The discovery of the critical assemblage lawsonite + quartz + calcite indicates that a significant part of the Cenozoic Olympic subduction complex of northwestern Washington State formed by underplating at a depth of about 11 km. The deep structural level exposed in this area is attributed to the presence of a 10-km-high arch in the underlying Juan de Fuca plate. We postulate that this arch was formed when the southern Cordilleran coastline swung westward as a result of middle Miocene to recent extension in the Basin and Range province.

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