Abstract

Immediately north of Big Piney in western Wyoming, the Moxa arch is modeled as emplaced during the Late Cretaceous along an east-dipping, low-angle thrust (Moxa thrust) that has Precambrian basement and Paleozoic and younger cover rocks in the hanging wall. The west-verging Moxa thrust cut up-section from the basement-cover contact and flattened to the west along a detachment in Lower Triassic rocks (Thaynes detachment). During motion on the Moxa thrust, the leading edge of the hanging wall wedged westward along the Thaynes detachment, peeling back Triassic and younger rocks and thrusting them relatively eastward along the ancestral Prospect thrust. The 5.3 km of westward movement along the Moxa thrust was matched by eastward movement along the west-dipping ancestral Prospect thrust. The ancestral Prospect thrust moved an additional 5.1 km during the late Paleocene when thrust-belt deformation progressed eastward to the Moxa arch. It appears that in the Snider Basin area, the Prospect thrust does not share a ramp with the Darby thrust and that the emplacement of the Moxa arch and Prospect thrust determined the position of the later and more westerly Darby thrust.

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