Sheep Mountain Anticline Research Articles

Timing of sulfuric acid speleogenesis (SAS) as an indicator of canyon incision rates of the Shoshone and Bighorn rivers, Wyoming, USA

Two sulfuric acid speleogenesis (SAS) cave sites in northern Wyoming host speleogenetic byproducts that have been dated using the uranium-series method and thus provide the timing of the SAS events. Gypsum crusts that formed as byproducts of SAS in Lower Kane Cave, exposed by Bighorn River incision into the Little Sheep Mountain anticline, yield uranium-series dates of 20.5 ± 0.2 ka to 16.2 ± 0.2 ka that represent the timing of late stage speleogenesis of that cave. A calcite mammillary engulfed by elemental sulfur that formed during SAS of Shoshone Canyon Conduit Cave, exposed by Shoshone River incision of the homonymous canyon, yielded a uranium-series age of 174 ± 5 ka and represents late stage speleogenesis of that cave, although the process continued after the cave was elevated above the water table until at least 100 ± 5 ka. In addition to the timing of speleogenesis, the U-series results provide incision rates for these two canyons. The rate of canyon incision for the last 200 kyr by the Bighorn River in Little Sheep Mountain Canyon is measured as 190 ± 50 m/Ma, and for the Shoshone River in Shoshone Canyon as 400 ± 40 m/Ma. Using the Bighorn River incision rate, Upper Kane Cave located 33 m above Lower Kane Cave has a projected age of 170 ± 60 ka. We suggest that climatically driven hydrologic mechanisms rather than magmatic ones likely produced the pulses of SAS at these two cave sites.

Influence of mechanical stratigraphy on clastic injectite growth at Sheep Mountain anticline, Wyoming: A case study of natural hydraulic fracture containment

Sandstone injectites ranging from 1 km in outcrop length intrude the Cretaceous Mowry Formation in the vicinity of Sheep Mountain anticline (Bighorn Basin, Wyoming, USA). These injectites were sourced from the Peay Sandstone Member of the overlying Cretaceous Frontier Formation and represent a significant possible fluid pathway through impermeable shales. Sand injection occurred along dikes and sills, interacting with bedding discontinuities and preexisting joints in the Mowry Formation during the early folding of Sheep Mountain anticline. We argue that, in contrast to the passive sweeping of sediments into fissures characteristic of Neptunian dike formation, downward intrusion of the Peay sand was forceful and made possible by a highly stratified horizontal stress field resulting from the deposition, burial, and lithification history of the rock units in the area. The internal structure of the injectites is dominated by two sets of mutually offsetting deformation bands. The deformation bands have shear and compaction components, exhibiting significant porosity loss, as well as cataclasis and minor pressure solution. After formation of the deformation bands, subsequent faulting was localized along the margins of deformation bands, evidenced in the field by slickensided surfaces. A detailed kinematic analysis of slickenline lineations yields shortening and extension axes consistent with deformation band formation during early Laramide–oriented shortening, and continuing through the folding of Sheep Mountain anticline. Beyond the formation and deformation of these sandstone injectites, this study highlights the importance of mechanical stratigraphy in the containment of hydraulic fractures.

The Fold Analysis Challenge: A virtual globe-based educational resource

We present an undergraduate structural geology laboratory exercise using the Google Earth virtual globe with COLLADA models, optionally including an interactive stereographic projection and JavaScript controls. The learning resource challenges students to identify bedding traces and estimate bedding orientation at several locations on a fold, to fit the fold axis and axial plane to stereographic projection data, and to fit a doubly-plunging fold model to the large-scale structure.The chosen fold is the Sheep Mountain Anticline, a Laramide uplift in the Big Horn Basin of Wyoming. We take an education research-based approach, guiding students through three levels of difficulty. The exercise aims to counter common student misconceptions and stumbling blocks regarding penetrative structures. It can be used in preparation for an in-person field trip, for post-trip reinforcement, or as a virtual field experience in an online-only course. Our KML scripts can be easily transferred to other fold structures around the globe.

Evolution of pore-fluid pressure during folding and basin contraction in overpressured reservoirs: Insights from the Madison–Phosphoria carbonate formations in the Bighorn Basin (Wyoming, USA)

Reconstructing the evolution of paleofluid (over)pressure in sedimentary basins during deformation is a challenging problem, especially when no hydrocarbon-bearing fluid inclusions are available to provide barometric constraints on the fluid system. This contribution reports the application to a natural case (the Bighorn Basin) of recent methodological advance to access fluid (over)pressure level prevailing in strata during sub-seismic fracture development. The fluid pressure evolution in the Mississippian-Permian Madison–Phosphoria limestone reservoir is tentatively reconstructed from the early Sevier Layer Parallel Shortening to the Laramide folding in two basement-cored folds: the Sheep Mountain Anticline and the Rattlesnake Mountain Anticline. Results point out that supra-hydrostatic pressure values prevail in the limestone reservoir during most of its whole Sevier–Laramide history. The comparison of the reconstructed fluid overpressure values within situ measurements in various overpressure reservoirs in other oil-producing basins highlights that the supra-hydrostatic fluid pressure gradually reaches the lithostatic value during the whole basin contraction and fold development. During most of the LPS history, however, overpressure level can be defined by a mean gradient. Among the factors that control the pressure evolution, the mechanical stratigraphy, the stress regime under which fractures developed and regional fluid flow are likely dominating in the case of the Bighorn Basin, rather than classical factors like disequilibrium compaction or fluid generation during burial. A coeval evolution between fluid overpressure and differential stress build-up is also emphasized. The approach presented in this paper also provides estimates of strata exhumation during folding.

Comparative FEM and DEM modeling of basement-involved thrust structures, with application to Sheep Mountain, Greybull area, Wyoming

Abstract Basement-involved fold and thrust systems have been widely studied in several locations, through field and seismic investigations. However, our understanding of the geometries of the faults that drive folding, and the mechanical conditions within the resulting folds, is still limited. Simulations run using the finite element and discrete element methods have great potential to help determine the mechanical evolution of structures at all scales. Both methods have been employed to explore the structural evolution of the Laramide-age Sheep Mountain anticline, located in the eastern Bighorn Basin of Wyoming. Both sets of simulations are constrained by experimental and well log data, and use the same size model domain and mechanical stratigraphy. The underlying basement–sediment boundary was deformed by a set of prescribed displacements. The results of the two simulations are geometrically similar, and match previous field-based interpretations of the Sheep Mountain fold to the first order. Stresses calculated by the two methods were also very similar, thus verifying the mechanical comparison. Differences in the geometric details, however, provide important perspectives on the capabilities of the two methods. For example, the major faults producing the Sheep Mountain fold develop slightly higher dips in the DEM simulation than the FEM simulation, yielding a better match with published interpretations. The crest of the anticline is characterized by distributed thinning of shallow strata in the FEM simulation, and by local extensional faulting in the DEM simulation. Neither simulation reproduced the north–south fold splay observed in the field, suggesting that this minor feature results from preexisting structure or 3D effects not modeled here.

Controls of basement faults on the geometry and evolution of compressional basement-involved structures

Foreland basement-involved structures commonly occur in front of major fold-thrust belts as irregular chains of uplifts. The variation in the map and three-dimensional (3-D) geometries of individual structures, and their mutual spatial and angular relationships, are strongly suggestive of the influence of preexisting basement discontinuities. Three-dimensional experimental models were constructed to determine the function of preexisting frontal and lateral discontinuities in determining the geometry of the structures. The models consisted of two layers, with stiff clay representing the basement and soft clay representing the sedimentary cover. Laser scanning and 3-D surface modeling were used to determine the map geometry to compare the models with examples of natural structures from the Rocky Mountain foreland in Wyoming. Planar discontinuities result in doubly plunging structures terminating against a frontal fault with a linear trace (Sheep Mountain anticline), whereas intersecting fault sets with sharp or curved intersections result in trapdoor geometries (Grass Creek anticline). Opposite-dipping faults result in uplifted blocks with varying relief and orientations of the structures, depending on the relative orientations of the two fault trends. Significant strike-slip faulting along the frontal fault only occurs when the angle between the two fault trends exceeds 30°. Chains of structures show offsets or relay patterns, commonly accompanied by changes in the orientations of adjacent structures. The Sage Creek–Steamboat Butte chains, the Hamilton Dome–Wagonhound anticline chain, the Maverick Springs–Circle Ridge chain, and the Grass Creek–Walker Dome trend in the Rocky Mountain foreland are examples of these configurations. These patterns suggest the influence of preexisting frontal or lateral discontinuities on the trends and locations of structures. Lateral discontinuities are either preexisting, controlling the lateral extent of structures, or form during deformation to accommodate the formation of structures formed along offset frontal faults. Preexisting lateral faults that penetrate the entire basement commonly result in surface lateral faults bounding the surface structures. However, reverse faults in the basement connected by buried lateral faults result in surface structures with overlaps between them. A relay of reverse faults in the basement not connected by lateral faults may develop parallel or oblique structures with transfer zones between them. The mapped geometries of the experimental models, and the orientations of secondary faults, provide predictive analogs that can be used in the interpretation of surface and subsurface structures.

Impact of fracture stratigraphy on the paleo-hydrogeology of the Madison Limestone in two basement-involved folds in the Bighorn basin, (Wyoming, USA)

Based on the study of the Madison Limestone at Sheep Mountain and Rattlesnake Mountain, a unique outcrop dataset including (1) facies and diagenetic analyses, (2) vertical persistence and cement stratigraphy of vein sets and (3) fluid inclusions thermometry are used to demonstrate the impact of folding and fracturing on paleo-hydrogeology. Quantification of the vertical persistence of fractures shows that Sheep Mountain and Rattlesnake Mountain differ by the vertical persistence of the pre-folding Laramide vein sets, which are strictly bed-confined in Sheep Mountain but cut across bedding at Rattlesnake Mountain, whereas the syn-folding veins are through-going in both. The emplacement chronology and the various sources of the fluids responsible for the paragenetic sequence are based on isotope chemistry and fluid inclusions analysis of the matrix and vein cements. At Sheep Mountain and Rattlesnake Mountain, the cements related to the burial are characterized by isotopic signatures of marine formation waters that were diluted during the karstification of the Madison Platform at the end of Mississippian. Meteoric fluids, presumably migrating during the Cenomanian from Wind River Range and Teton Range, recharge zones located in the south-west of the Bighorn Basin, were remobilized in the early bed-confined and through-going syn-folding veins of the Sheep Mountain Anticline. The former vein set drained only local fluids whose isotopic signature relates to an increase of temperature of the meteoric fluids during their migration, whereas the latter set allowed quick drainage of basinal fluids.

Structural and microstructural evolution of the Rattlesnake Mountain Anticline (Wyoming, USA): New insights into the Sevier and Laramide orogenic stress build-up in the Bighorn Basin

The Rocky Mountains in western US provide among the best examples of thick-skinned tectonics: following a period of thin-skinned tectonics related to the Sevier orogeny, the compressional reactivation of basement faults gave birth to the so-called Laramide uplifts/arches. The Bighorn basin, located in Wyoming, is therefore a key place to study the transition from thin- to thick-skinned tectonics in orogenic forelands, especially in terms of microstructural and stress/strain evolution. Our study focuses on a classic Laramide structure: the Rattlesnake Mountain Anticline (RMA, Wyoming, USA), a basement-cored anticline located in the western part of the Bighorn basin. Stress and strain evolution analysis in folded sedimentary layers and underlying faulted basement rocks were performed on the basis of combined analyses of fractures, fault-slip data and calcite twinning paleopiezometry. Most of the fractures are related to three main tectonic events: the Sevier thin-skinned contraction, the Laramide thick-skinned contraction, and the Basin and Range extension. Serial balanced cross-sections of RMA and displacement profiles suggest that all thrust faults were coeval, evidencing strain distribution in the basement during faulting. The comparison of RMA with another structure located in the eastern edge of the Bighorn basin, i.e. the Sheep Mountain Anticline (SMA), allows to propose a conceptual model for the geometric and kinematic evolution of Laramide-related basement-cored anticlines. Finally, the stress evolution is reconstructed at both the fold scale and the basin scale. We show that the evolution of stress trends and magnitudes was quite similar in both structures (RMA and SMA) during Laramide times (thick-skinned tectonics), in spite of different stress regimes. During Sevier (thin-skinned tectonics) and post-Laramide times, stress trends and fracture patterns were different in these two structures. These results suggest that the distance to the orogenic front influenced the fracture patterns but not the foreland stress magnitudes, which were likely controlled by the structural style.

Paleostress magnitudes in folded sedimentary rocks

Using Sheep Mountain Anticline (Wyoming, USA) as a case study, we propose a new approach to quantify effective paleo-principal stress magnitudes in the uppermost crust. The proposed mechanical scenario relies on a well-documented kinematic and chronological sequence of development of faults, fractures and microstructures in the folded strata. Paleostress orientations and regimes as well as differential stress magnitudes based on calcite twinning paleopiezometry are combined with rock mechanics data in a Mohr construction to derive principal stress magnitudes related to the successive steps of layer-parallel shortening and to late stage fold tightening. Such quantification also provides original insights into the evolution of the fluid (over)pressure and amount of syn-folding erosion.

Sedimentary and diagenetic controls on the multiscale fracturing pattern of a carbonate reservoir: The Madison Formation (Sheep Mountain, Wyoming, USA)

A multidisciplinary approach has been applied to characterize the Madison Formation in the Sheep Mountain anticline at different scales, in order to illustrate the relationship between sedimentology, diagenesis and fracturing pattern. Sedimentary facies are organized into three types of elementary facies sequences showing different vertical evolutions of their petrophysical properties. This is controlled by the double influence of the initial sedimentary facies and the diagenetic evolution. Three main sets of fractures (related to the Sevier and Laramide compressive phases and to the folding of Sheep Mountain) have been described at three orders of scale. At small scale, the Sevier-related set I and folding related set III are the most abundant. Set II, dated from the early stage of the Laramide compression, exhibits the highest fracture intensity. There is also a clear faciologic control on the fracture intensity. Small-scale mechanical units correspond to entire facies sequences when no or little petrophysical property contrasts between beds are observed. Conversely, small-scale mechanical units correspond to each bed of a facies sequence in the case of high contrasts of petrophysical properties between beds. At intermediate scale, set II is not present and sets I and III show the highest fracture intensities. The latter are partly controlled by the mechanical unit thickness. Intermediate and large-scale mechanical unit distribution is mainly controlled by the overall faciologic vertical stacking pattern and by the position across the Sheep Mountain anticline (forelimb or backlimb). Sheep Mountain outcrops provide a good spatial representation of the carbonate reservoir heterogeneities from the micro- to the field scale, illustrating the complexity of fractured-carbonate reservoirs.

Fracture-controlled paleohydrogeology in a basement-cored, fault-related fold: Sheep Mountain Anticline, Wyoming, United States

New geochemical and microstructural data constrain the origins and pathways of paleofluids during the growth of Sheep Mountain Anticline, Wyoming, United States. Oxygen, carbon, and strontium isotope studies were performed on prefolding and fold-related calcite veins and their sedimentary host rocks and combined to fluid inclusion microthermometry results. We show that most of the cements precipitated from Paleogene meteoric fluid. Stable isotopes and fluid inclusion homogenization temperatures further indicate that most veins were mineralized from upward moving fluids after these fluids were heated at depth (T > 110°C). This implies that fluids migrated along the basement thrust underlying the fold and/or at the base of the cover. Above the fault tip, the fluids circulated rapidly in the diffuse synfolding (and early folding) fracture network. The zone of preferential migration of the warm fluids is currently located in the backlimb of the fold, which supports some of the previously published structural interpretation of the subsurface. This study also highlights the potential of combined fracture analysis and geochemical analyses of paleofluid flows in fractures to constrain both the deformation history and the fluid flow during basement-involved shortening in Laramide-style forelands.

Fold geometry at Sheep Mountain anticline, Wyoming, constructed using airborne laser swath mapping data, outcrop‐scale geologic mapping, and numerical interpolation

Constraints available at the Earth's surface on the geometry of geologic folds often are discontinuous, consisting of scattered bedding surface outcrops representing the position and orientation of different stratigraphic surfaces. For this reason, characterization of fold geometry from surface data, by methods such as sequential balanced cross sections, remains a subjective and interpretive process. We present a new method for modeling the geometry of kilometer‐scale folds using dense, precise topographic data available from airborne laser swath mapping (ALSM), outcrop‐scale geologic mapping, and a reproducible numerical interpolation method that is free of subjectivity. Geologic mapping at Sheep Mountain anticline, Wyoming, enables the association of hundreds of thousands of ALSM data points with 14 weathering‐resistant bedding surfaces from seven different stratigraphic units ranging in age from Mississippian to Cretaceous. These data, supplemented by several hundred strike and dip measurements constraining local orientation of folded strata, are projected to the stratigraphic position of a single surface and used as constraint for a smoothed minimum curvature spline interpolation to generate a continuous representation of fold geometry. Both strengths and weaknesses of the new method are discussed in the context of comparison between the new surface model and an existing model constructed by traditional methods.

Insights into fold growth using fold-related joint patterns and mechanical stratigraphy

Despite how common folds are as structural features, along-strike fold propagation has proven elusive to document. However, if a fold grows laterally along its axis, early-formed fold-related joints may differ significantly in orientation from joints that form later. In this paper, we integrate mechanical stratigraphy with joint pattern analysis to determine relative timing of jointing. Additionally, we demonstrate that joint patterns can be related to stresses on both the top and bottom of the bed during flexure. We present joint data from eight sedimentary beds on the fold terminus at Sheep Mountain Anticline, Wyoming, USA. The joint patterns around the terminus show two distinct patterns: joints in six of the beds show a radial pattern around the terminus whereas joint patterns in the two remaining beds differ from proximal units, despite being in the same structural position. Fracture resistance calculations confirm that the beds with mis-oriented fractures are less resistant to fracturing than other units in the study, and therefore would have fractured earlier in fold growth history. We present a plate bending model that illustrates potential joint patterns around a plunging fold nose from stresses along both the top and bottom of the bed. The joint strike predictions for the area in front of the inflection line on the fold nose match the orientations in our less resistant beds, which are now positioned behind the inflection line, suggesting that the fold grew laterally. The combined analysis of fracture pattern and mechanical stratigraphy provides a new way to investigate fold evolution.

Constraints on deformation mechanisms during folding provided by rock physical properties: a case study at Sheep Mountain anticline (Wyoming, USA)

SUMMARY The Sheep Mountain anticline (Wyoming, USA) is a well-exposed asymmetric, basementcored anticline that formed during the Laramide orogeny in the early Tertiary. In order to unravel the history of strain during folding, we carried out combined anisotropy of magnetic susceptibility (AMS), anisotropy of P-wave velocity (APWV) and Fry strain analyses. The results are compared to previously published stress–strain data from calcite twins at the microscopic scale and from fracture sets at the mesoscopic scale, and are used to discuss the kinematics and mechanics of forced folding. The results obtained in sandstone and carbonate lithologies demonstrate a good agreement between (1) the principal axes of the AMS and APWV tensors, (2) stress–strain tensors derived from calcite twins, (3) Fry strain axes and mesoscopic fracture sets. Furthermore, these tensors are coaxial with the main structural trends of the anticline. The differences between AMS and APWV fabrics on one hand, and the differential stress values of the forelimb and the backlimb on the other hand, emphasize how the macroscopic asymmetry of Sheep Mountain anticline affects the strain pattern at the microscopic scale. The data set presented in this paper offers a consistent mechanical scenario for the development of Sheep Mountain anticline.

Stress and strain patterns, kinematics and deformation mechanisms in a basement-cored anticline: Sheep Mountain Anticline, Wyoming

[1] In order to characterize and compare the stress-strain record prior to, during, and just after folding at the macroscopic and the microscopic scales and to provide insights into stress levels sustained by folded rocks, we investigate the relationship between the stress-strain distribution in folded strata derived from fractures, striated microfaults, and calcite twins and the development of the Laramide, basement-cored Sheep Mountain Anticline, Wyoming. Tectonic data were mainly collected in Lower Carboniferous to Permian carbonates and sandstones. In both rock matrix and veins, calcite twins recorded three different tectonic stages: the first stage is a pre-Laramide (Sevier) layer-parallel shortening (LPS) parallel to fold axis, the second one is a Laramide LPS perpendicular to the fold axis, and the third stage corresponds to Laramide late fold tightening with compression also perpendicular to the fold axis. Stress and strain orientations and regimes at the microscale agree with the polyphase stress evolution revealed by populations of fractures and striated microfaults, testifying for the homogeneity of stress record at different scales through time. Calcite twin analysis additionally reveals significant variations of differential stress magnitudes between fold limbs. Our results especially point to an increase of differential stress magnitudes related to Laramide LPS from the backlimb to the forelimb of the fold possibly in relation with motion of an underlying basement thrust fault that likely induced stress concentrations at its upper tip. This result is confirmed by a simple numerical model. Beyond regional implications, this study highlights the potential of calcite twin analyses to yield a representative quantitative picture of stress and strain patterns related to folding.

Mechanical models of fracture reactivation and slip on bedding surfaces during folding of the asymmetric anticline at Sheep Mountain, Wyoming

We use finite element methods to investigate the reactivation of fractures (opening and shearing) and the development of bedding-surface slip during the deformation of the asymmetric anticline at Sheep Mountain, Wyoming. A series of numerical simulations were run to show the effect of mechanical stratigraphy, layer thickness, slip on bedding surfaces, and tectonic shortening of the fold on the response of a representative three-layer (ductile, brittle, ductile) two-dimensional system. The model uses large deformation frictional contact mechanics to capture the response of existing fractures and slip along bedding surfaces, and considers both elastic and elastoplastic layer properties. The computational results demonstrate the relationships among overall configuration of the multilayer, slip on bedding surfaces, and the sequence and mode of deformation (opening versus shearing) of bed-perpendicular fractures. We show that fractures located in the hinge are mainly reactivated as joints and that those in the forelimb are predominantly reactivated as thrust faults. A flexural-slip mechanism develops during folding when the layers bounded by frictional bedding surfaces have similar stiffnesses. In contrast, when the difference in the layer stiffnesses is significant (softer outer layers) the deformation is accommodated within the softer units without exceeding the frictional strength of the bedding surfaces. A reduction of the middle layer thickness from 100 m to 10 m has a minor quantitative effect on the slip along the bedding surfaces. We compare the numerical results with fracture data collected at Sheep Mountain Anticline, and discuss the similarities and differences between the field observations and the model results.

Mechanical aspects of thrust faulting driven by far-field compression and their implications for fold geometry

In this paper we present a mechanical model that intends to captures the kinematical aspects of thrust fault related folds induced by regional-scale far-field contraction. Fold shapes may be the only surface evidence of the geometry of underlying faults, so complex fault interactions are assessed in terms of how they influence fold geometry. We use the finite element method to model the fold and finite deformation frictional contact to model the activation and evolution of slip throughout preexisting faults. From several simulated 2D fault patterns we infer how one may form an anticline similar to that observed at Sheep Mountain Anticline, Wyoming.

Curvature and fracturing based on global positioning system data collected at Sheep Mountain anticline, Wyoming

We investigate the curvature-fracture relationship at Sheep Mountain anticline, Wyoming, by coupling fracture mapping with structural mapping using high-precision global positioning system data. Carrier-phase post-processing techniques of spatial data collected across patches of bedding surfaces result in a high-resolution data set. Differential geometry tools form the basis for curvature analysis, allowing for a quantitative understanding of the shapes of these surfaces. Comparison of principal curvature magnitudes with fracture measurements indicates that greater curvature correlates with greater spherical variance of fracture set orientations. Fracture intensities, however, correlate only loosely with curvature, because fracturing mechanisms other than curvature of bedding must be taken into account.

The role of fractures in the structural interpretation of Sheep Mountain Anticline, Wyoming

The development of fractures in the sedimentary layers of Sheep Mountain Anticline, a Laramide asymmetric fault-cored fold of the Bighorn Basin, is documented and interpreted to constrain the kinematic evolution of the fold. The fracture pattern is interpreted to identify five main fracture sets. A fracture set striking 110° (Set I), oblique to the future fold trend, is interpreted as a regional fracture set that predates Laramide compression. A joint set (Set II), striking 045° and present in the hinge and backlimb, is related to the NE-oriented Laramide compression just prior to and during initial anticline growth. Joints striking 135° (Set III), parallel to the fold trend, are found within the hinge and are interpreted to have developed in response to the bending of layers. The two youngest fracture sets are attributed to a late stage of fold growth: a joint set (Set IV) in the backlimb striking parallel to the Set I fractures, but vertical; and a fracture set in the forelimb consisting of Set I fractures reactivated as reverse faults. The relative chronology, mode of formation (opening vs. shearing), and structural locations of these fractures suggest the following kinematics of folding at Sheep Mountain Anticline: little or no lateral fold propagation; no hinge migration; and limb rotation with only minor limb flexure and stretching.

From spatial variation of fracture patterns to fold kinematics: A geomechanical approach

A vertical, compression‐parallel joint set apparently formed at Sheep Mountain Anticline (SMA) during the early Laramide orogeny, prior to significant folding. Field data indicate that these joints occur in fewer locations in the forelimb than in the hinge and backlimb. Using three‐dimensional elastic dislocation models, we show that early slip along an underlying thrust fault can locally perturb the surrounding stress field, inhibiting joint formation above the fault. Thus, we are able to constrain the forelimb kinematics finding that the forelimb was originally located in the hangingwall, above the thrust fault. Furthermore the mode of folding at SMA is constrained: the anticline developed with a fixed hinge and rotating limbs, while the underlying basement rocks deformed internally.