The Raft River extensional shear zone is exposed in the Albion-Raft River-Grouse Creek metamorphic core complex. Several studies of ductile deformation have shown that it accommodated crustal stretching in Tertiary time during late orogenic collapse of the thickened Cordilleran crust. Progressive deformation that results from mixed pure and simple shear produces a complex strain pattern along the shear zone. We propose a numerical kinematic model that relates strain variations in the shear zone to the different amounts of extension between the brittlely extended upper plate rocks and the undeformed lower plate rocks. A possible hypothesis for postorogenic crustal extension involves diffuse heterogeneous extension and flow in the ductile lower crust and localized extensional shear zones in the middle and upper crust.

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