Abstract

Geologic relationships of the contact between highly deformed Mesozoic strata and weakly deformed Cenozoic strata in Sevier Valley, Utah, have been variously interpreted to reflect the Sevier unconformity, surficial gravity gliding, salt diapirism, large-volume pressure solution, and a rolling-hinge normal fault. Field study and structural analysis were used to evaluate these five alternative hypotheses. Slickenlines, sheared and offset clasts, and secondary mineral growth that all define a consistent top-to-the-east shear sense indicate that the contact is a fault rather than an unconformity, a pressure-solution surface, or the roof of one or more salt intrusions. The interpretation is confirmed by tilted Cenozoic strata that dip discordantly into the contact and by downward termination of Cenozoic strata and faults at the contact. The absence of a contractional toe, and mapped continuity of the contact with the West Sanpete Valley fault to the north in Sanpete Valley, indicate that the contact is a rooted normal fault rather than a surficial glide surface. Balanced cross sections across Sevier Valley and adjacent ranges confirm the presence of a fault interpretation and indicate that the heave across the fault, here termed the Salina detachment, is approximately 3 km. The present domal shape of the fault may be interpreted either in terms of a rolling hinge, or as reactive salt and shale diapirism caused by tectonic removal of overburden. Both terms refer to footwall ascent driven by buoyancy forces, either generated (rolling hinge) or triggered (reactive diapirism) by tectonic exhumation, and therefore to distinguish the two processes is difficult. The unconventional geometry of Sevier Valley, in which an extensional basin formed on the footwall rather than a hanging wall of the master normal fault, results from the mechanical weakness and relative buoyancy of the Arapien Shale in the footwall which facilitated rolling-hinge footwall rebound as the hanging wall was tectonically removed.