ABSTRACT

New road cuts of U.S. Route 14a along the western flank of the Bighorn Mountains of Wyoming create nearly continuous outcrops through the edge of a Laramide uplift. Exposed granitic basement in the range-front area is divided into three large, differentially rotated blocks separated by shattered, poorly exposed zones. Within each of these rotated blocks are three populations of faults: (1) an early set of dominantly strike-slip faults at high angles to the front with either polish or smearing of hematite, epidote, and/or chlorite, (2) a slightly younger set of faults with mostly similar motion and mineralization, but no smear or polish, and (3) a young set composed mostly of thrust faults marked by gouge and pervasive shattering.

Stress tensor analysis on these three fault populations shows that maximum compressive stress (sigma 1) acted N25E-S25W, normal to the local range front, but changed plunge with time. In the early buckling stages, sigma 1 plunged at about 30° in both directions away from the future buckle. Subsequently, sigma 1 assumed a relatively uniform 30° plunge into the range, presumably as a result of reduced resistance to buckling. Finally, under extremely brittle conditions associated with regional uplift and erosion, sigma 1 assumed a relatively flat plunge through the rotating blocks. The exposures support models of basement “folding” by subhorizontally driven tectonic forces creating an early buckle zone that expanded into a small number of rotated blocks with subsequent, beanbag-like, minor reshaping of the rotating blocks by many small faults.

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