RELATIONSHIP BETWEEN SHALLOW SUBDUCTION OF THE PELONA AND RELATED SCHISTS AND DISMEMBERMENT OF THE MOJAVE/SALINAN MARGIN OF SOUTHERN CALIFORNIA

Central and southern California preserve one of the best ancient examples of shallow subduction and associated tectonic underplating. The defining features of this Late Cretaceous – early Cenozoic shallow subduction zone are the Pelona-Orocopia-Rand schists and correlative rocks in the Salinian block. These rocks broadly resemble the Franciscan Complex, but were emplaced directly beneath the mid-Cretaceous batholith and adjoining cratonal areas of southeastern California and southwestern Arizona. Emplacement of the schists is widely believed to have been linked with dismemberment of the continental margin batholith and its forearc region along the Mojave/Salinian segment of the California margin. In southern California and its displaced correlatives within the central Coast Ranges west of the San Andreas fault, the convergent margin belts are severely disrupted by the Nacimiento fault, which separates the Salinian block on the northeast from the Nacimiento block to the southwest. The Salinian block is cored by granitoid plutons similar in age (100–75 Ma) and composition to those of the central belt of the Sierra Nevada and regions to the east, yet lacks an analog to the western foothills belt of the Sierra. The Nacimiento block, in contrast, is underlain by the Franciscan Complex and remnants of sedimentary sequences equivalent to the distal parts of the type Great Valley Group, but is missing forearc units indicative of a proximal setting. By comparison with the Franciscan–Great Valley–Sierran triad east of the San Andreas fault, the juxtaposition of the Salinian and Nacimiento blocks along the Nacimiento fault implies the removal of a width of 150 km or greater of formerly intervening westernmost arc and inner to central forearc basin. Although recent efforts based partially on geo-thermochronometry suggest that removal of the Mojave/Salinian segment of the margin likely occurred between 75 and 60 Ma, the mechanism involved remains unclear. Previous interpretations include (1) large-scale underthrusting, possibly achieved via west-directed gravitational collapse; (2) large-magnitude, sinistral strike slip faulting; and (3) very significant displacement dextral strike-slip faulting (e.g., Mojave-BC hypothesis). This talk discusses ongoing research aimed at evaluating these possibilities.