CONTRASTING VOLCANIC STYLES IN THE CUESTA RIDGE OPHIOLITE REMNANT: EVIDENCE FOR SSZ FORMATION AND RIDGE COLLISION

The Coast Range Ophiolite (CRO) is a crucial geologic entity of oceanic character that must be interpreted correctly to fully understand the tectonic evolution of the Cordilleran margin of California. Previous studies have suggested formation at mid-ocean spreading centers, back-arc basins, or in a supra-subduction zone (SSZ). Recent work from the Cuesta Ridge ophiolite remnant, combined with previous studies of other CRO remnants, provides a unique opportunity to study the origin of the CRO.

Geologic mapping of the Cuesta Ridge ophiolite revealed a nearly complete ophiolite sequence sitting synclinally above the Franciscan Complex. The section consists of a 1.5 km thick mantle section of serpentinized harzburgite and dunite, intrusive non-layered gabbros, sills of wehrlite and pyroxenite, calc-alkaline and arc volcanics, a sheeted sill complex of quartz diorite, and a late stage of basaltic dikes.

At Cuesta Ridge there are two contrasting volcanic styles present. The most abundant and stratigraphically lowest have Ti/V ratios (11-20) and trace element concentrations (e.g. low HFSE, low Zr/Y) that are atypical of MORB and suggest formation in a SSZ setting. Forty percent of the volcanics are boninitic with high MgO (11-14 wt%), Cr, and Ni abundances, suggesting formation in the fore-arc region. The presence of high silica quartz diorites (52-75 wt% SiO2) is also atypical of mid-ocean ridge formation. The late stage dikes and uppermost volcanics, however, have Ti/V ratios (20-27), Ti-Zr plots, and Zr/Y-Y plots that suggest genesis at a mid-ocean spreading center.

The new field and chemical data from Cuesta Ridge point toward formation in a SSZ setting above the east-dipping proto-Franciscan subduction zone due to the onset of subduction and subsequent slab rollback. Multiple stages of magmatism followed, until the injection of the last late stage MORB dikes. These dikes are most likely the product of a ridge collision event that thermally disrupted the margin and ended igneous activity in the ophiolite. Finally, the ophiolite was raised via accretionary uplift of the buoyant Franciscan Complex that was subducted beneath it.