Geodynamic controls on slab window volcanism: Insights from the spatiotemporal evolution of the Coast Range Volcanics, California, USA

Intracontinental volcanism inboard of growing transform boundaries has often been associated with the opening of slab windows following the collision of an oceanic spreading ridge with the continental margin. Yet, the specific drivers of supra-slab-window volcanism remain unresolved, partly due to uncertainties surrounding the spatiotemporal evolution of volcanic activity inboard of the evolving transform boundary. We report the distribution, age, and geochemical evolution of volcanism above a slab window by integrating 33 new zircon U-Pb ages and paired zircon Hf isotope measurements from the Coast Range Volcanics of California (USA) into a paleogeographic reconstruction of western North America from 27 Ma to the present. A cluster of 18−15 Ma volcanic centers in central California are interpreted to have been driven by extension of the Transverse Ranges above a condensed geotherm, while a northwest-younging linear progression of volcanism, spanning ca. 27−3 Ma, is proposed to be associated with the vertical vorticity component of toroidal flow of the shallow mantle around the northwest-migrating trailing edge of the subducting Gorda slab. Furthermore, our model suggests that a 22−12 Ma gap in the northwest-younging volcanic trend was driven by occlusion of the Gorda slab edge by a thick crustal root in the southern Sierra Nevada. This occlusion likely also interrupted or redirected regional eastward shallow-mantle flow beneath the continental margin, putatively reducing asthenospheric traction on the base of the crust below western North America and driving increased rates of extension and volcanism in the Basin and Range province.