ABSTRACT The Bighorn uplift, Wyoming, developed in the Rocky Mountain foreland during the 75–55 Ma Laramide orogeny. It is one of many crystalline-cored uplifts that resulted from low-amplitude, large-wavelength folding of Phanerozoic strata and the basement nonconformity (Great Unconformity) across Wyoming and eastward into the High Plains region, where arch-like structures exist in the subsurface. Results of broadband and passive-active seismic studies by the Bighorn EarthScope project illuminated the deeper crustal structure. The seismic data show that there is substantial Moho relief beneath the surface exposure of the basement arch, with a greater Moho depth west of the Bighorn uplift and shallower Moho depth east of the uplift. A comparable amount of Moho relief is observed for the Wind River uplift, west of the Bighorn range, from a Consortium for Continental Reflection Profiling (COCORP) profile and teleseismic receiver function analysis of EarthScope Transportable Array seismic data. The amplitude and spacing of crystalline-cored uplifts, together with geological and geophysical data, are here examined within the framework of a lithospheric folding model. Lithospheric folding is the concept of low-amplitude, large-wavelength (150–600 km) folds affecting the entire lithosphere; these folds develop in response to an end load that induces a buckling instability. The buckling instability focuses initial fold development, with faults developing subsequently as shortening progresses. Scaled physical models and numerical models that undergo layer-parallel shortening induced by end loads determine that the wavelength of major uplifts in the upper crust occurs at approximately one third the wavelength of folds in the upper mantle for strong lithospheres. This distinction arises because surface uplifts occur where there is distinct curvature upon the Moho, and the vergence of surface uplifts can be synthetic or antithetic to the Moho curvature. In the case of the Bighorn uplift, the surface uplift is antithetic to the Moho curvature, which is likely a consequence of structural inheritance and the influence of a preexisting Proterozoic suture upon the surface uplift. The lithospheric folding model accommodates most of the geological observations and geophysical data for the Bighorn uplift. An alternative model, involving a crustal detachment at the orogen scale, is inconsistent with the absence of subhorizontal seismic reflectors that would arise from a throughgoing, low-angle detachment fault and other regional constraints. We conclude that the Bighorn uplift—and possibly other Laramide arch-like structures—is best understood as a product of lithospheric folding associated with a horizontal end load imposed upon the continental margin to the west.

ABSTRACT We report the results of 167 calcite twinning strain analyses (131 limestones and 36 calcite veins, n = 7368 twin measurements) from the Teton–Gros Ventre (west; n = 21), Wind River ( n = 43), Beartooth ( n = 32), Bighorn ( n = 32), and Black Hills (east; n = 11) Laramide uplifts. Country rock limestones record only a layer-parallel shortening (LPS) strain fabric in many orientations across the region. Synorogenic veins record both vein-parallel shortening (VPS) and vein-normal shortening (VNS) fabrics in many orientations. Twinning strain overprints were not observed in the limestone or vein samples in the supracrustal sedimentary veneer (i.e., drape folds), thereby suggesting that the deformation and uplift of Archean crystalline rocks that form Laramide structures were dominated by offset on faults in the Archean crystalline basement and associated shortening in the midcrust. The twinning strains in the pre-Sevier Jurassic Sundance Formation, in the frontal Prospect thrust of the Sevier belt, and in the distal (eastern) foreland preserve an LPS oriented approximately E-W. This LPS fabric is rotated in unique orientations in Laramide uplifts, suggesting that all but the Bighorn Mountains were uplifted by oblique-slip faults. Detailed field and twinning strain studies of drape folds identified second-order complexities, including: layer-parallel slip through the fold axis (Clarks Fork anticline), attenuation of the sedimentary section and fold axis rotation (Rattlesnake Mountain), rotation of the fold axis and LPS fabric (Derby Dome), and vertical rotations of the LPS fabric about a horizontal axis with 35% attenuation of the sedimentary section (eastern Bighorns). Regional cross sections (E-W) across the Laramide province have an excess of sedimentary veneer rocks that balance with displacement on a detachment at 30 km depth and perhaps along the Moho discontinuity at 40 km depth. Crustal volumes in the Wyoming Province balance when deformation in the western hinterland is included.

Abstract A synthesis of low-temperature thermochronologic results throughout the Laramide foreland illustrates that samples from wellbores in Laramide basins record either (1) detrital Laramide or older cooling ages in the upper ~1 km (0.62 mi) of the wellbore, with younger ages at greater depths as temperatures increase; or (2) Neogene cooling ages. Surface samples from Laramide ranges typically record either Laramide or older cooling ages. It is apparent that for any particular area the complexity of the cooling history, and hence the tectonic history interpreted from the cooling history, increases as the number of studies or the area covered by a study increases. Most Laramide ranges probably experienced a complex tectono-thermal evolution. Deriving a regional timing sequence for the evolution of the Laramide basins and ranges is still elusive, although a compilation of low-temperature thermochronology data from ranges in the Laramide foreland suggests a younging of the ranges to the south and southwest. Studies of subsurface samples from Laramide basins have, in some cases, been integrated with and used to constrain results from basin burial-history modeling. Current exploration for unconventional shale-oil or shale-gas plays in the Rocky Mountains has renewed interest in thermal and burial history modeling as an aid in evaluating thermal maturity and understanding petroleum systems.This paper suggests that low-temperature thermochronometers are underutilized tools that can provide additional constraints to burial-history modeling and source rock evaluation in the Rocky Mountain region.

The incorporation of increasingly multidisciplinary aspects of geoscience curricula into a traditional geology field camp requires compromises. Among these, decisions about projects to reduce or eliminate and course prerequisites are two of the most challenging. Over the past 10 yr, the University of Missouri’s geology field camp has completed a two-stage plan to expand our projects in hydrology and geophysics while maintaining traditional aspects of our course and our standard prerequisites. The first stage added projects in surface and groundwater hydrology, seismic refraction, and surficial mapping during the fifth week of our six-week course, replacing an existing mapping project. The second stage added advanced project options that students can select to complete during the last week of the course. Advanced projects in hydrology and geophysics were added as alternatives to the existing hard-rock structural analysis project that had been the sixth-week project for all students. This staged addition has allowed us to: (1) integrate these projects into a curriculum that maintains a strong emphasis on historical bedrock geology, geologic mapping, and three-dimensional visualization; and (2) accommodate differences in the coursework that students have completed prior to beginning the field camp. Rather than requiring students to have prerequisite courses in hydrogeology or geophysics in order to select these advanced project options, we include sufficient instruction during the fifth and sixth weeks that builds upon previous projects to provide the required background. To set up the context for our expanded hydrology and geophysics projects, this paper briefly describes our traditional field projects and our instructional philosophies. We describe the expanded projects that have been implemented during the fifth and sixth weeks of our course, project objectives, and the ways that these projects reinforce lessons learned during traditional field projects. We present the results of student surveys that have been used to evaluate the success of these efforts, and we discuss the personnel and equipment expenses required.

Continental crust subjected to horizontal contraction in convergent settings deforms in a variety of styles. In many instances, it is useful to consider the deforming crustal sections in terms of crystalline basement rocks underlying incipiently undeformed sedimentary strata. Three deformation styles are commonly found in such settings. The structural style referred to as thin-skinned tectonics encompasses a stack of thrust sheets composed of non- or weakly metamorphic sedimentary rocks. The associated thrust faults usually level off in a mechanically weak décollement horizon along which a substantial amount of displacement occurs in the course of the formation of the fold-and-thrust belt. Thrust faults may also cut down into the crystalline basement and level off a few kilometers beneath the basement-cover interface. The term basement-involved thin-skinned tectonics is proposed to describe this style of continental contraction. This style, too, is characterized by stacks of thrust sheets. In many cases however, such nappe stacks are overprinted by pervasive folding of a crust thermally weakened by magmatic activity or regional burial metamorphism. Thick-skinned tectonics seems less common. This style implies that thrust faults cut across the entire upper crust (and possibly the lower crust). The associated continental contraction is smaller, and the ensuing deformation is characterized by warping of the basement-cover interface. Displacements accumulated in a major basal detachment horizon may connect into mantle by means of a subduction zone. However, under elevated temperatures, pervasive deformation of the hanging wall and footwall rocks may compensate large displacements over relatively short distances. Thin-skinned fold-and-thrust belts are common on both sides of collisional orogens. Noncollisional orogens tend to be more asymmetric.