New Insights On Franciscan Complex Geology, Architecture, Depositional Age, And Provenance For The Western Mt. Tamalpais Area, Marin County, California

New geologic mapping of the Franciscan Complex in the western Mt. Tamalpais area northwest of San Francisco, California, reveals a tectonostratigraphic stack of accretionary units that young westward and structurally downward. Serpentinite-matrix melange at the top of the section, beneath a possible basal fragment of Coast Range Ophiolite, and a structurally low black-rock-bearing, mudrock-matrix Ocean Plate Stratigraphy (OPS) melange located near the base of the section, bracket lithic and feldspathic metasandstone-dominated accretionary units. The ages of the metasedimentary accretionary units are constrained by U-Pb age dating of detrital zircons. Maximum depositional ages fall between 124 Ma and 88 Ma. Sediment provenance is dominantly in the Sierra Nevada arc to the east, but minor numbers of detrital zircons were derived from Paleozoic and Precambrian sources. The provenance age profiles (cumulative distribution plots) reveal changing age components of zircon populations over time, reflecting the history of unroofing in the Sierra Nevada arc. An unusual post-accretion thrust fault and atypical greenschist facies metamorphism uncharacteristic of the Franciscan Complex, plus probable Late Cenozoic faults, complicate the character of the geology of the Mt. Tamalpais area.