations are cor- fault-propagation models (ii), fold forelimbs should be steeper and undergo related with changes in structural trend, provide a quantitative method to varying amounts of shear during progressive tilting, related to the ratio of test models (Fig. 2A). fault propagation to slip rate in trishear models. High ratios, as in overall

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A (1) Temporal Changes in (2) Transpression (3) Regional Shortening with Shortening Directions Stress-Strain Refraction en fold earl N echelon N linkage N folds y fold linkage horizontal shear trajectory /ε 3 σ 1 fold overprint Map-Scale late

fo Pmag late

Pmag shor LPS+ relate ld Pmag shear shea ear LPS

r l te late y

shear n d Scale late LPS early LPS LPS+ p- shear Pmag ro

early fold relate Outc Pmag LPS shor ten d

Pmag P-Pr LPS L-Lr Pmag P-Pr LPS L-Lr Pmag P-Pr LPS L-Lr strike test strike test strike test strike test strike test strike test ests T

S-Sr S-Sr S-Sr S-Sr S-Sr S-Sr Strike slope = 0 bimodal slope >0.0 slope >0.0 slope ~0.0 slope >0.0 B (i) Detachment/ buckle folding (ii) Fault propagation folding (iii) Drape folding (tri-shear) anking folds anking anking cover cover folds cover folds

basement basement arch basement arch block uplift Cross sec tion

C (a) Single fold-fault system (b) En echelon fold-fault system (c) Seperate fold-fault systems low S/L moderate S/L high S/L w L early early early propagate and bypass age S propagate inter propagate inter and partly link inter and link late late late S-seperation distance iew Link late V inter inter inter L-fault segment length w-fold width S/L- separation-length ratio Map early early early n n n te te te late late late shor shor shor inter inter inter early early early along-strike length along-strike length along-strike length

Figure 2. (A) Idealized kinematic models for development of variably trending Laramide arches include: (1) temporal changes in shortening directions; (2) transpression with horizontal shear along differently oriented zones; and (3) regional shortening with refraction of paleostress/strain directions partly related to basement weaknesses. Schematic map-scale patterns of faults and folds, outcrop-scale patterns of layer-parallel shortening (LPS) fabrics and vertical-axis rotations of paleomagnetic (Pmag) declinations, and idealized strike tests that correlate variations in LPS directions relative to a reference shortening direction (L-Lr), variations in paleomagnetic declinations relative to reference pole (P-Pr), and structural trend (S-Sr) are indicated for each model. Each model predicts different patterns that can be tested by integrated structural and paleomagnetic studies. Modified from Weil et al. (2014). (B) Idealized models for development of flanking fold systems in cross-section view include: (i) detachment and buckle folding of cover rocks; (ii) fault-propagation folding related to basement reverse faults; and (iii) drape folding above steep basement faults. (C) End members for lateral propagation and linkage of fold-fault systems in map view include: (a) a single straight system for linkage of colinear segments; (b) an en echelon system with partial linkage across a saddle (relay zone) for segments with moderate S/L and associated fold width (w); and (c) separate (isolated) systems for bypass of segments with large separation distance. Idealized fold/fault shortening-length relations during propagation and variable linkage of segments also indicated, based on model of Gupta and Scholz (2000).

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strong basement, favor concentrated fault slip, whereas lower ratios, as in 2004; Yonkee and Weil, 2015). Development of the orogenic system weak sedimentary cover, favor distributed shear and folding (Allmendinger, occurred during a time of northwestward to westward drift of the North 1998). The fold-thrust model (Berg, 1962) combines early buckling of both America plate and increased rates of relative motion with the subduct- basement and cover, followed by fault propagation. Fault segments and ing Farallon and related oceanic plates. Directions of relative motion associated folds may also propagate laterally to form (a) a single, straight ranged from approximate west-east in the Early Cretaceous, to southwest- fold-fault system with along-strike linkage of colinear segments; (b) an en northeast in the Late Cretaceous to Paleogene (Doubrovine and Tarduno, echelon fold-fault system with partial linkage of segments having moderate 2008; Wright et al., 2016). A flat-slab segment likely developed along the separation distances across structural saddles that represent relay zones; Mojave sector ca. 90 Ma, and then propagated northeastward under the and/or (c) separate fold-fault systems that bypass each other for segments Laramide foreland by later Cretaceous to Paleogene time (Fig. 1; Saleeby, with large separation distances (Fig. 2C; Davis et al., 2005; Grasemann and 2003). The presence of a flat-slab segment is indicated by development of Schmalholz, 2012). Varying interactions between propagating and linking a magmatic gap along the Mojave to Sierra Nevada arc sectors (Dickinson segments also result in distinctive along-strike gradients in fault displace- and Snyder, 1978; Cross and Pilger, 1982), inboard shift of limited mag- ment and shortening (Gupta and Scholz, 2000). Seismic and drill hole data matic activity along the Colorado Mineral Belt (Chapin, 2012), forearc have revealed moderate-dipping reverse faults along steep forelimbs of disruption and underplating of the Pelona-Orocopia-Rand schists (Jacob- some flanking folds, local detachment folds, and complex structural saddles son et al., 2011), presence of eclogite xenoliths (subsequently erupted in between domes (Berg, 1962; Brown, 1988; Erslev, 1993; Stone, 1993), but volcanic centers in the Colorado Plateau) that likely represent remnants the importance and styles of buckling, basement faulting, and lateral fold- of subducted oceanic crust and altered mantle lithosphere (Smith and fault linkage remain debated (Petersen, 1983; Gay, 1999; Tiffany, 2011). Griffin, 2005), and increased Late Cretaceous subsidence followed by The Dallas–Derby–Sheep Mountain (D-D-SM) fold system, located uplift that propagated northeastward from Arizona to Wyoming (Liu et al., along the southeastern margin of the Wind River arch, is an ideal location 2010; Heller et al., 2013). Although multiple lines of evidence indicate the to test models for evolution of flanking folds because of the following: former presence of a flat slab beneath the Laramide belt, geodynamic pro- red beds of the Triassic Chugwater Group are widely exposed and carry cesses that led to thick-skin crustal deformation are debated, including end a high-fidelity record of the paleomagnetic field, providing a marker to loading along a lithospheric keel, increased basal traction, and enhanced estimate vertical-axis rotation (Van der Voo and Grubbs, 1977; Shive et asthenosphere flow (Bird, 1984; Livaccari et al., 1981; O’Driscoll et al., al., 1984); red beds typically have subtle anisotropy of magnetic suscep- 2009; Jones et al., 2011; Yonkee and Weil, 2015). Laramide deforma- tibility (AMS) fabrics that can be used to estimate layer-parallel shorten- tion partly overlapped with later phases of Sevier thin-skin thrusting, but ing (LPS) directions and compared with shortening directions estimated shortening directions differed. Regional west-southwest–east-northeast from minor faults in interlayered Triassic and Jurassic limestone beds shortening in the Laramide was at low angles to the direction of relative (Weil and Yonkee, 2012; Weil et al., 2014, 2016); structural geometry of motion between the Farallon and North American plates during the Late the fold system is well constrained by surface exposures and drill hole Cretaceous (Wright et al., 2016), supporting models of increased coupling data (Willis and Groshong, 1993; Brocka, 2007; Alward, 2010; Hilmes, during flat-slab subduction (Weil and Yonkee, 2012). In contrast, the 2014); and basement rocks are exposed nearby in the core of the Wind Wyoming salient of the Sevier belt underwent regional east-west shorten- River arch (Frost et al., 2000) and imaged geophysically along the arch ing with radial dispersion of shortening directions along arcuate thrusts, flank (Gay, 1995), allowing evaluation of basement heterogeneities and supporting wedge models with a component of topographic-driven stress weaknesses. By integrating detailed structural and paleomagnetic studies along a curved mountain front and the presence of a weak basal décolle- this paper addresses the following questions. ment (Yonkee and Weil, 2010a). 1. What are the large-scale structural geometries, internal strain patterns, The study area is located along the southern part of the northeast flank and nature of vertical-axis rotations in the D-D-SM flanking fold system? of the Wind River arch in central Wyoming (Fig. 3). The arch is ~200 km 2. How do paleostress-strain directions estimated from different data long by ~50 km wide and trends overall northwest, in detail curving from sets (minor faults, extension fractures, AMS fabrics, and previously pub- north-northwest– to west-northwest–trending from the northern to south- lished calcite twin data) compare, and how did the local stress-strain field ern ends (Blackstone, 1993a). The southwest flank of the arch is bound evolve in relation to regional Laramide patterns? by the moderately dipping (~30°–40°) Wind River thrust, which has a net 3. What combination of models best explains development of the fold slip of ~25 km and cuts an overturned, sheared limb in the sedimentary system? cover (Brown, 1988). The thrust is imaged geophysically to mid-crustal If temporal changes in shortening directions were important during (~25 km) depth (Smithson et al., 1979), but evidence for offset of the development of the fold system, then early north-trending structures Moho is lacking and the fault may flatten within the lower crust, similar should be overprinted by later west-trending structures, and vertical-axis to interpretations for other arches (Erslev, 1993; Yeck et al., 2014). Toward rotations should be limited. If transpression was important, then vertical- the northern end, the Wind River thrust branches and slip is partly trans- axis rotations should be widespread. If regional shortening was dominant, ferred onto the White Rock thrust (Fig. 3A; Mitra et al., 1988). Toward then stress-strain directions and large-scale structures should record a the southern end, the Wind River thrust loses slip eastward as shortening single, evolving deformation field. If fault propagation and fold linkage increases along the Sweetwater arch (Weil et al., 2016). The Continental were important, then forelimbs should be steeper, reverse faults should normal fault system reactivated the southern part of Wind River thrust cut basement at depth, and structural saddles between fold culminations during subsequent partial collapse of the arch. The northeast flank of the should record shortening transfer. Wind River arch has overall gentle dips (typically 10°–15°) toward the Wind River Basin, but is locally deformed by flanking folds, including GEOLOGIC BACKGROUND the D-D-SM fold system that is the focus of this study. Total shortening across the arch and flanking folds is ~25 km (Fig. 3B). The Laramide belt comprises a distinctive part of the North American Basement rocks exposed near the study area within the southern part Cordilleran orogenic system, which developed during protracted Jurassic of the Wind River arch and western part of the Sweetwater arch com- to Paleogene Andean-style subduction and terrane accretion (DeCelles, prise older (ca. 3.3–2.7 Ga) polydeformed migmatitic orthogneiss with

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110o 109o 108o

-1500 1500 A 3000 -1500 0 Owl Creek Arch 1500 0 -1500 0

1500 -6000

White Ro -1500 -4500 Wind River Basin -3000 -9000 ck thrust 0 43o 1500 A’ -3000 Triassic red bed outcrops W ind Ri 4500 -1500 Precambrian basement -7500 3000 Dallas ver thrust 0 Latest Archean granite plutons Derby 1500 -6000 Wind Sheep Mtn Calc alkaline plutons 0 River 0 3000 Supracrustal belts Arch A ne e zo tur Bridger batholith ruc Sweetwater l st Arch rai Green n T Mt. Helen structural belt rego River O Continental 0

normal 0 N Basin fault -1500 -300 Older migmatite gneiss shear zone -7500 0525 0 100km structure contour lines for top of basement (m) 42o B A Wind River Arch Wind A’ anking River Basin folds T Mzl Klm 0 km T Ku -3 km Ku Pz

-6 km Klm W Wb Mzl ind River thrust -9 km Pz Wb -12 km Wb T Paleogene -15 km Ku upper Cretaceous Klm lower/middle Cretaceous -18 km Mzl lower Mesozoic -21 km 20 km Pz Paleozoic Wb -24 km undierentiated Precambrian basement

Figure 3. (A) Simplified geologic map of the Wind River arch and adjacent basins shows exposures of different Precambrian basement rock types and shear zones (modified from Frost et al., 2000), exposures of Triassic red beds, major thrust traces, and flanking fold systems, including the Dallas–Derby–Sheep Mountain system, the focus of this study. Structure contours for top of basement (in red) illustrate three-dimensional geometry of the arch and fold systems (modified from Blackstone, 1993b). Location of cross-section line A-A′ is indicated. (B) Cross-section A-A′ across southern part of the arch illustrates large-scale structure with most shortening accommodated by slip on the Wind River thrust and limited shortening along flanking fold systems. Section incorporates seismic data from Smithson et al. (1979).

disrupted lenses of supracrustal rocks; younger (2.7–2.6 Ga) supracrustal The sedimentary cover in the study area includes a Cambrian basal belts, including the South Pass belt that has east-northeast–striking shear sandstone; an overall incompetent interval of upper Cambrian shale and zones and foliations; younger (2.7–2.6 Ga) calc-alkaline plutons with minor limestone; a competent interval of Ordovician to Mississippian variably developed foliations; Neoarchean (ca. 2.6–2.5 Ga) nonfoliated massive carbonates; Pennsylvanian to Permian interlayered sandstone, granitic plutons; and limited 2.1 Ga and 0.8 Ga mafic dikes (Frost et shale, and carbonate; Triassic to Jurassic interlayered red beds, limestone, al., 2000). The Oregon Trail structural zone, which marks a fundamen- and minor evaporites; and an overall incompetent interval of Cretaceous tal change in basement rock types and lithosphere thermal history and shale and sandstone. contains east-northeast– to east-striking shear zones, extends from the Latest Cretaceous to early Eocene synorogenic strata were deposited northern margin of the Sweetwater arch, beneath the southern part of the within the Green River Basin and Wind River Basin to the southwest and D-D-SM fold system, into the Wind River arch (Fig. 3A; Chamberlain northeast of the arch, respectively, providing key constraints on timing of et al., 2003). The protracted tectonothermal evolution of these basement large-scale deformation (Keefer, 1970; Beck et al., 1988; Steidtmann and rocks led to development of crustal weaknesses with varying orientations Middleton, 1991). Synorogenic strata within the Green River Basin record that likely influenced the style and distribution of Laramide deformation. unroofing of the arch and include the Maastrichtian Lance Formation,

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which contains clasts eroded mostly from Mesozoic to Paleozoic strata; directions were also estimated using linked Bingham distribution statistics the Paleocene Fort Union Formation, which records early influx of base- with the FaulKin software (Allmendinger et al., 2012). ment granitic detritus; and the early Eocene Wasatch Formation, which Core samples for AMS and paleomagnetic analyses were drilled from records continued erosion of granitic material (Steidtmann and Middleton, well-exposed red beds using a portable gas-powered drill and a magnetic 1991). The Paleocene depositional axis of the Green River Basin was compass for orientation. We obtained 8–12 cores from multiple beds at located slightly southwest of and roughly coincident with the curved front each site in order to average out secular variation and potential variability of the Wind River arch, consistent with a component of flexural subsidence in AMS fabrics. AMS was measured for individual cores using an AGICO (Beck et al., 1988). Along the northeast flank of the arch and into the Wind Kappabridge KLY-3 susceptibility bridge operated at a frequency of 875 Hz River Basin, the Lance Formation has dips similar to those of the older and with a sensitivity of ~2.0 × 10−8 SI. Site mean eigenvectors/eigenval- strata around folds, whereas the base of the Paleocene Fort Union Forma- ues and uncertainties were estimated from core data using tensor methods tion is marked by an angular unconformity, indicating slightly younger and bootstrap resampling following the methods of Constable and Tauxe onset of large-scale folding there (Keefer, 1970). The Eocene Wind River (1990) and Tauxe (1998). AMS is a second order tensor, [K], that relates Formation records continued folding with simultaneous exhumation of directional variability of induced sample magnetization in response to an the Wind River, Sweetwater, and Owl Creek arches. Little deformed Late applied magnetic field of varying orientation, providing a measure of over- Eocene to Oligocene strata onlap the Wind River thrust along the south- all preferred orientation of magnetic grains within a volume of rock (Bor- west flank (Steidtmann and Middleton, 1991) and overlie with angular radaile, 2001). The shape and orientation of the AMS ellipsoid are defined

unconformity folds on the northeast flank (Love, 1970), bracketing the by the three eigenvalues and eigenvectors, Kmax ≥ Kint ≥ Kmin. Because AMS end of large-scale Laramide shortening in the region. Apatite fission track measures the combined contributions from diamagnetic, paramagnetic, data from the Wind River arch provide additional constraints, with depth- and ferromagnetic minerals that may form at different times and by differ- age relations suggestive of ~3 km of exhumation and rapid cooling from ent mechanisms, care is needed to document relations of microfabrics to ca. 65 to 50 Ma (Peyton et al., 2012; Stevens et al., 2016). measured AMS ellipsoids. Detailed sampling, AMS analysis, and scanning electron microscope (SEM) imaging of multiple lithologies with varying METHODS grain size and mica content were completed for two sites. Cores were thermally demagnetized in an Analytical Service Com- Detailed structural, AMS, and paleomagnetic analyses were completed pany demagnetizer and measured with an AGICO JR-6a spinner magne- for 61 sites within red beds of the Red Peak Formation and overlying Alcova tometer in a low-field magnetic cage in order to determine characteristic Limestone of the Triassic Chugwater Group, along with structural analy- remanent magnetization (ChRM). ChRM directions were calculated for sis of 28 sites within limestone beds of the Jurassic Sundance Formation each core using principal component analysis (Kirschvink, 1980), based (Fig. 4A; Table DR11). Sites were distributed along the length of the D-D- on linear demagnetization-step trajectories extracted from NEV (north, SM fold system and gently northeast-dipping limb of the Wind River arch. east, vertical) component plots (Zijderveld, 1967). Site mean paleomag-

The Red Peak Formation, interpreted as Early Triassic based on regional netic vectors and α95 confidence cones were calculated from core data correlations, is composed mostly of subarkosic to quartzose, variably mica- using Fisher (1953) statistics. The Super IAPD2000 software package was ceous, sandstone and mudstone (Fig. 5A) that contain fine-grained hematite used for paleomagnetic data analyses (Torsvik et al., 2000). A total of 20 that formed during early diagenesis (Picard, 1965; Weil and Yonkee, 2012). cores were also selected for rock magnetic experiments to characterize The Alcova Limestone, interpreted as latest Early Triassic to Middle Trias- remanence carrying mineral fractions and evaluate contributions of fer- sic based on 87Sr/86Sr ratios (Lovelace and Doebbert, 2015), is composed of romagnetic and paramagnetic minerals to susceptibility. Specimens were laminated, micritic limestone that forms a distinctive 1–5-m-thick marker chosen to represent a range of AMS fabric types and provide broad spatial unit. The Middle Jurassic Sundance Formation is composed of interbedded coverage. Acquisition of isothermal remanent magnetization (IRM) was marine sandstone, mudstone, and oolitic to bioclastic limestone. measured on 10 specimens using a Princeton Measurements Corporation At each site, detailed structural measurements were made of bed- MicroMag vibrating sample magnetometer in fields to 2.0 T. Three-axis ding, fracture sets, and minor faults (best developed within the Alcova thermal demagnetization of IRM was performed on 10 additional speci- Limestone and limestone beds of the Sundance Formation). Site mean mens. Applied fields of 2.0, 0.5, and 0.15 T were imparted along the three

orientations and α95 confidence cones of poles to bedding and fracture mutually perpendicular axes of each core (Lowrie, 1990) using a Magnetic sets were calculated using Fisher (1953) statistics. Idealized maximum, Instruments pulse magnetizer. All magnetic experiments were carried out

intermediate, and minimum (σ1, σ2, and σ3) stress vectors were estimated in the Paleomagnetic Laboratory at Bryn Mawr College (Pennsylvania). for minor faults using the method of Compton (1966) and a 25° fracture

angle, with site mean stress directions and α95 confidence ellipses calcu- RESULTS lated using the orientation tensor. Because this method does not account for potential reactivation of preexisting fractures, a stress inversion method Large-Scale Structural Geometry was also used that searched for the combination of principal stress direc-

tions and stress ratio, ϕ = (σ2 – σ3)/(σ1 – σ2), that minimized the absolute The D-D-SM fold system comprises a series of left-stepping, dou- value of angular misfit between observed slip lineations and directions bly plunging anticlines, which share a gently dipping backlimb, have of maximum resolved shear stress on the fault planes. Principal strain segmented, southwest-verging forelimbs cut by reverse faults, and are connected across structural saddles (relay zones) (Fig. 4A). A syncline 1 GSA Data Repository Item 2017299, Figure DR1: Examples of anisotropy of separates the fold system from the gently northeast-dipping homoclinal magnetic susceptibility (AMS) and rock magnetic characteristics for different limb of the Wind River arch. A series of cross sections across the system lithologies; Table DR1: Site location and structural data for Dallas–Derby–Sheep provide a framework to evaluate fold-fault relations, internal strain pat- Mountain area, Wyoming; Table DR2: Anisotropy of magnetic susceptibility data terns, and vertical-axis rotations (Figs. 4B–4F). for Dallas–Derby–Sheep Mountain area, Wyoming; Table DR3: Paleomagnetic data for Dallas–Derby–Sheep Mountain area, Wyoming, is available at http://www Dallas Dome, the site of the first producing oil well in Wyoming .geosociety.org/datarepository/2017, or on request from [email protected]. (Krampert, 1948), has an amplitude of 1.0 km with Triassic strata exposed

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W108.70o W108.60o W108.50o W108.40o A Q Quaternary T Paleogene N N42.80 Ku Upper Cretaceous Lower Cretaceous o T Kl Ku J Jurassic CH87 Dallas Jsm Sundance-Morrison Fms CH86 Dome Jn Nugget Fm G1 CH88 CH83 CH78 B2 CH84 Tr Triassic CH79 Backlimb CH81 CH82 thrust Pzu Permo-Pennsylvanian Kl Tr CH80 CH85 SD1221 SD1219 Pzl Cambrian-Mississippian J B1 CH283 C2 J SD1217 CH114 Wb Archean basement SD1303 SD1224 SD1222 SD1223 SD1216 CH115 SD1214 Derby Fault trace SD1218 U-upthrown Fold trace C1 SD1215 Dome D-downthrown N42.70 Tr CH74 D2 CH75 CH76 SD1302 CH35 Sample Site CH77 Sundance Formation

CH36 o CH117 CH40 CH38 Chugwater Group CH37 SD1213 D1 CH39 CH72 010 km CH71 CH73 Carr Reservoir SD1211 SD1209 thrust SD1212 SD1202 E2 SD1301 SD1206 SD1205 SD1210 CH116 CH52 SD1203 SD1208 CH53 CH51 Red CH54 Ku Pzu E1 Blu Sheep SD1207 CH41 CH56 Tr03 Mountain CH55 SD1204 CH70 Anticline Tr04 1-4 CH57 F2 Kl CH69 Pzl CH311 Tr01 CH58 N42.60 Tr02 CH59 J SD12 CH68 F1 CH62 SD1304 01 CH42 Tr 1-4 CH60 CH63 CH61 o CH312 T Pzu CH44 CH65 CH64 CH66 D Clear Creek U CH67 fault CH119 Wb Q Wb CH118 G2 Pzl Schoetlin Anticline

B B1 Dallas Dome backlimb B2 thrust 2000 m Ku Ku Kl Kl Jsm Jsm Jn Jn Tr 1000 m Tr Pzu Pzu Pzl Pzl Sea Level Wb Wb 0 2 km Figure 4 (on this and following page). C1 C2 C Dallas-Derby backlimb 2000 m structural saddle thrust

Ku Ku Kl Jsm Kl Jn Jsm 1000 m Jn Tr Tr Pzu Pzu Pzl Sea Level Pzl Wb Wb 0 2 km

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D D1 Derby Dome D2 2000 m

Ku Kl Jsm Kl back backlimb Jsm thrust Jn Jn thrust 1000 m Tr Tr Pzu Pzu Pzl Sea Pzl Level Wb Wb 0 2 km

E E1 Derby-Sheep Mountain E2 structural saddle Carr Reservoir 2000 m thrust Jsm Kl Ku N Jn Kl Tr Jsm Jn * Carr Reservoir * thrust 1000 m Tr Pzu ** bedding Pzu Pzl

Pzl Sea Wb Level WF1 WF2 Wb 0 2 km

minor fold axis * tectonic stylolite pole F F1 Sheep Mountain Anticline F2 2000 m Jsm Jn Kl back Jsm Tr Jn thrust Pzu Tr 1000 m Pzu Pzl

Pzl Wb

Sea Level Wb

0 2 km

G Total shortening B1 C1 D1 E1 F1 large-scale fold-fault shortening internal shortening

shrotening Dallas Dome Derby Dome Sheep Mountain Anticline Total

1 km north south culmination culmination backlimb thrust Clear Creek G1 4 km along-strike distance G2

Figure 4 (continued). (A) Geologic map of Dallas–Derby–Sheep Mountain fold system shows main structural features and site locations in Triassic Chugwater Group and Jurassic Sundance Formation. Locations of cross-section lines B1-B2 to F1-F2, and along-strike profile G1-G2 are indicated. U—upthrown; D—downthrown. (B–F) Cross-sections B1-B2 to F1-F2 illustrate structural geometry of fold culminations and structural saddles. Folds have steeper forelimbs that developed in triangular regions above reverse faults that cut basement. Additional shortening was accommodated by backlimb thrusts and top-to-the-northeast backthrusts. Sec- tions incorporate interpretations of Willis and Groshong (1993), Brocka (2007), Alward (2010), and Hilmes (2014). Stereogram in E illustrates kinematics of minor faults and folds associated with the Carr Reservoir (Res) thrust that record west-southwest–oriented shortening. WF1, WF2 are conjugate wedge fault sets. (G) Along-strike profile G1-G2 shows lateral variations in estimated large-scale fold-fault shortening, internal shortening from backlimb thrusts and detachment folds, and total shortening at the level of the Triassic Alcova Limestone. Locations of cross-section lines are indicated. Large-scale shortening between section lines is estimated using forelimb dip and amplitude. Large-scale shortening decreases, internal shortening increases, and total shortening slightly decreases across structural saddles (relay zones). Lateral gradients in large-scale shortening are correlated with fold plunge.

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A Jgs Trcp B Jn

Trcp e Trca S0

Trcr cross-strik

high-angle W E C D

wedge faults

Figure 5. (A) View to north of Red Peak Formation (Trcr), Alcova Limestone ridge (Trca), and Popo Agie Formation (Trcp) of Triassic Chugwater Group and overlying Jurassic Nugget Formation (Jn) and Gypsum Spring Formation (Jsg) exposed along Derby Dome. Note repeat of Triassic strata along the backlimb thrust. (B) Fracture patterns in Alcova Limestone define two dominant sets: a cross-

strike set subperpendicular to the strike of bedding (S0) and a high-angle set subparallel to bed strike. (C) Wedge fault in the Alcova Limestone, with steps in calcite fibers giving sense of slip (black arrows). (D) Wedge faults in limestone of the Sundance Formation.

in the fold culmination, and accommodated 0.9 km of shortening (Fig. interpreted to offset basement at depth, and by a top-to-the-northeast 4B). The fold has an axial length of ~10 km, trends overall 340°/160°, backthrust that repeats Paleozoic to Triassic strata toward the fold hinge plunges ~10° northward along its northern end, and curves and plunges (Willis and Groshong, 1993). The backlimb dips 10°–15°NE and is cut 20° toward 140° at its southern end. The fold has a steep forelimb that by the backlimb thrust, which loses slip southward. dips 50°–90°SW and is cut by two top-to-the-southwest reverse faults that The Derby–Sheep Mountain saddle (relay zone) is marked by a ~2.5 offset basement at depth (Willis and Groshong, 1993). The backlimb dips km left step and curvature in fold trend. Fold amplitude is less (0.8 km), ~10°–15°NE and is locally cut by a backlimb thrust, interpreted to sole into with Jurassic to Cretaceous strata exposed across the saddle. In detail, Triassic strata. Small-displacement, east-striking normal faults accom- two anticlinal warps are present (Fig. 4E), which represent the tips of modated limited fold axis–parallel extension near the fold culmination. the Derby and Sheep Mountain folds. The forelimb dips 20°–40°SW and The Dallas-Derby saddle (relay zone) is marked by a ~1.5 km left bends in strike to 300°/120°. Additional shortening was accommodated step and curvature in fold trend. Fold amplitude is less (0.8 km), with by the northwest- to west-northwest–striking Carr Reservoir thrust that Jurassic to Cretaceous strata exposed across the saddle. The forelimb has top-to-the-west-southwest slip and associated northwest-trending dips 30°–50°SW and bends in strike to 310°/130°. Shortening across minor folds, which were tilted along the northern plunge of the Sheep the forelimb is smaller compared to Dallas Dome, whereas slip on the Mountain anticline; complex detachment folds of Cretaceous strata with backlimb thrust is greater (Fig. 4C). The forelimb is locally cut by a steep, northwest- and northeast-trending axes interpreted to reflect crowding east-striking fault with sinistral offset. This fault appears to partly cut and within the saddle; and the poorly exposed Red Bluff fault that thickens partly transfer slip onto forelimb reverse faults, and loses slip eastward Triassic strata. This fault may have top-to-the-west-southwest slip based toward the hinge zone of the saddle (Brocka, 2007). This fault lines up on kinematics of nearby minor faults. In comparison, Tiffany (2011) with an east-striking fault in the Wind River arch, and may reflect reac- interpreted the Red Bluff fault and northeast-trending folds to record an tivation of a basement weakness. episode of late north-south shortening. Derby Dome, the site of another oil field, has an amplitude of 1.0 km Sheep Mountain anticline has an amplitude of 1.3 km with Pennsylva- with Triassic strata exposed in the fold culmination, and accommodated nian strata exposed in its core, and accommodated 1.0 km of shortening 0.9 km of shortening (Fig. 4D). The fold has an axial length of ~8 km, (Fig. 4F). The fold has a total axial length of ~13 km, contains 2 culmi- trends overall 340°/160°, plunges ~10° northward along its northern end, nations linked across a saddle marked by a 1 km left step, and ranges in and curves and plunges 15° toward 150° at its southern end. The forelimb trend from 350°/170° to 310°/130° from the north-central to southern dips 40°–70°SW and is locally cut by top-to-the-southwest reverse faults parts. The fold plunges ~10° northward along its northern end, and the

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southern end terminates against the steep, east-northeast–striking Clear north-northwest–striking set of fractures subparallel to fold trend and at Creek fault that has left-lateral reverse slip (Abercrombie, 1989). This high angles to bedding (termed the high-angle set; Fig. 5B). Some sites fault continues to the west-southwest into the Wind River arch, subparal- displayed a set of east-southeast–striking extension fractures, and some lel to the Oregon Trail structural zone, and likely reflects reactivation of sites had additional oblique-striking fractures. Most sites in the Alcova a basement weakness. The forelimb dips 40°–60°SW and is interpreted Limestone and limestone beds of the Sundance Formation displayed to be cut by top-to-southwest reverse faults that offset basement at depth, minor conjugate reverse-slip (wedge) faults at acute angles (20°–30°) to based on fold asymmetry similar to that of Derby Dome. A top-to-the- bedding (Figs. 5C, 5D). Some sites also had minor conjugate strike-slip northeast backthrust repeats Paleozoic to Triassic strata toward the fold (wrench) faults at high angles to bedding with slip lineations subparallel hinge zone, and curves along the left step into an area of complex minor to the intersection of faults with bedding. Minor wedge and wrench faults folding within the forelimb. The south end of the northern culmination displayed consistent kinematic relations with respect to bedding around

plunges as much as ~30° southward along the left step. the large-scale anticlines and yielded best-fitσ 1 directions subparallel to The fold system displays systematic lateral variations in large-scale slightly less steep than bedding (Fig. 6), indicating minor faults formed fold-fault shortening concentrated along the forelimb and internal short- during early LPS, prior to, and during initiation of large-scale folding, ening by backlimb thrusts and minor folds (Fig. 4G). Changes in fold similar to interpretations of other studies (Erslev and Koenig, 2009; Weil plunge that record lateral gradients in large-scale shortening and varying et al., 2014, 2016). Magnitudes of LPS, based on minor fault spacing and styles of linkage are partly related to separation distances across saddles slip, were small (<5%). Rare minor folds and tectonic stylolites accom- that represent relay zones. The saddle between the two culminations of the modated limited additional LPS. Thin cross-strike veins and conjugate Sheep Mountain anticline has smaller separation (~1 km left step), shorter wrench faults accommodated limited (<2%) tangential (strike parallel) overlap (<2 km), increased fold plunge (as much as 30° toward the south extension. Paleostress-strain directions were estimated from minor faults end of the northern culmination), and consistent large-scale shortening. for 41 sites in the Alcova Limestone and for 25 sites in the Sundance For- The Dallas-Derby saddle has moderate separation (~1.5 km left step), hard mation. Other sites had too few (<5) measured minor faults for analysis. linkage along a steep, east-striking fault, increased fold plunge (as much as Paleostress and strain methods yielded nearly identical orientations of

20° toward the south end of Dallas Dome), and a decrease in large-scale principal compression (σ1) and shortening (ε3) directions at the site level. shortening partly balanced by increased internal shortening. The Derby– LPS directions obtained from the method of Compton (1966) had smaller Sheep Mountain saddle has larger separation (~2.5 km left step), longer uncertainties (typically <5° at the 1σ level) and are reported in Table DR1.

overlap (~4 km) with soft linkage distributed across minor folds and faults, Estimated stress ratios, ϕ = (σ2 – σ3)/(σ1 – σ3), were low (<0.2) for most slightly increased fold plunge (as much as 15° toward the south end of sites, consistent with development of both wedge and wrench faults that

Derby Dome), and a decrease in large-scale shortening along with a broad record switching of the σ2 and σ3 axes during early LPS and tangential zone of increased internal shortening. These varying patterns are interpreted extension. Some reverse faults within steep fold limbs cut bedding at to record early linkage of closer spaced basement faults between the two moderate to high angles and likely formed as bedding was tilted during culminations in the Sheep Mountain anticline; cross-strike linkage of mod- early to late stages of folding. Some strike-slip faults with gently raking erately separated basement faults along an east-striking basement weakness lineations also likely formed during folding. Widely spaced faults that across the Dallas-Derby saddle; and more distributed linkage between developed during continued deformation along steep forelimbs typically wider separated basement faults across the Derby–Sheep Mountain saddle. have poorly developed slip lineations.

Internal Strain Patterns of Fracture Sets and Minor Faults AMS Fabrics

Most sites in the study area had two dominant fracture sets in both Most sites from the study area had measureable AMS Kmax lineations, red beds and limestone: a northeast- to east-northeast–striking set of even though Triassic red beds typically lacked mesoscopic evidence of

extension fractures and thin (<1 cm thick) calcite-filled veins subper- internal strain. Samples had low mean susceptibility [Km = (Kmax + Kint + −5 pendicular to fold trend (termed the cross-strike set); and a northwest- to Kmin)/3], with most core and site Km values between 2 × 10 SI and 12

A WSW view to NNW back ENE

limb thrust Figure 6. Example minor fault and fracture, Js anisotropy of magnetic susceptibility (AMS), Jn and paleomagnetic data sets for two sites from the Alcova Limestone and adjacent red beds Js located on the forelimb (site CH40) and backlimb Trcp Jn (site CH36) of Derby Dome illustrate relative timing of deformation. (A) Oblique Google Earth Trca image looking northwest along crest of Derby Trcr Dome shows site locations, Alcova Limestone CH40 (Trca) that forms a resistant ridge (dashed line) CH36 separating Red Peak (Trcr) and Popo Agie (Trcp) formations, contact between Jurassic Nugget Js (Jn) and Sundance (Js) formations (dot-dash Trca line), and trace of backlimb thrust. (Continued on following page.)

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× 10−5 SI (Fig. 7A; Table DR2), typical for susceptibility dominated by values provided a crude proxy for deformation intensity and, following paramagnetic minerals. AMS ellipsoids had oblate to triaxial shapes, with Weil and Yonkee (2009), were divided into three types: type 1 with a

foliation values (F = Kint/Kmin) mostly from 1.020 to 1.080, and lineation dominant sedimentary fabric, L < 1.003, and >30° uncertainty in Kmax

values (L = Kmax/Kint) mostly from 1.002 to 1.020 (Fig. 7B). AMS ellip- trend; type 2 with a weak LPS fabric, 1.003 ≤ L ≤ 1.010, and a 10°–30°

soid shapes reflect a combination of primary sedimentary fabrics with uncertainty in Kmax trend; and type 3 with a distinct LPS fabric, L > 1.010,

magnetic foliation subparallel to bedding, and tectonic LPS fabrics that and <15° uncertainty in Kmax trend (Fig. 7C). Of the 61 sites in the study

give rise to Kmax lineations. SEM analysis showed preferred alignment of area, 5 sites had a dominant bedding fabric (type 1), 46 sites had a weak

phyllosilicates grains (biotite, chlorite, muscovite that are paramagnetic) Kmax lineation (type 2), and 10 sites had a distinct Kmax lineation (type along bedding, and microkinking of some grains that defines a zone axis 3). Detailed analysis of AMS fabrics for multiple lithologies sampled at

subparallel to measured Kmax lineations (Figs. 7D, 7E), similar to find- two sites revealed broadly similar AMS fabrics between samples, with ings by Weil and Yonkee (2009) and Weil et al. (2014, 2016). Lineation a tendency for better bedded, more micaceous layers to have stronger

in situ fault in situ AMS N in situ fault N in situ AMS B ST 335 N CH40 N CH40 ST 330 CH36 CH36 Bedding Bedd ing

K Kmax max K Kint int Wrd K Kmin σ min σ1= 26, 062 1= 14, 249 WF2 φ φ = 0.01 = 0.10 WF1 WF1 Wrs WF2

Be d Bedding ding Coulomb σ1 Coulomb σ1 Stress Inv σ1 Stress Inv σ1 Strain ε3 cross-strike fract pole Strain ε3 cross-strike fract pole restored fault restored AMS restored fault restored AMS C ST 335 N CH40 N CH40 ST 330 N CH36 N CH36

Kmax Kmax Kint Kint WF2 K WF2 K Wrd min min

WF1 Wrs WF1

LPS LPS

Coulumb σ1 -03, 247 Coulumb σ1 10, 242 Stress Inv σ1 -02, 247 Stress Inv σ1 06, 239 Strain ε3 -07, 249 Strain ε3 09, 243 Tr - reference N restored pmag Tr - reference N restored pmag D direction CH40 direction CH36

lower hemisphere lower hemisphere upper hemisphere upper hemisphere

Tr - reference Tr - reference direction direction Figure 6 (continued). (B) In situ data sets. Equal-area stereograms of minor fault data show conjugate wedge fault sets (WF1 with top to east-northeast

slip and WF2 with top to west-southwest slip) and wrench fault sets (WRs and WRd), slip lineations, σ1 estimated for idealized Coulomb failure of indi-

vidual faults and mean direction (circles), best-fit σ1 (star) estimated from stress inversion (Inv), and ε3 strain direction (hexagon) estimated from linked

Bingham distribution. Poles to cross-strike extension fractures (fract; squares) are also shown. Equal-area stereograms of AMS data show Kmax, Kint,

and Kmin axes for individual cores and site mean directions (see text). Minor fault and AMS fabrics display consistent relations with respect to bedding

across the fold. (C) Tilt-restored data sets. Minor fault data record west-southwest–east-northeast shortening subparallel to bedding. AMS Kmax lineations, as well as poles to cross fractures, are subparallel to structural trend and are consistent with LPS (layer-parallel shortening) directions estimated from minor faults. (D) Tilt-restored paleomagnetic data. Equal-area stereograms of paleomagnetic (pmag) vectors for individual cores (circles) and site

mean vector (star) and α95 confidence cones indicate limited vertical-axis rotation with respect to the Triassic (Tr) reference declination.

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A B 14 1.060 12 Triaxial

s 10 in t 1.040 8 /K max Prolat e

# of site 6

4 L = K 1.020 2 Oblate 0 1.000 246810 12 14 16 18 1.0001.020 1.0401.060 1.0801.100 -5 Mean Susceptibility (x 10 SI) F = Kint/Kmin Fabric type 1- Fabric type 2- Fabric type 3- C sedimentary weak K lineation distinct K lineation max max Structural trend

Kmin

Kint

Kmax CH79 CH81 CH55

1.06 1.06 1.06

L L L

1.00 F 1.10 1.00 F 1.10 1.00 F 1.10 DE

Bt Kf Qt

Mt Ms Qt

050 100 µm

Figure 7. Anisotropy of magnetic susceptibility (AMS) characteristics. (A) Histogram shows site average values of mean susceptibility

(Km). (B) Flinn diagram of site average values of foliation (F = Kint/Kmin) and lineation (L = Kmax/Kint). AMS ellipsoid shapes vary from distinctly oblate to triaxial. (C) Equal-area stereograms and Flinn diagrams show data for individual cores at sites representative of different AMS fabric types. Stereograms are for bedding restored to horizontal. Sites with type 1 fabrics have distinctly oblate

AMS ellipsoids related to sedimentary bedding fabric and lack Kmax lineation. Sites with type 2 fabrics have moderately oblate AMS

ellipsoids with a weak, but definable maxK lineation. Sites with type 3 fabrics have triaxial AMS ellipsoids with a well-defined maxK

lineation. Kmin axes (filled circles) are subperpendicular to bedding. maxK axes (filled squares) are subparallel to structural trend (gray arrows) for type 2 and 3 fabrics, related to intersection of bedding with weak LPS fabrics. (D, E) Scanning electron microscope images

of microkinked mica grains in red beds. The average zone axis of kinked micas is parallel to the AMS Kmax lineation. Bedding trace is subhorizontal in images. Bt—biotite, Ms—muscovite, Qt—detrital quartz grains, Kf—detrital K-feldspar grain, Mt—magnetite.

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AMS foliations and lineations, whereas the presence of minor magnetite trend. Several sites in fold forelimbs contained additional minor faults produced more variable AMS fabrics (Fig. DR1). Samples from intervals that formed during continued fold growth, with shortening directions of

with varying orientations of cross-beds and ripples showed similar Kmax ~210°–260°, subperpendicular to fold trend. orientations, confirming that AMS lineations are related to secondary LPS The dominant cross-strike extensional fracture and vein set also dis- fabrics rather than to primary current lineations. played consistent directions across the study area for both limestone and red beds, with site mean strikes ranging from 208° to 268° in Triassic Paleomagnetic and Rock Magnetic Characteristics red beds, from 210° to 260° in the Alcova Limestone, and from 216° to 268° in limestone beds in the Sundance Formation (Table DR1; Fig. 9B), ChRM vectors were determined for 56 of 61 sites in red beds across the with respective averages of 241°, 241°, and 243°, which are subparallel study area (Table DR3). Two magnetization components were observed: a to LPS directions estimated from minor fault data and AMS fabrics. The low-temperature unblocking component removed upon heating to 500 °C cross-strike set is interpreted to have accommodated tangential exten- and a high-temperature characteristic component that mostly unblocked sion mostly during early LPS, and partly during later fold axis–parallel from 600 to 680 °C (Fig. 8A). In the majority of core samples, the high- extension associated with three-dimensional (3-D) fold growth. Some temperature component decayed linearly during progressive thermal sites had an east-southeast–striking fracture set, best developed near the demagnetization and provided a stable direction for analysis. Of the 56 Clear Creek fault. Similarly oriented fractures in parts of the Big Horn sites with measured ChRM, 13 sites recorded mixed polarities, 25 sites Basin were interpreted to have formed during early west-northwest–east- recorded reverse polarity, and 18 sites recorded normal polarity magne- southeast compression prior to Laramide deformation (Bellahsen et al., tism. Paleomagnetic analysis was not completed for 3 sites due to disin- 2006; Amrouch et al., 2010; Beaudoin et al., 2012). The tectonic signifi- tegration of cores during sample preparation or spurious demagnetization cance of these fractures and comparison to calcite twin strain data are behavior, and 2 sites displayed high dispersion of core vectors with >15° further explored in the Discussion.

α95 cones and were not used for further analysis. Vertical-axis rotation estimated from ChRM declinations restored for A tilt test was completed to evaluate timing of ChRM acquisition. For bed tilt was statistically insignificant at most sites along the backlimb of the test, fold plunge was first removed and then bedding was progressively the fold system. Ten sites located near the east-northeast–striking Clear untilted toward horizontal. In situ ChRM vectors displayed moderate Creek fault and along more west-trending parts of fold forelimbs recorded dispersion, whereas restored ChRM vectors showed peak clustering at local counterclockwise rotations (Fig. 9C). ~100% untilting, indicating that the ChRM was acquired prior to folding (Fig. 8B). Structurally untilted vectors had gentle inclinations that ranged Strike Tests mostly from 0° to 30° with a mean inclination of 14°, consistent with the Triassic paleolatitude of Wyoming (calculated using the North America Correlations between estimated LPS directions and structural trend polar wander curve of Domeier et al., 2011) and minor (~5°) inclination were evaluated using the strike-test method of Yonkee and Weil (2010b). flattening related to sediment compaction. Of the 56 sites with measur- This method uses a weighted least-squares approach, and incorporates able ChRM, 44 sites lacked statistically significant differences between measurement uncertainty in estimated LPS directions (typical 1σ of ~5° observed declination and expected declination for the Triassic paleopole, for minor fault data and ~10° for AMS fabrics) plus random dispersion 10 sites had statistically significant counterclockwise vertical-axis rotation, from local stress-strain refraction and AMS fabric variability (taken as 4° and 2 sites had clockwise rotation. and 6°, respectively, similar to values of Yonkee and Weil, 2010b). Site IRM acquisition showed continuous magnetization acquisition to 2.5 structural trend was estimated from a combination of fold axis trend and T with convex-upward curves, indicating the presence of a high-coercivity local bed strike (corrected for fold plunge), with a typical uncertainty of magnetic mineral phase, likely hematite (Fig. 8C). Thermal demagnetiza- 5°. For strike tests, LPS directions relative to a regional shortening direction of three-axis IRM showed consistent slow decay in intensity for all tion of 240° were correlated with site structural trend relative to a regional three components to as much as 680° (Fig. 8D), confirming that hematite trend of 330°. A best-fit slope of 1.0 indicates that LPS directions are was the main carrier of remanence in red beds. Most of the remanence was perpendicular to structural trend, whereas a slope of 0.0 indicates that LPS carried along the axis imparted with a 2.0 T field. Some cores displayed directions are parallel to the regional shortening direction regardless of a minor dip in intensity for the 0.15 T field at ~560 °C, consistent with structural trend. Strike tests for combined minor fault data and AMS fab- the presence of minor magnetite. rics gave a slope of 0.6 ± 0.1 (at a 95% confidence level) for sites located along the forelimbs, and a slope of 0.9 ± 0.2 for sites located along the Spatial Patterns of LPS Directions and Vertical-Axis Rotations backlimb to hinge zone of the fold system (Fig. 10A), indicating that LPS directions were partly refracted with changes in structural trend along LPS directions estimated from both minor fault data and AMS fab- arcuate forelimbs and partly dispersed about the regional west-southwest rics, restored for bed tilt, displayed consistent relations across the study shortening direction along the backlimb. The mean square of weighted area. Within the backlimb to hinge of the fold system and the homoclinal deviates (MSWD), incorporating dispersion from nonsystematic stress- northeast limb of the Wind River arch, site shortening directions estimated strain refraction, was close to 1. Correlation of LPS directions estimated from minor faults ranged from 223° to 254° in the Alcova Limestone from AMS fabrics with directions estimated from minor fault data yielded (Chugwater Group) and from 231° to 253° in the Sundance Formation, a slope of 0.9 ± 0.1 (Fig. 10C) with an MSWD close to 1, confirming that with an average of 241° (Fig. 9A). Shortening directions estimated from AMS fabrics provided a robust estimate of LPS directions. AMS fabrics in red beds gave consistent estimated shortening directions, Correlations between paleomagnetic declination and structural trend ranging from 204° to 257° with an average of 237°. LPS directions were were also evaluated using the strike-test method, incorporating measure- subparallel to the average regional Laramide shortening direction of 240° ment uncertainty (typical 1σ of ~6° for paleomagnetic data) plus random (Erslev and Koenig, 2009; Weil and Yonkee, 2012). Within fold forelimbs, dispersion from nonsystematic local block rotation (taken as 6°). For this minor faults were slightly better developed and estimated LPS directions strike test, tilt-corrected paleomagnetic declinations relative to a reference ranged from 221° to 268°, partly related to map-view curvature of fold declination of 336° for the Early Triassic were correlated with site structural

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North,Up North,Up North,Up North,Up North,Up A Figure 8. Paleomagnetic characteristics. (A) Representative thermal demagnetization diagrams for in situ oriented core samples of red beds from the Chugwater Group. In each CH44Ea diagram, endpoints of core magne- West o 680 West o tization vectors are projected onto CH35Ga o 680 CH40Ga 665 o the horizontal plane (solid squares) o CH71Ia 680 665 West o o o and the vertical plane (open squares) 665 680 580 o o o West CH63Da 680 with demagnetization steps indi- 580 580 o West o 665 o cated in °C; tick marks on both axes 580 580 are 1.0 mA/m. All diagrams show lin- o inclination o 25 o o 25 o ear trends over temperature steps 25 declination 25 25 from 600 to 680 °C that record a North,Up North,Up North,Up North,Up North,Up characteristic remanence carried by o o hematite. Samples display normal 580 25o 665 o 25 o o and reverse polarity. (B) Equal-area o o 665o 580 25 25 665 o o o stereograms show in situ (on left) 680 680 665 o o West and tilt-corrected (bedding restored 665 680 CH57Ba o o West o 680 to horizontal, middle) site mean 680 West West CH82Ba 25 CH114Ba West CH61Fb paleomagnetic vectors (open circles CH39Ia indicate upper hemisphere; closed circles indicate lower hemisphere). Tilt test plot (on right) shows values of the τ1 clustering parameter (normalized principal eigenvalue of the orientation tensor) of site mean vectors for progressive incre- ments of untilting (black line for North 1.0 original data, dashed gray lines B * for bootstrap parasets). Maximum bootstrap 0.8 original para-samples clustering of original paleomag- data netic vectors is at ~100% untilting

original data (star), consistent with a pre-folding 100% tilt max clustering in situ e v magnetization acquisition. Cumula- corrected 0.6 r u c tive distribution curve for untilting n

o i τ 1 t values that maximizes clustering o u Dec = 155 b o 0.4 i parameter of bootstrap parasets Dec = 152 r o t Inc = -16 o s Inc = -14 i indicates a 95% confidence interval d k = 24.0 k = 30.1 e v N = 53 i of 37%–119% untilting. Dec—decli- N = 53 t la 0.2 u nation; Inc—inclination. (C) Plot of m α95 m 37-119 % u isothermal remanent magnetization c untiltng (IRM) acquisition for typical red bed 0.0 0204060 80 100120 samples shows acquisition of IRM % Untilting to applied fields >2.5 T, consistent CD1.0 with the presence of high coerciv- 1.0 ity hematite. (D) Typical diagram for 0.8 Z - 2.00 T thermal demagnetization of three- 0.8 component IRM acquired in fields of Y - 0.5 T 0.6 2.0 T, 0.5 T, and 0.15 T, following the 0.6 X - 0.15 T

ed Intensity approach of Lowrie (1990). Samples ed Intensity 0.4 0.4 display consistent slow decay in intensity during heating to as much 0.2 Normaliz 0.2 as 680 °C, confirming dominance of Normaliz hematite, with minor dip in intensity of the 0.15 T field at ~560 °C consis- 0500 1000 1500 2000 2500 0100 200300 400500 600700 tent with minor magnetite. Temperature (°C) Applied Field (mT)

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W108.70o W108.60o W108.50o W108.40o W108.30o A LPS direction from minor Q Quaternary faults - Chugwater Group T Paleogene N N42.80 N LPS direction from minor Ku Upper Cretaceous

faults - Sundance Fm. Kl Lower Cretaceous o J LPS direction from AMS Jurassic Tr Triassic Dallas Sedimentary AMS fabric Pzu upper Paleozoic Dome Pzl Cambrian-Devonian Tr Wb Archean basement J Anticline Ku Fault

Kl Derby T

Dome N42.70 J Figure 9. Spatial patterns of estimated shortening direc- Tr 052.5 10 km o tions and vertical axis rotations. (A) Simplified geologic map of study area with tilt-corrected Ku (bedding restored to horizontal) Pzu LPS (layer-parallel shortening) directions estimated from Sheep Mountain minor fault data in the Alcova Anticline Kl Limestone (red arrows), anisotropy of magnetic susceptibility Pzl J N42.6 0 (AMS) fabrics in red beds (white T bars), and minor fault data in

o the Sundance Formation (blue Tr arrows). LPS directions dis- Pzu play consistent patterns and are at high angles to structural Wb Wb Q Pzl trend defined by fold traces. (B) Simplified geologic map with tilt-corrected site-mean strikes of fracture (fract) and W108.70o W108.60o W108.50o W108.40o W108.30o vein sets for Alcova Limestone, red beds, and Sundance For- B Cross strike fract set Chugwater Gp mation. Most sites display an limestone N42.80 east-northeast–striking exten- N redbeds sion fracture/vein set that Cross strike fract set Sundance Fm o formed partly during early limestone LPS. Some sites, mostly near ESE fract set the Clear Creek fault, display Dallas limestone an east-southeast–striking Dome red beds fracture set that may record Tr LPS from calcite twin strain pre-Laramide stress directions. J WSW-oriented (Laramide) Shortening directions for cal- Ku ESE-oriented (pre-Laramide) cite twin strain in limestone reported by Willis and Gro- Kl Derby T

Dome SSE-oriented (fold parallel) N42.70 shong (1993) and Craddock and J Relle (2003) are also indicated

Tr 052.5 10 km o (directions for vein samples are not shown). (Continued on following page.)

Ku Pzu

Sheep Mountain Anticline Kl

Pzl N42.60

T o Tr

Pzu

Wb Wb Q Pzl

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W108.70o W108.60o W108.50o W108.40o W108.30o

C Paleomagnetic declination with N42.80 respect to reference direction. Cone is 95% con dence interval. o

site showing statistically signi cant rotation Dallas site showing no Dome signi cant rotation Figure 9 (continued). (C) Google Earth image of study area with tilt-corrected paleomagnetic declinations. Black arrows 052.5 10 km indicate site mean declination Derby relative to the reference decli- N42.70 Dome nation for the North America Early Triassic paleopole (i.e., a o N site with 0° vertical-axis rotation is represented by a line trending due north). Cones represent individual site mean 95% confidence intervals (red/ yellow cones indicate sta- Sheep Mountain tistically significant clockwise/ Anticline counterclockwise vertical-axis rotations). N42.60 o

Clear Creek fault

trend relative to a regional trend of 330° (Fig. 10B). Paleomagnetic decli- Maastrichtian and continued into the early Eocene, with most shortening nations were uncorrelated with structural trend for sites located along the from concentrated slip on the Wind River thrust (Smithson et al., 1979). backlimb with an estimated slope of 0.1 ± 0.2, but were correlated with Additional shortening was accommodated by folds on the northeast flank structural trend along arcuate forelimbs with an estimated slope of 0.9 ± of the arch that developed mostly during the Paleocene to early Eocene 0.3 and a statistically significant negative intercept of −10°, consistent with (Keefer, 1970), including the D-D-SM fold system. The system comprises limited counterclockwise rotation, partly related to sites near the Clear a series of doubly plunging, left-stepping anticlines, which are connected Creek fault. across structural saddles (relay zones). Surface geologic and drill-hole data indicate the anticlines have moderately to steeply southwest-dipping DISCUSSION forelimbs cut by reverse faults that offset basement at depth (Willis and Groshong, 1993; Hilmes, 2014). Within saddles, slip on basement reverse Structural, AMS, and paleomagnetic data sets record patterns of early to faults and large-scale fold amplitude decrease, whereas internal shorten- late shortening and vertical-axis rotations during evolution of the D-D-SM ing from backlimb thrusts and detachment folds increases. The saddles are fold system along the flank of the Laramide Wind River arch. We first sum- also locally cut by steep, east-striking faults that partly connected base- marize key constraints from these data sets. Next, orientations of paleostress- ment reverse faults at depth and likely reactivated basement weaknesses. strain directions estimated from various methods (minor fault kinematics, Minor wedge and conjugate wrench faults, best developed in lime- fracture orientations, AMS fabrics, and previously published calcite twin stone, accommodated widespread but limited (<5%) LPS. These wedge strain data) are compared and used to evaluate the evolving local deforma- and wrench faults display consistent relations with respect to bedding tion field in relation to regional Laramide patterns. An integrated model is around large-scale folds, indicating that they formed prior to and synchro- presented for the evolution of the D-D-SM fold system that is compared nous with the onset of large-scale folding. Fold forelimbs are locally cut with idealized end-member models for development of variably trending by additional younger minor faults that accommodated limb steepening Laramide arches and origins and linkage of flanking fold-fault systems. and shear during continued shortening. A cross-strike set of fractures and veins accommodated limited tangential extension during early LPS Summary of Key Data Sets and later 3-D fold growth. Red beds also underwent widespread, limited

LPS, recorded by AMS Kmax lineations. Early LPS directions estimated Large-scale structural relations (Blackstone, 1993a), thermochrono- from minor faults and AMS fabrics, corrected for bed tilt, trend gener- logic data (Stevens et al., 2016), and characteristics of sedimentary strata ally west-southwest–east-northeast along the backlimb of the fold sys- in adjacent basins (Beck et al., 1988; Steidtmann and Middleton, 1991) tem, subparallel to the regional Laramide shortening direction, and are indicate that large-scale uplift of the Wind River arch began during the partly refracted along arcuate forelimbs. Paleomagnetic analysis and rock

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40 40 40 40 A LPS vs Trend: forelimb LPS vs Trend: backlimb m = 0.6 (± 0.1), b = -2 m = 0.9 (± 0.2), b = -1 N = 50, MSWD = 0.9 N = 78, MSWD = 0.9 20 - Minor Fault data 20 20 - Minor Fault data 20 - AMS data - AMS data

0 0 0 0 AMS - 330 AMS - 330 Minor Fault - 240 Minor Fault - 240 -20 -20 -20 -20

-40 -40 -40 -40 -40 -20 02040 -40 -20 02040 Structural Trend - 330 Structural Trend - 330 B 40 40 Pmag vsTrend: forelimb Pmag vs Trend: backlimb m = 0.9 (±0.3), b = -10 m = 0.1 (±0.2) N = 18, MSWD = 0.7 N = 37, MSWD = 1.3 20 20

0 0 Declination - 336 Declination - 336 -20 -20

-40 -40 -40 -20 02040 -40 -20 02040 Structural Trend - 330 Structural Trend - 330

40 Figure 10. Strike tests for layer-parallel shortening (LPS) and paleo- C AMS vs Minor Fault magnetic data calculated using method of Yonkee and Weil (2010b). m = 0.9 (± 0.1) Example error bars show measurement uncertainty and structural N = 50, MSWD = 1.1 trend uncertainty. Best-fit slopes (m; 95% confidence interval in brack- ets), number of samples (N), and mean square of weighted deviates 20 (MSWD) are listed. (A) LPS directions estimated from both minor fault data (relative to a reference direction of 240°) and from anisotropy of

magnetic susceptibility (AMS) Kmax lineations (relative to 330°), plotted as a function of structural trend (relative to a reference trend of 330°), 0 define slopes of 0.6 (±.0.1) and 0.9 (±0.2), for forelimb and backlimb sites, respectively, indicating that LPS directions are partly correlated with structural trend along the forelimb and slightly dispersed about

AMS - 330 the regional Laramide shortening direction on the backlimb. (B) Paleo- magnetic (Pmag) declinations (relative to the reference declination of -20 336° for the North America Triassic paleopole) are uncorrelated with structural trend and record only limited, nonsystematic local rotations on the backlimb. Declinations on the forelimb give a poorly defined slope of 0.9 (±0.3) and have a statistically significant intercept (b) that records minor overall counterclockwise rotation. (C) Correlation of -40 LPS directions estimated from AMS fabrics and minor faults defines -40 -20 02040 a slope of 0.9 (±0.1), confirming that AMS provides a robust estimate Minor Fault - 240 of LPS directions.

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magnetic experiments indicate that Triassic red beds of the Chugwater 2016), leading to early LPS with development of minor fault networks Group have a near-primary remanent magnetization carried by hematite. and AMS fabrics, followed by large-scale fault propagation and folding. Paleomagnetic declinations, corrected for bed tilt, record limited coun- Similar patterns are observed regionally in the Laramide foreland. For terclockwise rotations near the east-northeast–striking Clear Creek fault example, calcite twin and fracture data from folds in the adjacent Big Horn and along more west-trending parts of forelimbs, whereas the backlimb Basin were interpreted to record pre-Laramide east-southeast–west-north- lacks statistically significant vertical-axis rotation. west–oriented principal compression, followed by increasing deviatoric stress with west-southwest–east-northeast–oriented early LPS and later Comparison of Estimated Paleostress-Strain Directions and large-scale folding and fault propagation during Laramide deformation Evolving Deformation Field (Amrouch et al., 2010, 2011; Beaudoin et al., 2012). Craddock and van der Pluijm (1999) compared calcite twin strain data from the Sevier belt, Quantitative comparison of LPS directions estimated from AMS fab- Laramide foreland, and continental plate interior (based on limited expo- rics and from minor fault kinematics yielded a slope of 0.9 ± 0.1, indicating sures of mid-Cretaceous limestone), which they interpreted to record early

that AMS Kmax lineations provide a reliable method to estimate shortening Sevier east-west principal compression that decreased in magnitude toward directions. SEM imaging and detailed analysis of different lithologies the plate interior, followed by early Laramide west-southwest–east-north- with varying sedimentary structures (planar bedding, cross-bedding, and east shortening, and late Laramide north-south shortening recorded in veins.

ripples) at two sites revealed similarly oriented Kmax lineations subparallel Although early east-southeast– to east-oriented paleostress-strain direc-

to the zone axis of microkinked phyllosilicate grains. In detail, AMS Kmax tions in the foreland have been interpreted as Sevier, these early calcite directions had slightly greater variability compared to minor fault LPS twin strains may instead record plate interior stresses related to lithosphere directions, possibly due to contributions from multiple paramagnetic and basal drag subparallel to the direction of North American plate motion ferromagnetic minerals that deformed differently. Average LPS directions during the mid-Cretaceous. The length scale for development of LPS of 240° and 237° estimated from minor fault data and AMS fabrics and fabrics (cleavage and tectonic stylolites) in front of the Sevier wedge average shortening directions of 241° to 243° estimated from extension was ~50–100 km and Sevier shortening was oriented overall west-east fractures in Triassic to Jurassic red beds and limestone are all consistent with radial dispersion likely related to topographic stresses along the with the regional average ~240° shortening direction for the Laramide curved wedge front (Mitra and Yonkee, 1985; Yonkee and Weil, 2010a; belt (Erslev and Koenig, 2009; Weil and Yonkee, 2012). Weil and Yonkee, 2012). Thus, we restrict use of the term Sevier to those Previously published calcite twin data, in comparison, record more features that formed near the fold-thrust wedge front. Mid-Cretaceous complex patterns during evolution of the fold system. Principal shortening Sevier deformation occurred during increased relative plate motion and directions for calcite twin strain reported by Willis and Groshong (1993) growth of an orogenic wedge, and also during increased west-northwest varied from west-southwest–east-northeast for samples near culmina- to west drift of the North American plate that may have enhanced basal tions of the D-D-SM system, interpreted to record initiation of Laramide drag and slightly increased plate interior stresses. Sevier deformation in LPS, to west-northwest–east-southeast for samples located away from the Wyoming salient continued into the later Cretaceous to early Eocene, fold culminations and along the homoclinal limb of the Wind River arch, temporally overlapping with Laramide deformation, but regional short- interpreted to record pre-Laramide stress. Shortening directions estimated ening directions differed, with west-southwest–east-northeast Laramide from calcite twin strain in limestone samples at Derby Dome reported by shortening interpreted to reflect increased plate coupling during flat-slab Craddock and Relle (2003) were mostly oriented north-northwest–south- subduction (Weil and Yonkee, 2012). southeast, subparallel to the fold axis, and were interpreted to record major rotation (>60°) of older Sevier-related east-west shortening fabrics during Model for Development of the D-D-SM Flanking Fold System younger Laramide deformation. Paleomagnetic data for red beds here, however, indicate that vertical-axis rotation was insignificant, and minor An integrated model for evolution of the D-D-SM fold system, based fault kinematics, cross-strike veins, and AMS fabrics indicate mostly west- on large-scale structural geometry (Fig. 4), minor fault kinematics, AMS southwest–east-northeast LPS directions. The unusual shortening direction fabrics (Fig. 9A), fracture set and calcite twin strain directions (Fig. 9B), for calcite twin strain in limestone and variable shortening directions for and paleomagnetic declinations (Fig. 9C), comprises three phases: (1) pre- calcite twin strain from vein samples at Derby Dome reported by Craddock Laramide west-northwest–east-southeast compression and development of and Relle (2003) could reflect local stress heterogeneities during folding. east-southeast–striking extension fractures and calcite twin strains away The west-northwest–east-southeast shortening direction for calcite from future fold culminations; (2) early Laramide west-southwest–east- twin strain away from fold culmination is consistent with the orientation northeast LPS with development of minor wedge and conjugate wrench

of the east-southeast–striking extensional fracture set, and is subparallel faults, AMS Kmax lineations, cross-strike veins and fractures, and addi- to the direction of North American plate motion during the mid-Creta- tional calcite twin strains; and (3) later Laramide propagation and linkage ceous (Torsvik et al., 2008), indicating that these structures may reflect of basement faults and folds (Fig. 11). During phase 1, the regional stress plate interior stresses. The east-southeast–striking fracture set is best field is interpreted to have been oriented subparallel to the direction of developed near the Clear Creek fault that is along the basement Oregon absolute plate motion and had relatively low differential magnitude, with Trail structural zone, which marks a change in lithospheric structure that locally enhanced fracturing along preexisting lithospheric structures such may have focused initial fracturing. The change in calcite twin strain as the Oregon Trail structural zone. The stress field was locally reoriented directions near future culminations may indicate local reorientation of to west-southwest–east-northeast above basement weaknesses (sites of the stress field above incipient basement reverse faults during initiation future fold culminations) during incipient Laramide deformation as a of Laramide deformation as a flat-slab segment began moving northeast- flat slab began propagating toward the foreland. Incipient deformation ward toward the foreland. During continued flat-slab subduction, devia- may have started ca. 75–70 Ma, based on subtle changes in thickness of toric stress increased and stress directions became regionally reoriented late Campanian strata near future culminations in southwest Wyoming west-southwest–east-northeast, at low angles to the direction of relative (Lopez and Steel, 2015). During phase 2, enhanced plate coupling related motion between the North American and Farallon plates (Wright et al., to flat-slab subduction led to increased deviatoric stress and regional

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(i) pre-Laramide: plate interior, incipient deformation

early ESE-striking Kmin fractures early calcite twin strain plate WNW-ESE interior oblate AMS Sundance Fm. σ1 stress ellipsoid Gypsum Spring Fm. mica basement Nugget Fm. weakness

hema- Alcova North American plate tite Limestone absolute motion basement calcite Chugwater Group incipient deformation weakness sedimentary AMS fabric onset at slab initial calcite twin strain relative motion (ii) early Laramide: LPS, onset folding HSh

AMS Kmax

VT LPS lineations wrench TE faults high angle fractures onic e ct te olit styl

Kmax triaxial AMS minor cross-strike ellipsoid folds local fractures LPS rotation LPS cross wedge WSW-ENE fractures wrench faults LPS veins faults

wedge onset

tures faults folding high angl e frac LPS AMS fabric LPS at slab calcite twin strain relative motion (iiia) late Laramide: large-scale folding, fault propagation

Figure 11. Schematic structural model for evolution of Derby-Dallas-Sheep Mountain flanking fold system. Block diagrams on left and right illustrate microscopic- mesoscopic and macroscopic structural relations, respectively, for red beds and limestone of the Triassic Chugwater Group and overlying Sundance Formation. AMS—anisotropy of magnetic tilted WSW-ENE wedge shortening susceptibility; LPS—layer- faults parallel shortening. Stages: late strike-slip faults (i) pre-Laramide intraplate stress field with development fault of east-southeast–striking propagation fracture set and earliest calcite twin strains, to Laramide (iiib) late Laramide: fold tightening, fault linkage initiation with incipient slip and stress refraction along late reverse faults basement weaknesses; (ii) early LPS, tangential exten- detachment sion (TE), vertical thickening late folds faults (VT), and limited horizontal backlimb shear (HSh) accommodated thrust by conjugate wedge and wrench faults and main fracture sets, along with early propagation of basement faults; and (iii) large-scale folding that tilts early structures, with development of late reverse- and strike-slip faults in steep forelimbs, and linkage of large-scale folds across saddles cut by east-striking faults and backlimb thrusts. at slab relative motion

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reorientation to west-southwest–east-northeast compression, subparal- D-D-SM system. Mazzoli et al. (2005) described en echelon fault-fold lel to the direction of relative plate motion, along with local stress-strain segments along a more mature thrust system in the Apennine belt of Italy, refraction related to basement weaknesses and propagating faults. LPS which, similar to the D-D-SM system, was marked by varying linkage and fabrics developed prior to and during onset of large-scale folding. Dur- increased lateral displacement gradients across relay zones. Such lateral ing phase 3, an en echelon fold-fault system formed as basement reverse linkage leads to longer fold-fault systems that may preferentially accrue faults propagated upward into steepening forelimbs, and laterally with continued shortening. Relations between fold-fault length and maximum partial connection across structural saddles (relay zones) locally cut by shortening for the D-D-SM system plot within the length-maximum dis- steep, east-striking faults. Backlimb thrusts and detachment folds helped placement field for reverse faults (Fig. 12; Torabi and Berg, 2011). Rela- accommodate shortening transfer across saddles. Early LPS fabrics were tions between the more youthful Ostler fault system, the D-D-SM system, tilted around large-scale folds that underwent additional minor faulting and major reverse faults, such as the Wind River thrust, are interpreted to along steeper parts of forelimbs. Large-scale folding in the D-D-SM sys- reflect multiple scales and cycles of fault propagation, interaction, and tem ended by 50 Ma (Keefer, 1970). linkage, similar to the numerical model of Cowie et al. (2000) for link- The integrated model is now compared with end-member models for age of normal fault segments into long systems that focus deformation. development of varying Laramide arch trends, styles of flanking folds, and Lateral variations of the D-D-SM system can also be compared with nature of fault-fold linkage shown in Figure 2. LPS directions estimated the numerical models of folding by Grasemann and Schmalholz (2012) from minor faults, AMS fabrics, and cross-strike extensional fractures that predict lateral propagation and partial linkage to form en echelon, and veins record a single, smoothly varying paleostress-strain field with doubly plunging folds for moderate ratios of fold separation to fold width. west-southwest–east-northeast shortening across the backlimb and partial Fold culminations in the study area have widths of ~4–5 km (measured refraction of early shortening along arcuate forelimbs, consistent with from the syncline trough on the southwest side to where dip shallows model 3 in Figure 2A. A component of reverse slip on the east-striking on the northeast backlimb) and separations of ~1–2.5 km between axial Clear Creek fault and local development of northeast-trending detachment traces, giving moderate separation-to-width ratios of ~0.2–0.6, consistent folds in saddles between culminations were interpreted by Tiffany (2011) with the numerical models. to record an episode of late north-south shortening, consistent with model 1. However, detachment folds in saddles may reflect local constrictional CONCLUSIONS deformation, and east-striking faults accommodated significant left-lateral slip and connected top-to-the-southwest reverse faults across relay zones, Integrated structural and paleomagnetic data sets record changing consistent with model 3. The development of east-striking faults that con- paleostress-strain patterns and partial fault linkage during evolution of a nected northwest-trending folds and reverse faults and the influence of east-trending basement weaknesses were also important in other parts of the Laramide belt (Paylor and Yin, 1993; Neely and Erslev, 2009; Weil et al., 2016). Paleomagnetic data indicate insignificant vertical-axis rota- WRT tions, except near the east-northeast–striking Clear Creek fault and more 104 west-striking parts of forelimbs, inconsistent with widespread sinistral Apennine

transpression and counterclockwise vertical-axis rotation expected for (m) link left-stepping en echelon folds in model 2. D-D-SM

Most shortening in the D-D-SM fold system was accommodated by -3 reverse faults that offset basement at depth and propagated into steep 2 forelimbs in the cover, consistent with a trishear model (Fig. 2B). Dallas 10 D/L=10

Dome has the steepest forelimb with reverse faults that reach the surface, -1

Derby Dome and the Sheep Mountain anticline have moderately steep link -5 forelimbs cut by reverse faults at depth, and structural saddles between D/L=10 ault Displacement -D culminations have less steep forelimbs with more limited faulting at depth D/L=10 (Figs. 4B–4F). These variations are interpreted to reflect decreasing ratios 100 of upward fault propagation to fault slip rate. Large-scale fold-fault short- Ostler link ening also decreases within saddles, whereas internal shortening accom- tening\ F compilation modated by backlimb thrusts and detachment folds increases, such that L-D ct thquake rupture L ear total shortening only varies slightly along the strike of the system (Fig. 4G). Shor intera These relations are consistent with lateral propagation and varying linkage reverse fault -2 compilation opagate of fault segments related to separation distances across saddles (Fig. 2C). 10 pr Although the results of this study are limited, they can be compared to fault linkage and displacement relations reported for other fault systems 10-1 101 103 105 L (m) and relay zones. Numerous studies have analyzed relay zones within nor- Figure 12. Relations between fold/fault length (L) and shortening/fault mal fault systems (see reviews by Peacock, 2002; Fossen and Rotevatn, displacement (D) for the Dallas–Derby–Sheep Mountain (D-D-SM) fold 2016), but studies of relay zones within reverse fault and associated fold system (open squares for individual folds and gray square for total length systems are more limited (Walsh et al., 1999; Nicol et al., 2002). Davis et of system), Wind River thrust (WRT, solid square), Apennine thrust fault al. (2005) described varying geometries of fault segment overlap, second- system (open circles; Mazzoli et al., 2005), Ostler fault-fold system (Davis ary faulting, and associated folding that helped accommodate shortening et al., 2005), and global compilation of length-maximum displacement (L-D) values for reverse faults and for single earthquake ruptures (Torabi transfer within the youthful Ostler thrust fault system in New Zealand, and Berg, 2011). Schematic arrows illustrate model of multiple cycles of which recorded varying linkage styles related to fault segment separa- lateral growth and increasing displacement along fault segments that tion, similar, but at shorter temporal and spatial scales compared to the interact and link to form longer systems.

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flanking fold system along the Laramide Wind River arch. Locally devel- Brown, W.G., 1988, Deformational style of Laramide uplifts in the Wyoming foreland, in Schmidt, C.J., and Parry, W.J., Jr., eds., Interaction of the Rocky Mountain foreland and oped east-southeast–striking fractures and early calcite twin strains record Cordilleran thrust belt: Geological Society of America Memoir 171, p. 1–26, doi:10 .1130 pre-Laramide west-northwest–east-southeast compression, interpreted to /MEM171-p1. reflect slightly enhanced plate interior stresses during increasing west- Chamberlain, K.R., Frost, C.D., and Frost, B.R., 2003, Early Archean to Mesoproterozoic evolution of the Wyoming Province: Archean origins to modern lithospheric architecture: northwest drift of the North American plate. Minor wedge and conjugate Canadian Journal of Earth Sciences, v. 40, p. 1357–1374, doi:10.1139/e03-054. wrench faults and AMS fabrics record early Laramide west-southwest– Chapin, C.E., 2012, Origin of the Colorado Mineral Belt: Geosphere, v. 8, p. 28–43, doi:10 oriented LPS prior to and during onset of large-scale folding, interpreted .1130/GES00694.1. Chapin, C.E., and Cather, S.M., 1983, Eocene tectonics and sedimentation in the Colorado to reflect increasing deviatoric stress during northeastward propagation Plateau–Rocky Mountain area, in Lowell, J.D., ed., Rocky Mountain foreland basin and of a flat-slab segment that enhanced end loading, basal traction, and/or uplifts: Denver, Colorado, Rocky Mountain Association of Geologists, p. 33–56. Compton, R.R., 1966, Analyses of Pliocene–Pleistocene deformation and stresses in northern asthenospheric flow. 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ACKNOWLEDGMENTS Craddock, J.P., and van der Pluijm, B., 1999, Sevier-Laramide deformation of the continental We thank Peiyeng Wen, Jamie Kendall, Amelia Lee Zhi Yi, Mary Schultz, Fern Beetle-Moorcroft, interior from calcite twinning analysis, west-central North America: Tectonophysics, v. 305, and Andriy Mshanetskyy for help in the field and laboratory. Student summer support was p. 275–286, doi:10.1016/S0040-1951(99)00008-6. partially funded by the Bryn Mawr College Summer Science Research Fellowship program Cross, T.A., and Pilger, R.H., 1982, Controls of subduction geometry, location of magmatic and its funding agencies and offices. We also thank John Spence and Betsy Spence for their arcs, and tectonics of arc and back-arc regions: Geological Society of America Bulletin, generosity and access to important outcrops. Careful and constructive reviews by two anony- v. 93, p. 545–562, doi:10.1130/0016-7606(1982)93<545:COSGLO>2.0.CO;2. mous reviewers improved this manuscript. 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A structural and paleomagnetic study of Laramide deformation in the southern Bighorn Yonkee, W.A., and Weil, A.B., 2010a, Reconstructing the kinematics of curved mountain belts: Arch, Wyoming: Geological Society of America Bulletin, v. 126, p. 415–437, doi:10.1130 Internal strain patterns in the Wyoming salient, Sevier thrust belt, U.S.A.: Geological So- /B30872.1. ciety of America Bulletin, v. 122, p. 24–49, doi:10.1130/B26484.1. Weil, A.B., Yonkee, A., and Schultz, M., 2016, Tectonic evolution of a Laramide transverse Yonkee, W.A., and Weil, A.B., 2010b, Quantifying vertical axis rotation in curved orogens: Cor- structural zone: Sweetwater Arch, south-central Wyoming: Tectonics, v. 35, p. 1090–1120, relating multiple data sets with a refined weighted least squares strike test: Tectonics, doi:10.1002/2016TC004122. v. 29, TC3012, doi:10.1029/2008TC002312. Willis, J.J., and Groshong, R.H., Jr., 1993, Deformational style of the Wind River uplift and Yonkee, W.A., and Weil, A.B., 2015, Tectonic transect of the Sevier and Laramide belts, North associated flank structures, Wyoming,in Keefer, W.R., et al., eds., Oil and gas and other American Cordillera: Evolution of a complex orogenic system: Earth-Science Reviews, resources of the Wind River Basin, Wyoming; Special symposium: Casper, Wyoming v. 150, p. 531–593, doi:10.1016/j.earscirev.2015.08.001. Geological Association, p. 337–375. Zijderveld, J.D.A., 1967, AC demagnetization of rocks: Analysis of results, in Collison, D.W., et Wright, N.M., Seton, M., Williams, S.E., and Muller, R.D., 2016, The Late Cretaceous to recent al., eds., Methods in paleomagnetism: New York, Elsevier, p. 254–286. tectonic history of the Pacific Ocean basin: Earth-Science Reviews, v. 154, p. 138–173, doi:10.1016/j.earscirev.2015.11.015. MANUSCRIPT RECEIVED 3 OCTOBER 2016 Yeck, W.L., Sheehan, A.F., Anderson, M.L., Erslev, E.A., Miller, K.C., and Siddoway, C.S., 2014, REVISED MANUSCRIPT RECEIVED 20 MAY 2017 Structure of the Bighorn Mountain region, Wyoming, from teleseismic receiver function MANUSCRIPT ACCEPTED 30 JUNE 2017 analysis: Implications for the kinematics of Laramide shortening: Journal of Geophysical Research, v. 119, p. 7028–7042, doi:10.1002/2013JB010769. Printed in the USA

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