Paleogeographic reconstruction of regional accretionary complex architecture, Franciscan Complex, northwestern San Francisco Bay Area, California, USA

Abstract

The Franciscan Complex of western California, USA, the archetypal subduction accretionary complex, cannot serve as a model subduction accretionary complex unless its local-to-regional architecture is clearly under- stood. Yet, architectural details are not clearly understood in many regions, including the northwestern San Francisco Bay Area. Here, Cenozoic-age, dextral strike-slip faulting on faults of the San Andreas System fragmented the original architecture, forming crustal blocks and juxtaposing fragments of accretionary rock of different tectonostratigraphy. One little-known Cenozoic-age fault and block boundary, the Tamarancho Shear Zone, separates northeastern crustal blocks that are dominated by Franciscan rocks from southwestern blocks with significantly different Franciscan accretionary tectonostratigraphy. The northeastern blocks have abbreviated accretionary stacks with at least one blueschist-facies accretionary unit, whereas the southwestern blocks lack blueschist-facies accretionary units and have either a westward-and-downward–younging Franciscan tectonostratigra- phy or thrust-sheet stacks composed of partial sequences of ocean-plate stratigraphy rocks. The northwestern San Francisco Bay Area Franciscan Complex is bounded on the southwest by the San Andreas fault (sensu stricto) and on the northeast by the Petaluma Valley–Point Richmond–Silver Creek fault. Using paleogeographic reconstruction, the original Franciscan Complex accretionary architecture of the northwestern San Francisco Bay Area can be partially reconstructed by removing block separations on San Andreas System faults and enhanced by unfolding Cenozoic folds. Accretionary units of the northwestern San Francisco Bay Area Franciscan Complex were originally assembled ~190 km southeast of their present locations, west of the southern Diablo Range. Reconstruction of the accretionary complex in that location and considerations of tectonostratigraphy require that the Novato Block, located northeast of the Tamarancho Shear Zone, and the Mt. Tamalpais Block, to its southwest, be separated along or across strike in the reconstructed accretionary complex. Either dual subduction zone or faulted plate geometries produced the northwestern San Francisco Bay Area segment of the accretionary complex, and each model highlights the possibilities of along- or across-strike variations in the structure and history of the accretionary complex.