Thrust sheet rotation and out-of-plane strains associated with oblique ramps: An example from the Wyoming salient U.S.A.

Abstract

Movement over oblique thrust ramps results in local thrust sheet rotation and principal strains which deviate from the transport plane. Resulting folds, faults and deformation fabrics are superposed on those associated with displacement on frontal ramps. The South Fork thrust, a hangingwall imbricate of the Absaroka thrust sheet, SW Wyoming, has a steeply-dipping (~70 °). small-displacement (~300 m), oblique ramp in the hangingwall and footwall, which makes an angle of 25 ° to the regional transport direction. Two penetrative deformation events are recorded by folding, stylolitic cleavage, and calcite twinning strains in the deformed hangingwall and footwall. The first is E- to SE-directed layer-parallel shortening prior to or during motion of the Absaroka thrust sheet. The second event indicates NE-directed shortening unique to the oblique ramp, at a high angle to ramp strike. At the trailing frontal ramp-oblique ramp intersection, folds in the footwall and normal faults in the hangingwall reflect local strike-parallel shortening and extension, due to out-of-plane deflection of the wallrock during displacement. The out-of-plane strains arc consistent with those predicted by kinematic and mechanical models. In the hangingwall of the oblique ramp, cross-strike fractures and bedding are rotated ~30° counterclockwise about a vertical axis with respect to structural domains to the north and south due to differential displacement on the South Fork thrust. Stylolitic cleavage and caleite strains unique to the oblique ramp show a similar change in attitude indicating the rotation was the latest recorded stage of deformation. In general, if an oblique ramp is shallowly-dipping, the hangingwall may move from the lower flat, up the ramp, and onto the upper Hat. If an oblique ramp has a steep dip, as in the case of the South Fork thrust, relatively greater sliding and bending resistance may impede displacement up the ramp and upper flat. Further transport of the thrust sheet is then accommodated by footwall deformation and hangingwall rotation.