PILLOWS, SCHIST, AND A LOT OF WACKE: SUBDIVIDING THE FORMERLY "COHERENT" ANGEL ISLAND TERRANE OF THE FRANCISCAN COMPLEX

The Franciscan Complex of California hosts an assemblage of variably deformed and metamorphosed rock units that were part of a subduction zone along the western North American margin, from late Jurassic through to Tertiary. Within this complex, the terranes were progressively underplated, i.e. younger units lie structurally below older units, to form the accretionary prism. The Angel Island nappe, part of the Franciscan Complex, is the structurally highest coherent blueschist terrane within the San Francisco Block. Based on the syn-subduction accretion model proposed for the Franciscan Complex, Angel Island Nappe is hypothesized to host the deepest paleo-megathrust structures in the San Francisco region. Past studies have correlated Angel Island to the rest of the Franciscan Complex based on sandstone compositions, foliation patterns and metamorphic grades. The previous mapping project done on Angel Island interpreted the poly-lithologic unit as “coherent” based on consistent internal metamorphic grade, but the structurally complex interior of the nappe has never been mapped.

Preliminary mapping allowed us to update the existing map of the Angel Island, with the discovery of new units and fault zone structures. Our first-order interpretations revealed variations within the previously mapped metaclastics unit that warrant subdivision. We used a combination of analog and digital mapping techniques (Field MOVE) to create an updated geologic map of parts of the island. We also observed several low-angle and later, high angle faults displacing the blueschist and metaclastics units. These generations of fault zones control the outcrop pattern on the island, so more recent faults must be differentiated before interpreting structures formed during Mesozoic subduction. For further work, we hope to corroborate our field observations using petrographic studies of the meta-greywackes and add an age relation to these rocks using detrital zircon geochronology, and relate new discoveries of serpentinite-matrix melange containing blueschist and eclogite to other similar occurrences in the Franciscan.