Regional structural and stratigraphic relations in the northern California Coast Ranges require an episode of extensional tectonic unroofmg of coherent blueschist terranes of the Eastern belt of the Franciscan complex along a crustal-scale, low-angle, normal-fault system, here named the Coast Range detachment. This tectonic unroofmg occurred during the latest Cretaceous and early Tertiary, contemporaneous with the compressional Laramide orogeny of the North American interior. Dynamic models of accretionary wedges predict that shallowing of subduction, as has been inferred during the Laramide orogeny, will impart extensional stresses in the wedge above the subducting plate. We therefore suggest a single causal event—gradual shallowing of subduction in the latest Cretaceous and early Tertiary—to link the extensional fault uplift of regional Franciscan blueschists to Laramide compression in the North American interior.

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