Contrasting Settings of Serpentinite Bodies, San Francisco Bay Area, California: Derivation from the Subducting Plate vs. Mantle Hanging Wall?

Abstract

Although the best-known outcrops of serpentinite in the California Coast Ranges are part of the Coast Range ophiolite (CRO), significant serpentinite bodies crop out within the Franciscan subduction complex, and others have ambiguous affinity. These serpentinites provide evidence of subduction-accretion processes. The Hunters Point shear zone (HPSZ), an intra-Franciscan structural horizon that consists of both a regionally extensive serpentinite body and shale matrix mélange, extends tens of km along strike, and has a structural thickness of 1 to 1.5 km. The HPSZ exhibits little mixing of serpentinite and shale. The HPSZ consists of a structurally high shale matrix mélange with mostly sandstone blocks, an intermediate zone composed of a serpentinite sheet (or sheets) containing metagabbro lenses, and a structurally low shale matrix mélange including a variety of different block types, but lacking serpentinite and gabbro. These field relations suggest that the serpentinite in the HPSZ became part of the subduction complex by offscraping of the remnants of a mantle core complex from the downgoing plate, rather than having originated from the upper mantle hanging wall of the subduction zone. In contrast, serpentinite exposed within a km east of the Hayward fault in the southern Hayward Hills is intermixed with shale on scales of tens of meters to centimeters. This serpentinite is present in shear zones, ranging from 50 m to several centimeters in structural thickness; these shear zones cut sandstones of the Great Valley Group (GVG), forearc basin deposits that locally overlie the Coast Range ophiolite in depositional contact. The shear zones also include blocks of of basalt, blueschist, amphibolite, and gabbro. Serpentinite bodies in the southern Hayward Hills may have been derived from the mantle hanging wall of the subduction zone relatively early in the history of the subduction zone before much tectonic underplating occurred; after substantial underplating, the mantle hanging wall would have been largely blocked from contributing material to the subduction channel. The serpentinite and shale matrix were exhumed to approximately the same crustal level as the CRO and GVG, before a final stage of faulting mixed pieces of both into the mélange and emplaced the mélange into shear zones cutting GVG rocks.