ABSTRACT: Overview of the Tectonic Setting of the Great Valley Ophiolite, California

ROBERTSON, ALASTAIR, University of Edinburgh, Edinburgh, United Kingdom

Combined field, petrological, and geochemical evidence suggests that the 169-161 Ma Great Valley Ophiolite (GVO) formed above a subduction zone during the early stages of genesis of a magmatic arc within the Jurassic Pacific Ocean. Key supporting evidence includes: (1) presence of primary acidic intrusives and extrusives (rather than altered MORB); (2) very depleted, immobile trace-element compositions of dikes and extrusives; and (3) the acidic, tuffaceous nature of the in situ sedimentary cover (including andesitic and intrusive calc-alkaline clasts). Absence of terrigenous material from the 165(?)-148 Ma sedimentary cover further suggests the ophiolite formed well away from continental margin influence. The localized presence of high-Mg andesitic extrusives, and comparisons with mo ern southwestern Pacific settings and Tethyan ophiolites, favor a fore-arc, rather than a back-arc or intra-arc setting for GVO genesis. The related oceanic arc is concealed by Great Valley sediments, but is locally exposed in the Sierra Nevada foothills (e.g., Copper Hill volcanics and Smartville arc-ophiolite?). The arc-fore-arc geography implies that the GVO was generated above an eastward- (continentward) dipping oceanic subduction zone. Around 155 Ma, the suprasubduction-zone ophiolite underwent pervasive crustal extension, followed by mass wasting of submarine fault scarps (exposed in northern California). By 148 Ma (middle Tithonian), the GVO had docked with the western America continental margin, probably following collision of the GVO oceanic arc with a separate Sierra Nevada co tinental margin arc to the east. The GVO ophiolite was then established as the Great Valley fore-arc basement. Franciscan-type melange (e.g., Paskenta) was accreted below the slab in the west, and terrigenous and tuffaceous sediments accumulated above, forming the base of the Great Valley sequence.

AAPG Search and Discovery Article #91016©1992 AAPG-SEPM-SEG-EMD Pacific Section Meeting, Sacramento, California, April 27-May 1, 1992 (2009)