Back-thrusting along the western flank of the Bighorn Mountain, Bighorn County,Wyoming

Field evidence of westward back-thrusting, compartmentalized faulting, and tectonically-thinned fault wedges supports a thrust-generated model of uplift for the Bighorn Mountains. Encompassed within the Leavitt Reservoir Quadrangle are structures suggestive of westward basement-involved thrusting antithetic to the main eastward thrust direction of the Bighorn uplift. The mountain-flank region in this area is characterized by reverse-faulted blocks of Precambrian basement which are draped by a deformed Paleozoic cover. A reentrant along the mountain front coincides with an east-west compartmental boundary separating two distinct areas of structural discordance. North of this fault boundary, compression appears to have been accommodated by folding of the hanging wall block. To the south, Precambrian rocks exposed at the mountain-front overlie, and are in fault-contact with a tectonically-thinned, overturned wedge of Paleozoic and Mesozoic strata. Structures i the basinal portion of the Leavitt Reservoir Quadrangle include small-scale folds and a thrust-faulted anticline paralleling the NW-SE trend of the larger mountain-flank structures. Decollement may have occurred parallel to bedding planes within multiple stratigraphic horizons. This deformation probably represents concomitant basinward ramping of the sedimentary sequence in response to westward back-thrusting of Precambrian blocks during the Laramide Orogeny.