Provenance evolution of the Late Cretaceous--Middle Eocene Gualala basin, California: Geochronologic, geochemical, and isotopic evidence from conglomerate clasts

This dissertation documents a detailed geochemical and isotopic provenance investigation into the origin and evolution of source terranes of Upper Cretaceous and Eocene conglomerates from the Gualala basin of coastal northern California. The basin, currently located at the northern extremity of the displaced Salinian block, has been offset from its depositional location by about 465 km on the Neogene San Andreas and San Gregorio-Hosgri fault systems. Geologic data suggests that additional, pre-Neogene displacement (on the order of tens to a few hundreds of km) is necessary to juxtapose the basin with appropriate sediment sources. In contrast, paleomagnetic data from the same units have been interpreted to suggest 1800 km of northward transport since the Eocene, and possibly up to 4000 km since the Juristic.; Coordinated study of the petrographic, geochemical, geochronologic, and isotopic characteristics of Gualala basin conglomerate clasts offers new insight into the changing provenance of the basin between the Late Cretaceous and Middle Eocene. Upper Cretaceous conglomerates record an initial influx of granites and rhyolites that have mid-Cretaceous crystallization ages, derived from high levels of the inboard side of a continental magmatic arc. Clasts of this type mix with, and finally give way upsection to gabbroic clasts that have oceanic isotopic compositions and Late Jurassic crystallization ages. The Upper Cretaceous clast population is interpreted to be derived from the northern margin of the westward collapsed Salinia-Mojave segment of the California batholithic belt; the only area throughout the North American Cordillera where the appropriate lithologies are tectonically juxtaposed adjacent to the forearc. Eocene conglomerate clasts were derived from west-central portions of the Early to mid-Cretaceous batholiths and record a change in provenance, which may reflect early Paleogene tectonism. The provenance constraints established by this study of Gualala basin conglomerate clasts are inconsistent with the large displacements (>1000 km) implied by paleornagnetic data. Furthermore, the conglomerates in the Gualala basin yield no definitive evidence for the passage of a “Baja BC” terrane; rather, they are firmly linked to a provenance in North America.