DETRITAL ZIRCON GEOCHRONOLOGY AND GEOCHEMISTRY OF SOUTHERN CALIFORNIA CRETACEOUS FOREARC STRATA, WESTERN TRANSVERSE RANGES

Provenance studies of forearc strata provide insight into the timing of uplift and exposure of adjacent lithotectonic belts, and growth and deformation of convergent margin structures. In particular, detrital zircon geochronology and geochemistry has become widely used because it can determine the maximum depositional age of the strata, as well as characterize ages of the source rock. Furthermore, recent compilations of zircon geochemical data allow differentiation of discrete zircon geochemical populations correlated with continental versus oceanic source rocks through comparison of trace element ratios (e.g., U/Yb).

Previous provenance studies from Nacimiento forearc strata exposed along the central California coast suggest a period of uplift and erosion of ophiolitic crust within the forearc and isolated from the arc during the Albian–early Cenomanian. This study presents detrital zircon age and geochemical data from the western Transverse Ranges south of the Santa Ynez River fault. Although these samples are now adjacent to the Nacimiento block, they represent an area of the forearc that formed over 100 km south of the southernmost samples from the Nacimiento block because the western Transverse Ranges along the Santa Barbara coast rotated clockwise ~90 degrees during Neogene deformation. Our preliminary results data from the western Transverse Ranges yield maximum depositional ages of 103 Ma for the Lower Cretaceous Espada Formation and 69 Ma for the Upper Cretaceous Jalama Formation. Importantly, the Espada Formation includes a 176–159 Ma low U/Yb zircon that is not present in the younger Jalama Formation, and indicates the presence of a primitive, erosional source to the southern California forearc basin during the Aptian. This supports results from the Nacimiento forearc and suggests that uplift and erosion of Jurassic Coast Range ophiolite in the forearc may have been widespread throughout the southern California Mesozoic forearc. Future work will examine petrographic properties of western Transverse Ranges forearc strata and expand the dataset to explore provenance variability throughout the Cretaceous depositional history of the Espada Formation.