Initiation of Franciscan subduction along a large-offset fracture zone: Evidence from mantle peridotites, Stonyford, California

We report here on the radiogenic isotope characteristics of peridotites from one ophiolite locale, Stonyford, in the Coast Range ophiolite (CRO) of California, spanning the entire observed range of upper mantle peridotites from both abyssal and subduction-zone settings. Based on the Sr-Nd-Pb-Hf isotopic compositions and estimated equilibration temperatures, we suggest that the abyssal peridotite block represents a remnant of large-offset transform oceanic lithosphere at >172 Ma. East-dipping, proto-Franciscan subduction is likely to have been initiated along this fault zone during a plate reorganization event, which produced the supra-subduction zone (SSZ) peridotites in the CRO. We propose that the remnant abyssal peridotites represent a snapshot of the mantle wedge composition prior to overprinting of large sectors by hydrous melting during the subsequent subduction-controlled SSZ processes. © 2008 The Geological Society of America.