Abstract: Coast Range Ophiolite: Paleoequatorial Ocean-Ridge Lithosphere

HOPSON, C. A., J. M. MATTINSON, B. P. LUYENDYK, W. BEEBE, E. A. PESSAGNO, JR., D. M. HULL, I. M. MUNOZ, and C. D. BLOME

The succession of sediments trapped within and accumulated on a mobile oceanic plate comprise its plate stratigraphy and provide a travel history from its birthplace. Plate stratigraphy at Point Sal, Stanley Mountain, Cuesta Ridge, and Llanada ophiolite (CRO) remnants shows that the oceanic crustal rocks originated beyond reach of terriginous or volcanic arc sedimentation and then moved progressively closer to the coeval Jurassic arc that fringed western North America. The stratigraphy records (1) middle Jurassic (approx. 164-170 Ma) oceanic lithosphere construction in an open-ocean ridge setting with pelagic carbonate sedimentation (interlava limestone) followed by sea-floor spreading transport progressively through (2) a deep-water region of starved pelagic sedimentation (Callovian-lower Oxfordian hiatus), (3) a submarine distal tephra fringe downwind of an active arc (thin Oxfordian-Tithonian tuff radiolarian chert strata), (4) the proxymal volcaniclastic apron of the arc (Tithonian tuff, volcanic sandstone, submarine debris-flow breccia and chert at Llanada), and (5) burial beneath terriginous muds and distal turbidites (Upper Tithonian basal part of Great Valley Group) that advanced over the seafloor following the Nevadan orogeny. CRO pillow lavas (Point Sal, Llanada remnants) formed near the mid-Jurassic paleoequator (paleomagnetic data) and were carried progressively northward, through Central and North Tethyan faunal provinces into Southern Boreal province (radiolaria and mollusca data), by mid to late Jurassic seafloor spreading. There, basaltic to silicic magmatism beneath the deep sea floor (new ridge-tip propagation?) took place in the late Jurassic, marked by basaltic to silicic sills/dikes and hydrothermal alteration in the CRO and its unconformable volcanopelagic cover (Cuesta Ridge and Llanada CRO remnants). Those events preceeded forearc basin development.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California