Cenozoic volcanic rocks of coastal California were erupted west of the magmatic arc trend related to subduction along the continental margin. Two assemblages, representing discrete pulses of mid-Tertiary (Oligocene–Miocene) and mid-Miocene volcanism, occupied relatively compact tracts, as restored palinspastically prior to disruption and dispersal by tectonic displacements within the Neogene San Andreas transform system. A third assemblage records migratory post–mid-Miocene volcanism at centers located progressively farther north. The distribution and petrologic character of all three assemblages reflect slab-window magmatism triggered by decompression melting of upwelling mantle.

Paleogeographic reconstructions, based on analysis of magnetic anomaly patterns offshore and restoration of on-land features prior to San Andreas transform slip and associated transrotational deformation, indicate the paleotectonic positions of the Cenozoic volcanic fields before structural disruption. The pulses of mid-Tertiary and mid-Miocene volcanism were related to transient episodes of mantle upwelling generated successively by rise-trench encounters associated with subduction of the Vancouver-Farallon and Monterey-Arguello plates, respectively, along the continental margin off southern California. Migratory post–mid-Miocene volcanism in central California accompanied the incremental expansion of a growing slab window as the Mendocino triple junction migrated northward along the continental margin.

Mid-Miocene Columbia River basalt volcanism in the Pacific Northwest was coeval with the mid-Miocene pulse of coastal volcanism and may have reflected tectonism induced by the final demise of offshore microplates to allow initial full integration of the San Andreas transform as a coherent plate boundary. Columbia River volcanism may have stemmed from mantle perturbation caused by torsional deformation of the continental block in response to shear imposed by the Pacific plate.

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