OLDEST AND MOST RECRYSTALLIZED OLISTOSTROMAL HORIZON IN THE FRANCISCAN SUBDUCTION COMPLEX, FOUND IN THE SKAGGS SPRINGS SCHIST OF SONOMA COUNTY CALIFORNIA

The mechanisms of mixing blocks into a mélange are still strongly debated and detailed studies of block-matrix relationships in the Franciscan Subduction Complex (FSC) have been confined to units with non-recrystallized to partly recrystallized matrix. The Skaggs Springs Schist (SSS) of the FSC is composed primarily of completely recrystallized quartz-rich glaucophane-lawsonite-phengite schist. Jadeitic clinopyroxene and garnet are present in some samples and epidote relics in lawsonite are common. The primary exposure belt of the SSS extends along strike for 70 km with a width of up to 3 km in Sonoma County of the northern California Coast Ranges. The SSS is the oldest FSC unit of significant size with Ar-Ar phengite ages of approximately 132 Ma and a maximum depositional age of 144 Ma (based on a single zircon). The SSS had been proposed as a recrystallized olistostromal matrix based on the large concentration of high-grade (eclogite, amphibolite, coarse blueschist) exotic blocks within the Cazadero area (separate from the main outcrop belt), but block-matrix contacts were not found. Recent fieldwork has identified SSS exposures of the main exposure belt above Warm Springs Creek that are made up of conglomerate with a variety of clasts and blocks up to .5 m in a matrix composed of SSS. The conglomerate horizon is approximately 1 m thick, whereas most of the 2 km thickness of the SSS along the Warm Springs Creek transect lacks exotic blocks. The SSS is interpreted as subducted trench fill consisting of sandstones and shales (block-free parts) with olistostromal conglomerate horizons. The field relationships suggest exhumation of high-grade Franciscan material (high-grade metamorphic ages of approximately 176 – 155 Ma) to the sea floor by 144 to 132 Ma.