Abstract

The area between the northern, Crawford, and southern, Meade, thrust sheets (southeastern Idaho-northwestern Wyoming) is a region of stratigraphically controlled thrust-to-thrust displacement transfer. Balanced, restored cross sections through the area demonstrate that the Meade and Crawford plates have unique structural detachment levels and are, in fact, separate thrust sheets.

The Meade thrust places Madison Limestone (Mississippian) over Twin Creek Limestone (Jurassic) in the north and Madison Limestone over Phosphoria Formation (Permian) in the south as the thrust detachment cuts down stratigraphically in the footwall south toward Montpelier, Idaho. The Crawford thrust places Wells Formation (Pennsylvanian) on Cokeville Formation (Cretaceous) in the south and cuts down stratigraphically in the hanging wall to the north, terminating in an overturned anticline. Detachments exposed in the Crawford thrust sheet give way to displacement on the Meade thrust to the northwest, folding of the upper Meade plate, refolding above and minor eastward displacement on the Twin Creek detachment.

The sequence of deformation recorded in carbonate rocks within the Crawford plate shows that early east-west layer-parallel compression resulted in the formation of a bedding-normal solution cleavage, followed by calcite twinning. Subsequent folding above a ramp in the Crawford plate caused folding of the first cleavage, development of a weak second cleavage, bedding-plane slip in argillaceous units and, finally, forelimb thrusting. In the Meade plate petrofabric analysis of twinned calcite also shows an early east-west layer-parallel shortening. Twinning strains in younger, syntectonic veins are more complex due to fold-related bending strains and formation of a solution cleavage.