DATING FARALLON SHALLOW-ANGLE SUBDUCTION: CONSTRAINING THE AGE USING U-PB ZIRCON GEOCHRONOLOGY OF THE SCHIST OF SIERRA DE SALINAS

The schist of Sierra de Salinas of central California represents subduction accretion assemblages formed along the western margin of North America. A sandstone protolith was underthrust during a period of shallow-angle subduction of the Farallon plate, bringing the sandstone in direct contact with recently extinct granitic plutons in the upper plate and leading to high-temperature metamorphism. In this study, we use U-Pb zircon geochronology to assess the maximum depositional age of the protolith and the timing of metamorphism of the schist, thereby placing tight constraints on the timing of the protolith’s underthrusting. Oscillatory zoned detrital zircon domains from two samples were analyzed using LA-ICP-MS, yielding a main age peak centered at ca. 90 Ma along with lesser peaks scattered through Paleozoic and Proterozoic ages. Maximum depositional ages of ca. 80 Ma were calculated from both samples. Depth profiling of metamorphic rim domains, recognized as bright convolute zones in cathodoluminescence images, was conducted via SHRIMP-RG analysis and yielded weighted mean ages of ca. 78 Ma, with a small subset of ages scattering from 78 to 60 Ma. Slab shallowing is widely regarded as a consequence of impingement of a large igneous province with the margin of North America, hence, metamorphic ages of ca. 78 Ma likely indirectly date the timing of collision. In aggregate, our results require that deposition of the protolith of the schist was essentially synchronous with its underthrusting, each occurring close to 80 Ma. This indicates that the collision of the large slab along the North American margin was the driving factor of upper plate uplift and a subsequent increase in erosion to create the protolith of the schist. Emplacement of the schist of Sierra de Salinas and correlative schists of southern California records a profound mode shift from non-accretion to tectonic erosion and significant accretion. We posit that shallow-angle subduction drove upper plate uplift, erosion, and a flood of detritus into the trench, which switched the margin into an accretionary mode.