ABSTRACT: Formation and Deformation of the Coast Range Ophiolite and Related Rocks Near Paskenta, California
BLAKE, M. C., JR., A. S. JAYKO, and B. L. MURCHEY, U.S. Geological Survey, Menlo Park, CA, and D. L. JONES, University of California at Berkeley, Berkeley, CA
In the Paskenta area, the Coast Range Ophiolite clearly is the oceanic basement for the Great Valley sequence. To the north and south along the western margin of the Great Valley, the ophiolite and much of the lower Great Valley sequence have been removed by faulting. Our detailed mapping and paleontological studies, plus published sedimentologic and isotopic data, suggest the following complex history for the Paskenta area, and perhaps the entire Sacramento Valley.
(1) Formation of the ophiolite about 165 Ma, probably at a spreading center, based on K-Ar and U-Pb dating of intrusive gabbro and plagiogranite.
(2) Dispersal to an active arc by 155 Ma to account for dated (K-Ar) diorite and quartz dikes that intrude the ophiolite.
(3) Extensional faulting, uplift, and erosion of the ophiolite, deposition of the Crowfoot Point ophiolitic breccia, and formation of the fore-arc basin (155-150 Ma).
(4) Deposition of the Great Valley sequence in the forearc basin (150-80 Ma).
(5) Extensional faulting related to underplating of the Franciscan subduction complex beneath the ophiolite and formation of the Round Mountain serpentinite melange.
(6) Left-lateral tear faulting along the Paskenta fault zone concurrent with westward thrusting of the ophiolite and Great Valley sequence probably due to eastward tectonic wedging of the Franciscan that began in the Eocene.
(7) Eastward thrusting of the Franciscan over the ophiolite along the Coast Range fault and ophiolite over Great Valley sequence along the Stony Creek fault; probably active today, but could be associated with point 6.