Origin of Miocene Volcanism near the Coast In Central and Southern California, Revisited

Intrusive and extrusive igneous rocks of mostly middle Miocene age crop out in widely separated areas near the coast in central and southern California and the offshore islands (e.g., Dickinson, GSAB 109, 1997). These generally consist of bimodal basalt to rhyolite suites of mostly calc-alkalic compositions but also include some alkalic basalts. Although the rocks typically are not geochemically depleted, their proposed source is the Pacific asthenosphere that upwelled beneath the continental margin due to e.g., subduction of the East Pacific Rise when the spreading center intersected the trench, rifting and rotation of the western Transverse Ranges block, and filling of the slab gap that developed behind the captured Monterey microplate. It is generally believed that decompression melting of the upwelling depleted asthenosphere produced primitive basaltic magmas that intruded the overlying crust where they underwent fractional crystallization and contamination by arc-related materials.

The region offshore central and southern California has abundant intraplate seamount volcanoes and available data suggest that the episode of volcanism in this region is generally coeval with that of the middle Miocene onshore volcanism (Davis et al., GSAB 114, 2002). Although the offshore intraplate lavas are chemically diverse, generally enriched, and lack a subduction component signature, their composition also overlaps with that of the middle Miocene onshore lavas. Here I propose that the upwelling of the depleted Pacific asthenosphere beneath the cold western margin of North America alone could have not caused the majority of both offshore and onshore middle Miocene magmatism. The upwelling upper mantle had to contain easily meltable, geochemically enriched components that have a lower melting temperature than the depleted peridotite asthenosphere. Melting of such components can explain the temporal and compositional similarity between onshore and offshore volcanic lavas as well as the generally wide and sporadic distribution of onshore Miocene igneous rocks.