Kinematics of Franciscan Complex exhumation: New insights from the geology of Mount Diablo, California

摘要

Three major lithotectonic units of an ancient arc-trench system are present in western California. In ascending structural order, they include: the Franciscan Complex, a remnant accretionary prism partly including high-pressure (blueschist-facies) metamorphic rocks in a subduction mélange; the relatively unmetamorphosed Coast Range ophiolite; and the Great Valley Group forearc sediments. In the Mount Diablo study area east of San Francisco, the subduction complex and ophiolite are juxtaposed across the Coast Range fault, and together they represent a structurally attenuated section through the ancestral forearc crust exposed by uplift and folding of the late Cenozoic Mount Diablo anticline. The Coast Range fault is locally associated with a sheared zone in the lower part of the attenuated ophiolite. Shear-sense indicators in the serpentinite, such as S-C fabrics, rotated clasts, and slick-enfibers, record the deformation kinematics and consistently indicate that the ophiolite moved down in a normal sense relative to the Franciscan rocks in the modern reference frame. Based on these kinematic relations and metamorphic facies contrasts between the Franciscan and the Coast Range Ophiolite, as much as 6–18 km structural attenuation has occurred along the Coast Range fault. After restoration of vertical axis rotation and late Cenozoic fold deformation, the Coast Range fault dips at a low angle toward the northeast, and shear-sense data restore to top-to-the-northeast motion on the Coast Range fault, implying normal displacement of the ophiolite with respect to the structurally underlying Franciscan Complex. These relations are consistent with the hypothesis that the Franciscan rocks were exhumed by crustal attenuation along a low-angle fault system. These data suggest that the Coast Range fault dominantly accommodated normal displacement and ductile thinning, and that these processes were primarily responsible for the structural exhumation of high-pressure assemblages in the Franciscan Complex relative to the structurally overlying forearc crust.