Much is published about the Cretaceous history of the California Great Valley forearc basin, but little is widely known about the structure, composition and initial exhumation history of its crystalline basement. This talk integrates the results of a long-term petrologic and geochronologic study of basement cores from the Great Valley with: 1) published data on the geophysically imaged structure of its basement; 2) the subsidence and structural history of nonconformably overlying Upper Cretaceous strata; 3) the initial exhumation history of the adjacent Sierra Nevada batholith; and 4) the initial structural assent of the Franciscan subduction complex in the adjacent Coast Ranges. The most critical data to be presented shows that much of the Great Valley basement;is the western margin of the Early Cretaceous Sierra Nevada batholith, along with enclaves that are typical of the western Sierra metamorphic belt. Out of ~300 basement cores studied ~35 per cent are mylonites and chlorite retrograded cataclasite breccias. These findings as integrated with topics 1, 2 and 3 (above) seem to indicate that much of the Great Valley basement is characterized by Late Cretaceous west-dipping low-angle normal faults cut by superposed early syn-depositional high-angle normal faults. These findings integrated with topic 4 render a regional crustal structure that resembles late Cenozoic supra-subduction core complexes of the Aegean forearc.
The synthesis presented above is integrated with published and ongoing findings on the geologic history of an 5000 km2 expanse of lower crustal batholithic and underplated Franciscan assemblages exposed in the southern Sierra, western Mojave and pre-Neogene restored Salinia region. This region was exhumed in the Late Cretaceous even more deeply and rapidly than the Great Valley-Franciscan segment to the north. Differential exhumation and extension were partitioned across the Kern Canyon-White Wolf crustal scale transfer zone that developed over a northward descending lateral ramp in the Rand-Coast Range subduction megathrust system. A case can be made from plate kinematic reconstructions, and accreted pelagic limestone blocks in the Franciscan complex that core complex segmentation resulted from the subduction of the conjugate to the Shatsky Rise oceanic LIP that was embedded in the Farallon plate, and that the megathrust lateral ramp mimics the (conjugate’s) steep bathymetric gradient between the principal Shatsky LIP and its broad northern rift shoulder. Large magnitude supra-subduction extension can be correlated to an intense pulse of focused slab rollback as the trailing edge of the oceanic LIP subducted, similar to that which has been proposed for the subduction of a northern Gondwanna rifted continental ribbon actively driving extension in the Aegean system.