ABSTRACT: Postulated Ridge Subduction in California During the Late Jurassic: Unifying Hypothesis to Explain the Nevadan Orogeny and the Formation of the Great Valley Fore-Arc Sequence
MURCHEY, BENITA L., and M. C. BLAKE, JR., U.S. Geological Survey, Menlo Park, CA
Oceanic crust (basalt and overlying chert) in the two major accretionary complexes of California, the Klamath-Sierra complex and the younger Franciscan Complex, exhibit opposite younging directions. In the Klamath Mountains, Permian to Jurassic oceanic rocks of the Klamath-Sierra complex young to the west, indicating accretion of progressively younger crust with time. In the Franciscan Complex, Lower to Middle Jurassic radiolarian cherts directly overlying basalt young to the east and record accretion of progressively older crust until the middle Cretaceous. We suggest that the oceanic rocks in the two accretionary complexes formed on opposite sides of a spreading ridge subducted beneath California during the Late Jurassic. The Coast Range Ophiolite formed (approximately 165 Ma) as th ridge approached North America, possibly forming at the ridge itself. Nearly synchronous formation of Klamath-Sierra complex ophiolites probably occurred within the Jurassic arc complex. The Nevadan orogeny and related deformation in the Klamath-Sierra complex may have happened as the arc system collapsed behind the advancing ridge.
Regardless of whether the Coast Range Ophiolite initially formed at the subducting ridge, it apparently became part of a volcanically active fore arc by 155 Ma. The beginning of Franciscan Complex subduction and Kimmeridgian(?) and Tithonian initiation of Great Valley sequence fore-arc sedimentation followed ridge subduction. These events coincide with a Late Jurassic (late Kimmeridgian to early Tithonian) cusp in the North American apparent polar wander path and may coincide with the beginning of major plate reorganization (140-130 Ma) in the Pacific basin.
AAPG Search and Discovery Article #91016©1992 AAPG-SEPM-SEG-EMD Pacific Section Meeting, Sacramento, California, April 27-May 1, 1992 (2009)