Chasing the Franciscan megathrust and its implications for Deep Subduction Zone Dynamics: Insights from Angel Island, Franciscan Complex.

Abstract:

Deformation in blueschist facies rocks is linked to the source region of episodic tremor and slip (ETS). Angel Island, California is composed of blueschist facies metasedimentary and metabasic rocks containing potential paleomegathrust faults. Based on the syn-subduction accretion model proposed for the Franciscan Complex, Angel Island is hypothesized to host the deepest intact exhumed paleo-megathrust horizon in the Bay Area . Past studies have correlated Angel Island to other Franciscan Complex units based on sandstone compositions, foliation, and metamorphic grade but the structurally complex interior of the unit has never been mapped.

The previous mapping project done on Angel Island interpreted the poly-lithologic unit as “coherent” based on consistent internal metamorphic grade. 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 also observed several low-angle and higher angle faults displacing the blueschist and metaclastics units. We believe that the low angle faults are indicative of deeper, syn-subduction fault zones. The higher angle faults could represent the shallower, San Andreas related faulting patterns. 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. We used fault rock characteristics, fault orientation, and syn-kinematic high pressure minerals as an identifier for syn-subduction faults. For the next aspect of this project, we hope to verify our unit designations using detrital zircon dating and conduct detailed mapping to identify potential tremor and slip source structures.