FRANCISCAN SHEAR ZONES BETWEEN COAST RANGE OPHIOLITE AND GREAT VALLEY GROUP ROCKS: EVIDENCE OF MéLANGE DIAPIRISM?

Shear zones with Franciscan Complex blocks that cut Coast Range Ophiolite (CRO) and Great Valley Group (GVG) rocks may provide insight into mélange genesis and deformation of the overriding plate during subduction. We examined examples of such shear zones in Hayward Hills of the San Francisco Bay area, California, where they crop out east of the late Cenozoic Hayward fault. These shear zones strike predominantly NW, sub-parallel to the well bedded rocks of the GVG. These mélange zones consist of a sheared shale and serpentinite matrix with a diverse block population which includes; CRO gabbro, GVG sandstone, serpentinite, actinolite schist, blueschist, amphibolite, and garnet amphibolite. Some of the serpentinite within the mélange contains antigorite, tremolite, and talc, indicating a higher grade of metamorphism than found in the lizardite-dominated serpentinites of the CRO. The presence of coherent GVG rocks of identical lithology structurally above and below the mélanges suggests a minimal amount of displacement between each sheet of GVG. However, the presence of high-grade Franciscan rocks in the mélange suggests a minimum of 20 km of differential exhumation was necessary to place the blocks at the same crustal level as the GVG and CRO which have undergone negligible burial metamorphism. We offer two alternative hypotheses to explain this complex geology: (1) One is emplacement by mélange diapirism. This may include earlier exhumation of high P metamorphic rocks to shallow depths by diapirism itself or other mechanisms, then diapirism to emplace the mélange between GVG sheets. (2) Another alternative is a sedimentary (olistostromal) origin for these mélange zones. This model includes exhumation of high P Franciscan rocks by multiple viable mechanisms, followed by erosion and deposition of Franciscan clastic material as a layer within the GVG. Subsequent crustal shortening and thrust faulting may repeat the mélange layer between the same GVG units and impart shearing to the matrix. In both alternatives the amount of displacement between coherent sheets of GVG is minimal. Whether the origin of these mélange zones is diapiric, olistostromal, or a combination will be determined by further field evidence such as shear sense indicators along the margins of the mélanges.