Abstract

Exposed along the Hoback Canyon in the western Wyoming thrust belt is the highly deformed footwall of the Bear thrust, an imbricate fault within the Prospect thrust sheet. Pennsylvanian sandstones here structurally overlie contorted Triassic redbeds which, in this outcrop, can be divided into upper and lower structural “panels” on the basis of unique folding styles. Evidence from field relations and dated thrust faults suggests that this exposure first developed as a large north-south trending anticline when the Prospect thrust sheet impinged on the Gros Ventre foreland buttress to the northeast. This fold was then faulted and truncated as the Bear thrust fault propagated to the east along an upper detachment within Triassic shales above the Prospect thrust. The sequence of folding, fold locking and subsequent thrusting followed by a final episode of folding indicates that the footwall of the Bear thrust fault has been shortened, thus changing the shape of the lower plate cutoff. Assumptions concerning constancy of ramp geometry are not true for this — and perhaps other — frontal thrust sheets.