OVERVIEW: Tectonic implications of Cenozoic volcanism in coastal California

Abstract

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.