Oligocene and Miocene blowtorch volcanism within the California Borderland and its influence on the plate-boundary transition

The California Borderland is a uniquely broad and complex region of the North American−Pacific transform margin. Oligocene to Miocene structural reorganization, including large-magnitude extension and vertical-axis rotation of crustal blocks, formed a network of submerged fault-bounded basins, ridges, and islands offshore of southern California during its evolution from a convergent to transform boundary. Here, we report new 40Ar/39Ar incremental heating age determinations and geochemical analyses of volcanic rocks in the California Borderland, as well as a reconciled regional tectonic model. The California Borderland volcanic rocks are calc-alkaline in composition and mostly silica-oversaturated and represent relatively large-degree partial melts of the asthenosphere that incorporated hydrated forearc lithospheric components. A temporal reconciliation of these records shows that subduction of an active spreading center starting in the Oligocene and again in the Miocene was the critical control on the observed volcanism at ca. 31−30 Ma and 19−13 Ma. We posit that prolonged trench-ridge interaction generated a blowtorch effect, which resulted in widespread forearc volcanism and weakening of the crust to facilitate the unique deformation styles documented in the borderland that did not occur elsewhere along the North American−Pacific plate boundary.