ABSTRACT: Coast Range Ophiolite of the Sierra Azul Block Southwest of Los Gatos, California

MCLAUGHLIN, R. J., R. W. KISTLER, and J. L. WOODEN, U.S. Geological Survey, Menlo Park, CA, and C. R. FRANCK, University of Washington, Seattle, WA

Near Los Gatos, an approximately 2300-m-thick section of Jurassic Coast Range Ophiolite is overlain by Tithonian-Valanginian and Campanian strata of the Great Valley sequence, and by Eocene to early Miocene strata. The ophiolite questionably was obducted eastward and attenuated in the Late Jurassic, and underwent additional extensional pulses at 60-52 Ma and 30-20 Ma. Beginning about 18 Ma, extensional structures were overprinted by northeast-vergent thrusting and dextral slip. Removing 145 km of right slip of the San Andreas fault system places these rocks near Cholame, California, at 20 Ma. Earlier right slip probably also occurred at 70-55 Ma along a suture cut and offset by the San Andreas fault system. Segments of the suture now align with the Sur-Nacimiento fault and faults alon the southwestern side of the Diablo Range.

A composite section of the ophiolite consists in ascending order of 610 m ultramafic cumulates (no tectonic hz); 152 m gabbro cumulates; 1200 m microgabbro to diorite dikes and sills; 300 m spilitic flows and flow breccias; 5 m laminated andesitic tuff with radiolarian chert near base; and greater than or equal to 7 m upwardly coarsening quartz-keratophyre pyroclastic breccia. The dikes and sills and extrusive lavas are low in Ti (less than or equal to 9000 pm), and high in SiO{2} (greater than or equal to 52%). Geochemical plots indicate that the ophiolite predominantly is arc like, with minor MORB, and (or) within-plate components. {87}Sr/{86}Sr ratios are 0.706-0.707 for the quartz keratophyre breccia, 0.704-0.705 for the underlying lavas, dikes and sills, and gabbro cumulates, and 0.703-0.7055 for ultramafic cumulates. A zircon separate from hbl-alb dikelets in the dikes and sills includes clear and metamict populations, suggesting inheritance from highly evolved wall rocks. These data suggest that the ophiolite comprises intrusive roots and higher level extrusives of an arc that formed above an off-ridge extended margin setting.

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