LATE CRETACEOUS STRUCTURE AND TECTONICS OF THE CALIFORNIA GREAT VALLEY (GV) FOREARC, AND ITS RELATIONSHIPS WITH THE SOUTHERN SIERRA NEVADA-SALINIA SUPRA-SUBDUCTION ZONE CORE COMPLEX (SSCC)

Petrography on >250 GV basement cores, complimented by U/Pb zircon ages and Sr-Nd isotopes show that the Sierra Nevada batholith (SNB) extends westwards to at least the axis of the GV beneath Upper Cretaceous forearc basin strata. This westernmost zone of the SNB yields ages of 115 to 140 Ma, has depleted mantle isotopic signatures, and consists of abundant hornblende rich mafic cumulates, as well as diorites and tonalites. Metamorphic pendants are typical of the western Sierra Foothills, including Paleozoic abyssal ophiolite, and Jurassic arc-forearc igneous suites and slaty turbidites. Coupled gravity-magnetic highs arise from belts of coalesced mafic batholithic ring dike complexes. An axial GV magnetic high arises from structural relief on its regionally W-dipping mafic basement surface, which extends westwards beneath the eastern Coast Ranges (CR) along young E-directed blind thrusts. Rare low-grade basites occur in the deepest westernmost cores, which could correlate to the CR ophiolite. Geophysical data suggests a steep (intrusive) contact between the low-grade ophiolitic domain and the GV-SNB batholith, and a mid-crustal remnant upper mantle layer for the western ophiolitic domain that is underlain by a low velocity/density layer interpreted as underplated Franciscan-related lower crustal channel remnants. The GV-western SNB was rapidly exhumed to 2-3 kb levels at 100-90 Ma, and then covered by GV marine strata by 90-85 Ma. Subsidence curves for these strata resemble 1-D crustal stretching models. Seismic structure across the basement nonconformity commonly shows syn-depositional normal faults, and at deeper crustal levels pervasive lamination typical of extensional provinces. Retro-deformation of E-directed blind thrusts yields a CR-GV basement structure that resembles the Cycladic core complex of the modern Aegean forearc. The hypothetical CR-GV core complex is partitioned from the more highly extended SScc by the Kern Canyon-White Wolf transfer zone, which developed over a lateral ramp in the CR-Rand subduction megathrust system. A model is presented whereby regional Late Cretaceous SSZ core complexes developed as the trailing edges of the conjugates to the principal massif (SScc), and the broad northern shoulder (CR-GV core complex) of the Shatsky Rise subducted beneath California.