PROVENANCE AND DEPOSITIONAL AGES OF THE GREAT VALLEY FOREARC AND RELATIONSHIP TO THE UNDERLYING COAST RANGE OPHIOLITE COMPLEX AT DEL PUERTO CANYON, CENTRAL CALIFORNIA

The Great Valley forearc basin in California formed during convergence of the Farallon-North American Plates during Late Jurassic-Eocene time. Along its western margin, forearc sediments accumulated on top of ophiolitic basement, collectively termed the Coast Range Ophiolite (CRO). Although the Great Valley forearc has been studied for decades, there are still uncertainties on the mechanisms of basin formation and the timing between the emplacement of the CRO and initial basin sedimentation. In Del Puerto Canyon, ~ 150 km east of San Jose, forearc strata is part of the Jurassic-Cretaceous Stony Creek Formation, which is overlain by the Upper Cretaceous Panoche Formation. The CRO remnants preserve an almost complete oceanic sequence of ultramafic rocks, gabbro, dikes and volcanics nearly 5 km thick. Preliminary field observations suggest a locally preserved depositional contact between the CRO and basal forearc strata, but in other locales within the canyon the contact is faulted against the Panoche Formation. We collected and analyzed 3 sandstone samples from the forearc stratigraphy immediately overlying the CRO for detrital zircon U-Pb geochronology (n = 245, 238, and 249 grains). The maximum likelihood depositional ages (MLAs) for three samples mapped as Panoche Formation are 102.62 ± 0.97 Ma, 144.61 ± 1.15 Ma, and 145.07 ± 0 .22 Ma. These are significantly older than the cited age for the unit, which is Campanian (~83.5-73.5 Ma), suggesting strata at two of these localities (144 Ma and 145 Ma) may be part of the basal Stony Creek Formation and record the early stages of forearc deposition. However, age spectra from all samples contain one unimodal Cretaceous-Jurassic peak, indicating proximal sources of the coeval Sierra Nevada magmatic arc and no input from older North American sources. This contrasts the age spectra of other latest Jurassic-earliest Cretaceous forearc strata, which commonly consist of >50% pre-Mesozoic ages derived from the retroarc region. Ongoing work seeks to understand whether these MLAs represent true depositional ages or are significantly older due to the age of source material available. These age relationships have important implications for understanding the joint evolution of the forearc and its underlying basement.