ABANDONED TRIANGLE ZONE IN A BASEMENT-CORED UPLIFT—EXAMPLE FROM THE WESTERN CASPER ARCH, WYOMING

Structural wedging that results delamination of the foreland basin and the development of a triangle zone is a common process in thin-skinned thrust belts, but they have not been observed in basement-cored (thick skinned) structural settings. The Casper Arch is a NNW trending basement-cored uplift in the central Rocky Mountains of Wyoming. Displacement on the bounding fault zone increases northward from the latitude of Casper toward the intersection of the Arch with the EW-oriented Owl Creek Mountains.

Recent, high quality 3D seismic and well data have revealed a triangle zone that formed during thrusting and uplift of the Casper Arch, by structural wedging and delamination of Cretaceous sediments in the Wind River Basin. This triangle zone formed as the west-directed, uplift-bounding fault flattened into the Cretaceous marine sedimentary section and the hanging wall wedged itself for approximately 1.5km into the the basin. The wedge has an east-directed roof thrust that formed as the stratigraphic section in the Wind River Basin was delaminated and uplifted by the westward insertion of a wedge of deformed rocks. The triangle zone was abandoned as the uplift bounding fault broke back and cut through the back part of the wedge. The deformation occurred during the Late Cretaceous and Early Cenozoic, and the timing is recorded by growth geometry of the rocks deposited during deformation. These growth geometries are imaged on the seismic data and in exposures of progressive unconformities on the surface.