Early Cretaceous transition from nonaccretionary behavior to strongly accretionary behavior within the Franciscan subduction complex

During subduction at the Franciscan trench beginning at 170-160 Ma and continuing to the present, marine sedimentary and lesser volcanic rocks have been underthrust, accreted, and metamorphosed to form the Franciscan accretionary wedge. The South Fork Mountain Schist (SFMS) forms the eastern margin and structural top of the wedge and so was apparently the first unit of substantial size to accrete into the Franciscan. New U-Pb ages of detrital zircons and 40Ar/39Ar ages of metamorphic white micas indicate that the SFMS accreted surprisingly late in the time span of Franciscan subduction, probably at ca. 123 Ma. At least parts of the Valentine Spring Formation and Yolla Bolly terrane accreted soon thereafter. Therefore, accretion of the SFMS apparently marks a fundamental transition from virtually nonaccretionary behavior to strongly accretionary behavior in the Franciscan forearc. About 75% of modern forearcs are nonaccretionary, so recognition of an early nonaccretionary period is consistent with modern examples. The Franciscan mafic high-grade exotic blocks (and related very small slabs), which yield ca. 132 to 169 Ma metamorphic ages, are the only well-documented rock record of the early approximate to 40-m.y.-long nonaccretionary period and represent very sparse accretion during that interval. The transition into an accretionary regime at ca. 123 Ma was essentially synchronous with the end of the Late Jurassic-Early Cretaceous magmatic lull in the Sierra Nevada magmatic arc, with the waning of Early Cretaceous faulting and warping within the Great Valley forearc basin, and possibly with changes in motion vectors of the Pacific and Farallon plates. Citation: Dumitru, T. A., J. Wakabayashi, J. E. Wright, and J. L. Wooden (2010), Early Cretaceous transition from nonaccretionary behavior to strongly accretionary behavior within the Franciscan subduction complex, Tectonics, 29, TC5001, doi:10.1029/2009TC002542.