THE COAST RANGE OPHIOLITE, CALIFORNIA: MULTISTAGE ORIGIN OF A SUPRA-SUBDUCTION ZONE OPHIOLITE

The middle Jurassic Coast Range ophiolite (CRO) of California is one of the most extensive ophiolite terranes in North America, extending over 700 km from northern California to the southern Transverse Ranges. The CRO represents a major but short-lived episode of oceanic crust formation that affected much of western North America just prior to the Nevadan orogeny. Formation of the CRO was associated with convergent plate tectonics, as shown by its tectonic position overlying the Franciscan subduction complex, its stratigraphic position beneath fore-arc sediments of the Great Valley Series, and the chemical composition of lavas and plutons which comprise the ophiolite. Geochemical studies over the last 20 years have shown that the overwhelming bulk of all volcanic rocks erupted within the CRO formed by arc-related volcanism. Chemical characteristics common to all suites include low TiO2 (<1.3 wt%), low Nb (<3 ppm), Ti/V <22, Zr/Y <4, a range in SiO2 from basalt to rhyolite, and slightly depleted to enriched LREE. Volcanic rock associations and cross-cutting relationships in the plutonic section demonstrate three major igneous episodes: (1) eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex, (2) extensional deformation of older plutonics, intrusion of refractory wehrlite-cpxites, and eruption of refractory lavas, (3) eruption of silicic lavas and intrusion of hornblende-quartz diorite. This sequence represents the progressive stages of ophiolite formation in a rifted fore-arc setting. The last magmatic event to effect the CRO (stage 4) is “oceanic” in character, typically represented by basaltic dikes with MORB geochemistry (TiO2>1.4 wt%, Ti/V=25-30, Nb=6-14 ppm) or by OIB seamounts that overlie the ophiolite (Stonyford). We correlate this stage with a ridge collision event that resulted in a major reorganization of plate motions and the Nevadan orogeny.

This sequence of events is observed in many supra-subduction zone (SSZ) ophiolites, and suggests that all SSZ ophiolites form in response to a common process. The existence of this consistent progression implies that ophiolite formation is not a stochastic event, but is a natural consequence of the SSZ tectonic setting.