TRENCHWARD-DIPPING SEGMENT OF THE COAST RANGE FAULT EXPOSED ALONG THE WESTERN MARGIN OF THE DIABLO RANGE ANTIFORM, CALIFORNIA: REGIONAL-SCALE COMPONENT OF AN EXTENSIONAL DOME THAT ACCOMMODATED EXHUMATION OF HIGH-PRESSURE METAMORPHIC ROCKS

The Coast Range Fault (CRF) places forearc strata, Great Valley Group (GVG), and/or the Coast Range Ophiolite (CRO) with burial ≤10 km depth over the Franciscan Complex (FC), high-pressure/low-temperature metamorphic rocks with ≥20 km burial. The CRF was a syn-subduction exhumation fault, responsible for the uplift of the high-pressure FC relative to the structurally overlying CRO and GVG. Recent work in the northern Diablo Range at Mount Diablo, California, the CRF appears to originally have a trenchward-dip on the south flank of the window and arcward-dip on the north flank. Original dip directions and fault vergence was determined via fault cut-off relationships and the GVG dip fanning. This leads to the question whether the Mount Diablo structure is local or a regional feature in the Diablo Range. To address this question, fieldwork is being conducted on the west flank of the regional-scale Diablo Range antiform in which FC in the core of the range is bounded to the east and west by GVG and/or CRO.

Field work has shown the GVG to be west-dipping, with no over-turned beds and increasing bedding thickness towards the CRF trace, pointing to this reach of CRF originally dipped west (trenchward). The CRF can be followed over topography due on the basis of surface outcrops and difference in geomorphologic expression between the serpentinite that bounds the Franciscan on the west. Petrologic analysis shows that unfoliated prehnite-pumpellyite facies overlies jadeite+lawsonite or lawsonite+albite facies, suggesting there were extensional fault splays within the FC on a larger scale than seen at Mount Diablo. To better characterize the CRF and further support field work findings, geochronology and metamorphic petrologic data will be gathered at a later date.