Geometry and Tectonic Setting of Sea-Floor Spreading for the Josephine Ophiolite, and Implications for Jurassic Accretionary Events along the California Margin

Abstract

Ophiolites with Jurassic petrogenetic ages occur in the western Sierra Nevada and Klamath Mountains, California Coast Ranges, and probably underlie much of the California Great Valley. The Josephine ophiolite of the western Klamath Mountains represents one of the most complete, intact, and best studied of the
Jurassic ophiolites. A fracture zone history of the ophiolite is indicated by a thin crustal section, highly fractionated ferrobasalts, and a distinctive olistostrome locally overlying the ophiolite interpreted as the sediment fill of a fracture zone valley. Orientations of sheeted dikes, transform remnants, and upper mantle flow fabrics all indicate a sea-floor spreading direction and transform motion parallel to the continental margin. Likewise, some dike orientations and fracture zone features of the Sierra Nevada and Coast Range ophiolites suggest spreading and transform motion parallel to the continental margin. The Great Valley is a major morphotectonic feature that may have originated as a large transform valley that in later Cretaceous time became a forearc basin. Geophysical lineaments of the Valley along with fracture zone features of the northern Coast Range ophiolite and southern Sierra Nevada suggest a left-stepping family of fracture zones in and around the margin of the Great Valley. It is suggested that Jurassic ophiolites of California record major sinistral-sense transform motion and related oblique spreading oriented parallel to the continental
margin. Such motion may be linked to the Mojave-Sonora megashear and reflects decoupling between North American and Pacific basin plates during rapid northwestward motion of the North American plate.

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