Age, provenance, and geochemical relationships amongst the Great Valley Group, Coast Range Ophiolite, and Franciscan subduction complex at Del Puerto Canyon, central California

Abstract

The Great Valley Forearc (GVF) basin of California preserves an extensive rock record of subduction zone processes, including evidence for the earliest states of forearc evolution. During initiation of the basin in the latest Jurassic-earliest Cretaceous, the sedimentary succession comprising most of the GVF, the Great Valley Group (GVG), was deposited atop the Coast Range Ophiolite (CRO) and later, atop older GVG strata and the Franciscan subduction complex along the GVF's eastern margin. We apply U-Pb geochronology, zircon and whole rock geochemistry, and petrographic analyses to rocks of the GVG, Franciscan subduction complex, and CRO in the northern San Joaquin Basin to better understand the timing and tectonic location of initial forearc sedimentation, and how sediment routing systems may have evolved during Early to Late Cretaceous time. At Del Puerto Canyon, the oldest strata of the GVF basin are preserved as a series of black shales, with subordinate sandstone and limestone concretions, that accumulated during the earliest Cretaceous, between ~145-140 Ma. These strata, referred to as the Knoxville Formation, are separated by a ~40 myr unconformity with the overlying Upper Cretaceous deep-water turbidites of the Panoche Formation, which were deposited between ~96- 85 Ma. Pre-Mesozoic age zircon grains are present in both the Knoxville and Panoche Formations, but sparse (0-7%) compared to other GVG sandstones. Zircon geochemical analyses support a predominantly felsic source (Th/U 0.9-0.2) during both periods of deposition, but Epsilon Hf signatures reveal a shift from contributions from juvenile to more evolved sources, broadly consistent with metavolcanic signatures of the Sierra Nevada magmatic arc, and more so with the Sierra Nevadan western metamorphic belt. Whole-rock geochemistry shows increasing compositional maturity from crystallization of the CRO during the latest Jurassic to deposition of the Knoxville and Panoche Formations during the Early and Late Cretaceous, respectively. Integrating these data, we present a tectonic model for the GVF basin at the latitude of Del Puerto Canyon that highlights the potential mechanisms to trap sediment during forearc initiation and hypothesize as to mechanisms to explain the ~40 myr unconformity and return to high-flux sedimentation during the earliest Late Cretaceous.