Early Onset of Franciscan Subduction

The Franciscan subduction complex of California is considered a type example of a subduction-accretion system, yet the age of subduction initiation and relationship to the tectonic history of western North America remain controversial. Estimates for the timing of Franciscan subduction initiation are largely based either indirectly on regional tectonic arguments or from the ages of high-grade blocks within mélange. Many of the high-grade blocks record counterclockwise pressure-temperature paths with early amphibolite overprinted by later eclogite and blueschist; however, their origin and significance with respect to subduction initiation have been debated. In contrast, some high-grade blocks show evidence for clockwise pressure-temperature paths and an early eclogite assemblage overprinted by later amphibolite Zircon U-Pb ages from inclusions in garnet and Lu-Hf estimates of initial garnet growth ages from these samples record early eclogite metamorphism at ~176 Ma. Matrix zircon U-Pb ages and Lu-Hf estimates of final garnet growth ages record a barroisite-amphibolite assemblage overprint of eclogite at ~160 Ma. Combined with petrologic data and existing geochronology, the data suggest that (1) Franciscan subduction was underway by no later than 180 Ma, (2) continuous subduction metamorphism occurred for at least 100 Ma, and (3) Franciscan subduction initiation predated the formation of the overlying Coast Range Ophiolite, supporting models that form the ophiolite above an east dipping Franciscan subduction zone