Recent geologic studies in the western United States Cordillera provide evidence of extensional deformation in Cretaceous time, during development of the Sevier foreland fold and thrust belt. This evidence comes from the region roughly bound on the east by the fold and thrust belt and on the west by the Mesozoic continental arc. We postulate that extension in this Internal Zone accommodated gravitational collapse of the. evolving Sevier orogen. Major surface-breaking normal faults and extensional basins that characterize other regions of syncompressional gravitational collapse (such as the Himalayan orogen) have not been documented in the Cretaceous record of the Internal Zone. Consequently, if extension was widespread in the Internal Zone, then it had little surface expression. We propose a conceptual model of the Sevier orogen, consistent with geologic constraints and supported by simplistic rheological arguments, in which westward movement of a mid-crustal extensional allochthon was driven by buoyancy stresses in the over-thickened Internal Zone. Bound at the top by the master décollement 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.

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