A TALE OF TWO MECHANISMS: NEW U-Pb AGES, ZIRCON HF ISOTOPIC COMPOSITIONS, AND SPATIOTEMPORAL CONTEXT OF THE COAST RANGE VOLCANICS, CALIFORNIA

Volcanism in the Coast Ranges of California, spanning the latest Paleogene and the entirety of the Neogene, has been associated with mechanisms affiliated with the incipience and evolution of the transform margin between the North American and Pacific plates. These proposed mechanisms include the development of a northward-transient slab window, extensional magmatism associated with the development of the Basin and Range province, and/or the rotation of the Transverse Ranges. Here, we present a compilation of LA-ICP-MS U-Pb ages and eHf compositions of zircon from 35 Coast Range volcanic localities. Pairing of these datasets with a Paleogene-Neogene paleogeographic reconstruction of California reveals two distinct regimes of volcanism in the Californian Coast Ranges. The first regime encompasses a suite of localities, spanning ca. 3-28 Ma with a gap between ca. 12-22 Ma, that become both progressively younger and increasingly eHf-positive along a northward trajectory. The paleogeographically-reconstructed localities of eruptive centers within this regime reflect a northward migration, including across the ca. 12-22 Ma gap, that is broadly consistent with modern relative plate velocities across the Pacific-North American transform boundary. This trend likely reflects the northward migration of a slab window source. Spatially, the paleogeographically-reconstructed gap in volcanism along this northward track is broadly coincident with the present-day extent of high-velocity seismic zones around 70km deep (the Isabella Anomaly and Transverse Range Anomaly) that have been suggested to represent foundered slabs. We posit that these structures may have occluded mantle heat flux through the transient slab window, resulting in the temporary cessation of slab-window volcanism. The second regime comprises a cluster (in paleogeographically-reconstructed coordinates) of ca. 16-18 Ma eruptive centers all with similarly-positive (8-10) eHf values that do not conform to the aforementioned northward-younging trend. This second regime of Californian Coast Range volcanism is putatively associated with the incipience of extension and rotation of the Transverse Ranges.