RADIOMETRIC AND BIOSTRATIGRAPHIC AGE RELATIONS IN THE COAST RANGE OPHIOLITE (CRO), NORTHERN CALIFORNIA: IMPLICATIONS FOR JURASSIC TECTONIC EVOLUTION OF THE WESTERN CORDILLERA

The Coast Range ophiolite (CRO) in northern California consists of two distinct remnants. The Elder Creek ophiolite is a classic suprasubduction zone ophiolite with three sequential plutonic suites (layered gabbro, wehrlite-pxite, quartz diorite), a mafic to felsic dike complex, and mafic-felsic volcanic rocks; the entire suite is cut by late MORB dikes and overlain by ophiolitic breccia. The Stonyford volcanic complex (SFVC) comprises three volcanic series with intercalated chert horizons that form a submarine volcano enclosed in sheared serpentinite. Structurally below this seamount are melange blocks of CRO similar to Elder Creek.

U/Pb zircon ages from plagiogranite/quartz diorites at Elder Creek range in age from 165 Ma to 172 Ma. U/Pb zircon ages obtained from CRO melange blocks below the SFVC are similar (166-172 Ma). 40Ar-39Ar ages of alkali basalt glass in the upper SFVC are all younger at=163 Ma. Chert lenses intercalated low in the SFVC volcanic section contain radiolarian assemblages similar to basal chert at other CRO locales, with a high diversity, polytaxic fauna. Chert intercalated high in the SFVC section, adjacent dated glass horizons, comprise a low diversity, oligotaxic fauna, similar to upper cherts elsewhere in CRO. Biostratigraphic ages are Bajocian-Callovian for the lower cherts and Oxfordian for the upper cherts.

These data show that (1) the CRO formed prior to the classic Nevadan orogeny by rapid forearc extension above a nascent subduction zone, (2) CRO spreading ended with subduction of an oceanic spreading center circa 163 Ma, coincident with the oldest high grade blocks in the Franciscan. We suggest that CRO formation began after the early Jurassic (175-180 Ma) collision of an exotic or fringing arc with North America, and initiation of a new east-dipping subduction zone. We further suggest that the “classic” Nevadan orogeny represents a response to spreading center collision, with shallow subduction of young lithosphere causing the initial compressional deformation, and the subsequent change in North American plate motion to rapid northward drift (J2 cusp) causing sinistral transpression and transtension in the Sierra foothills. These data are not consistent with models for late Jurassic arc collision in the Sierra foothills, or a back-arc origin for the CRO.