The curved east-facing salient of the Idaho-Wyoming thrust belt is a prime location to study well-dated interactions of foreland basement-cored uplifts and thrust belt development. Both field and fabric data, as well as analyses of twinned calcite, have been used to infer the deformation paths of rocks within the Absaroka, Darby, and Prospect thrust sheets as they approached the bounding foreland basement-cored uplifts. Each thrust sheet has a specific fracture pattern that maintains a constant orientation relative to the curvilinear strike of each thrust fault. Mean shortening directions computed from twinned calcite are layer-parallel, except in samples taken from sedimentary cover rocks of the basement uplifts, and mean compression and shortening axes are normal to local thrust strike. The joint and calcite fabric data indicate that the stress, strain, and presumably, deformation paths of each thrust sheet were normal to both the strike of the thrust fault, where it is exposed, and the base of the thrust ramp. Similarly, these data suggest that thrust sheet rotation, as much as 40° in a counterclockwise sense, took place as these thrust sheets approached the salient-bounding basement uplifts. South of Hoback Canyon, where there is no nearby basement uplift, a clockwise rotation of 36° is suggested. Interpretation of the thrust belt and Teton-Gros Ventre buttress tectonic history is based on deformation paths, cross sections, palinspastic maps, and well-documented timing relationships (Wiltschko and Dorr, 1983) of both terranes. Differential rotation and frontal imbrication are the primary responses of these thrust sheets to accommodate rotation into parallelism with the bounding basement uplifts.

Gravity-slide sheets are common in areas of sufficient topographic relief provided by extensional and compressional tectonism. They occur throughout the Basin and Range Province of Nevada and Utah but are perhaps best exposed in the thrust belt of southeast Idaho and northwest Wyoming. The Idaho-Wyoming slide sheets developed adjacent to extensional basin-bounding faults, which in turn were initiated at a ramp in the Absaroka thrust. Initial coherent sheets may have been up to 12 km 2 in aerial extent, but as a result of differential movement rates, these larger sheets were broken into smaller blocks by strike-parallel extension during emplacement. Folds on the down-slope edges of the sheets suggest movement analogous to a tank tread: Through interbed shear the limestone beds rolled forward and were dragged beneath the leading edge of each sheet. It is not known whether initial emplacement was catastrophic or by creep, but undisrupted cleavages within footwall Tertiary mudstones and coherent bedding within the sheets suggest creep for at least the later phases of movement.

A comparison of mesoscopic and microscopic structures in the Idaho-Utah-Wyoming thrust belt and the Rocky Mountain deformed foreland in Wyoming reveals regional variations in structural style and a decrease in regional shortening from the thrust belt (~60 percent shortening) to the foreland (~30 percent shortening). Deformation in the thrust belt is thin-skinned and is achieved by regional-scale simple shear in the sedimentary cover, which is separated from the basement by a regional décollement. Detailed studies on the Crawford thrust sheet show that large-scale shortening in the cover was produced by thrust faults and associated folds, while internal shortening was achieved by pressure solution (recorded by spaced cleavage), plastic deformation (recorded by deformed fossils), and cataclasis (recorded by contraction faults). In the foreland, both basement and cover are deformed by regional-scale pure shear, although this may be the secondary effect of even larger, lithospheric-scale thrusting. Detailed studies in the Wind River Mountains (of the Wyoming foreland) show that large folds in the cover are directly related to major deformation zones in the basement. Localized strain softening along these zones allowed large displacements by cataclastic and diffusional processes, while basement blocks between zones underwent only minor deformation by fracturing and faulting. Minor internal shortening in the cover was produced by contraction faults, buckle folds, and rare tectonic stylolites. The regional change in structural style from the thrust belt to the foreland may reflect a change in the physical conditions of deformation, caused (at least partly) by significant differences in the thickness of the sedimentary cover in the two areas.

Abstract This guide describes two areas of observation in the Idaho-Wyoming thrust belt. The first is a 3-mi (4.9-km) long section on Teton Pass, west of Jackson, Wyoming, along Wyoming 22 (Fig. 1). The geologic map (Fig. 2) and discussion below refer to a section that stretches from 2.5 mi (4.0 km) west of the summit of Teton Pass to 0.5 mi (8.0 km) to the east. The area is depicted on the Teton Pass and Rendezvous Peak 7¼-minute Geologic Quadrangle Maps (Schroeder, 1974, 1976). The second area is along the Hoback Valley and Canyon, about 10 mi (16 km) southeast of Jackson. Observations there are along U.S. 187–189, from Hoback Junction to Granite Creek, about 11.5 mi (18.5 km) east. The area is depicted on the Camp Davis and Bull Creek 7¼-minute Geologic Quadrangles (Schroeder, 1974, 1976). Two other locations in the area are briefly described. They are both on U.S. 26–89 in the Grand Canyon on the Snake River. The first is about 3 mi (4.8 km) southwest of Hoback Junction, where the Darby thrust is exposed, the other is at Keyser Creek, about 8 mi (12.9 km) east of Alpine Junction, where the Little Greys Anticline is exposed. All locations described herein are on public land.

Abstract Three stages are evident in the tectonic development of southeastern Idaho and western Wyoming. First are the changing patterns of tectonic elements during deposition; second, development of northward-trending folds and thrust faults; and third, development of block faults that produced horst ranges and graben valleys. During Paleozoic time about 50, 000 ft (15, 000 m) of marine sediments, mostly limestone and dolomite, were deposited in a miogeosyncline and about 6000 ft (1800 m) of mixed marine sediments were deposited on the shelf to the east. Detritus came from both east and west from Cambrian time onward. Starting in Mississippian time, the belt between shelf and miogeosyncline, where thicknesses increase markedly, shifted progressively eastward. During Mesozoic time about 35, 000 ft (10, 500 m) of marine and continental sediments were deposited in the western pan of the region and about 15, 000 ft (4500 m) in the eastern part, with terrestrial deposits becoming increasingly dominant. Western positive areas became the chief source of detritus. The belt of maximum thickening and the site of maximum deposition were relocated progressively eastward; maximum thicknesses of succeeding geologic systems are not superposed. In Late Triassic a belt on the west rose and the miogeosyncline started to break up. As Mesozoic time progressed the western high spread eastward, until by the end of the Jurassic the miogeosyncline gave way to intracratonic géosynclinal basins that received thick deposits, particularly in Cretaceous time. Cenozoic sedimentary rocks are products of orogeny in the region. The second stage, which overlapped the first, produced folds overturned toward the east and thrust faults dipping gently west in a zone, convex to the east, 200 mi (320 km) long and 60 mi (˜100 km) wide. Stratigraphie throw on many larger faults is about 20, 000 ft (6000 m); horizontal displacement is at least 10-15 mi (16-24 km). Lack of metamorphism and mylonite along the faults is striking. From west to east, the thrust faults cut progressivelyyounger beds, have progressively younger rocks in their upper plates, and are estimated to be successively younger.Thrusting started in the west in latest Jurassic and ended in the east perhaps as late as early Eocene time; detritus shed from emergent upper plates is preserved in coarse terrestrial strata of corresponding ages. West of the thrust belt is a northwestward-trending area underlain mostly by lower Paleozoic rocks and flanked on the east and west by upper Paleozoic and Mesozoic rocks. Scattered pieces of eastward-dipping thrust faults have been reported west of the older rocks. This central area of old rocks has been interpreted as: (1) part of a large continuous thrust sheet moved scores of kilometers from the west, or (2) an uplifted segment of the earth's crust from which thrust sheets on the east and west were derived. Both interpretations have defects; relative thrust ages are difficult to explain under the first; a large positive gravity anomaly, expectable under the second, is apparently absent. Block faulting, the third stage of tectonic development, started in Eocene time. Faulting has continued to Holocene time, as indicated by broken alluvial fans, displaced basalt flows less than 27,000 years old, and earthquakes. North-trending and east-trending fault sets are recognized. Old east-trending steep faults in the Bear River Range may be tear faults genetically related to thrusting. Movement along many faults has been recurrent. Patches of coarse Tertiary gravel on the flanks and crests of ranges, for which there is no provenance with present topography, may record reversed vertical movement along some north-trending faults. Present topographic relief of basins and ranges is tectonic.