Comparison of mesoscopic and microscopic deformational styles in the Idaho-Wyoming thrust belt and the Rocky Mountain foreland
Gautam Mitra, Joseph M. Hull, W. Adolph Yonkee, Gretchen M. Protzman, 1988. "Comparison of mesoscopic and microscopic deformational styles in the Idaho-Wyoming thrust belt and the Rocky Mountain foreland", Interaction of the Rocky Mountain Foreland and the Cordilleran Thrust Belt, Christopher J. Schmidt, William J. Perry, Jr.
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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.