Subduction architecture of Angel Island, San Francisco Bay, California: Contributions from Franciscan accretion and post-accretion strike-slip faulting

The architecture of accretionary complexes can preserve the history of plate convergence and reveal conditions at depth in subduction zones. Studies in the Franciscan Complex in California, USA, were central in establishing the geologic evidence for subduction processes. However, the accretionary architecture has been modified by syn- and post-subduction strike slip and exhumation, complicating the interpretation of subduction structures. Tectonic architecture in the northern Coast Ranges has been proposed to extend into the San Francisco Bay, although controversy remains about the timing and the role of the present-day structures in plate boundary dynamics. Angel Island, the largest island in San Francisco Bay, is a sparsely developed state park that allows for detailed mapping over a relatively large area (3 km2). Angel Island has a more diverse combination of rock units and variable metamorphic history than previously described. We show that Angel Island exhibits distinct trench sediments and oceanic units juxtaposed by major faults. Accretionary structures such as a fault-stack of trench sediments with variable metamorphic grade underlie eastern Angel Island. A steep NW-striking, tens-of-meters-wide serpentinite body bounded by late brittle faults juxtaposes the stack against blueschist-grade oceanic units on the western side of the island. These NW-striking faults offset accretionary structures and record brittle deformation on western Angel Island suggesting activity at shallow depths. Crosscutting relationships between generations of structures demonstrate that post-accretionary deformation modified the modern outcrop pattern and highlights the role of strike-slip overprinting that must be accounted for while reconstructing Franciscan accretionary architecture.