THE BIRTH OF THE GREAT VALLEY FOREARC BASIN, CALIFORNIA

The Great Valley forearc (GVF) basin of California is the archetypal accretionary forearc basin, yet our understanding of the timing (Jurassic or Cretaceous), process (extensional versus accretionary), and location (central or southern North America) of basin development remains controversial. Inspired by the contributions Dr. Eldridge Moores, we present new U-Pb ages from detrital zircon samples from the basal stratigraphy of the GVF, as well as new ages from its underlying basement, the Coast Range Ophiolite (CRO), exposed along the western outcrop belt. Eighteen of 20 samples (3711 new U-Pb ages) contain Cretaceous zircons, with 9 samples yielding statistically robust Cretaceous maximum depositional ages (MDAs), 2 with MDAs that overlap the J-K boundary, suggesting earliest Cretaceous deposition, and 9 with Jurassic MDAs, supporting Late Jurassic deposition. Two new titanite U-Pb ages from the top of the CRO are consistent with previous work and yield a ~165 Ma age for forearc basement in the northern GVF. When combined with our new detrital zircon U-Pb ages, the duration of the unconformity between the CRO and GVF in the northern part of the basin is <15-23 m.y.. We interpret this to indirectly reflect a period of sediment bypass where sediments eroded from the upper plate were transported to the trench and likely subducted. We interpret Jurassic and Cretaceous MDAs of the basal GVF as evidence for diachronous initiation of forearc sedimentation, reflecting the growth of isolated depositional centers which initially accumulated sediment at different times. Extensional processes appear to govern the early stages of forearc development, similar to other forearc systems that show diachroneity and fragmented depositional centers at basin inception. In addition, pre-Mesozoic age populations of our GVF samples contain a distinctly central North America source signal and do not require a southern provenance source.