Upper Cretaceous conglomerates in the Gualala basin, California, contain rhyolite and granite clasts that have been previously hypothesized to have disparate terranes of origin. New geochemical, age, and isotopic analyses of these conglomerate clasts suggest an igneous origin in the upper levels of the continental side of the Cretaceous Cordilleran magmatic arc. Sr-Nd isotope relations constrain a provenance in the pre-Neogene Salinia-Mojave arc segment. Given the tectonically reconstructed outboard depositional location for the Gualala basin, we interpret the initiation of rhyolite-granite conglomerate deposition in the Gualala basin to reflect the arrival of the westward-thrusted Salinian allochthon at the forearc, coincident with the collapse of the Salinia-Mojave segment of the magmatic arc. This interpretation constrains the age of batholithic collapse to be ca. 80 Ma and suggests that thrusting immediately followed or may have been contemporaneous with the youngest magmatic events in this part of the arc. The data do not support contributions from a northward-moving Baja British Columbia (Baja BC) terrane, or large-scale translation (∼2000 km) of the basin.