THE COAST RANGE OPHIOLITE (CRO) CALIFORNIA AND THE JURASSIC TECTONIC EVOLUTION OF THE WESTERN CORDILLERA IN NORTH AMERICA

The Coast Range ophiolite (CRO) of California is one of the most extensive ophiolite terranes in North America. It represents a major but short-lived episode of oceanic crust formation in the middle Jurassic, but its origin has been controversial. Proposals for its origin include mid-oceanic ridge, back-arc basin to an exotic arc that collided with North America in the late Jurassic during the Nevadan orogeny, and fore-arc spreading in a nascent arc along the cordilleran margin. Geochemical studies show that the overwhelming bulk of all volcanic rocks erupted within the CRO formed by arc-related volcanism. Volcanic rock associations and cross-cutting plutons suggest three major suites: arc tholeiites, refractory (boninitic), and calc-alkaline. This sequence represents progressive ophiolite formation in a rifted fore-arc setting, and rules out the mid-ocean ridge and back-arc basin models. MORB composition lavas are found in many locales, but only as late dikes that cross-cut arc suites, or as late flows. We correlate this stage with a ridge collision event that resulted in a major reorganization of plate motions. Modern U/Pb zircon ages show that most of the CRO formed over a short time range of 165-172 Ma, whereas 40Ar-39Ar ages of late oceanic basalts are 164 Ma. Biostratigraphic ages for chert overlying the ophiolite are Bajocian-Callovian for the lower cherts and Oxfordian for the upper cherts.

These data show that the CRO formed prior to the classic Nevadan orogeny by rapid forearc extension above a nascent subduction zone, and that CRO spreading ended with subduction of an oceanic spreading center circa 164 Ma, coincident with the oldest high grade blocks in the Franciscan. We suggest that CRO formation began after the mid-Jurassic (172-180 Ma) collision of an early Jurassic exotic or fringing arc with North America, and initiation of a new east-dipping subduction zone. We further suggest that the “classic” late Jurassic 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.