Extension in the Cretaceous Sevier Orogen, North American Cordillera

Evidence of extensional deformation in Cretaceous time, during development of the Sevier foreland fold and thrust belt comes from the region roughly bound on the east by the fold and thrust belt and on the west by the Mesozoic continental arc. It is postulated that extension in this "Internal Zone' accommodated gravitational collapse of the evolving Sevier orogen. A conceptual model of the Sevier orogen, consistent with geologic constraints and supported by simplistic rheological arguments is proposed in which westward movement of a mid-crustal allochthon was driven by buoyancy stresses in the overthickened Internal Zone. Bound at the top by the master decollement of the Sevier fold and thrust belt and at the bottom by a west-dipping, normal-sense shear zone, this extensional allochthon would have been effectively decoupled from the upper and lower crust and free to accommodate gravitational collapse during continued convergence across the orogen. -from Authors