Abstract

Prebatholithic rocks of Mesozoic age in the Sierra Nevada can be interpreted as remnants of ancient volcanic arcs, subduction complexes, and sequences of oceanic lithosphere. Two partly coeval subparallel volcanic arcs, one in the western foothills and the other in the northern and eastern Sierra Nevada, are juxtaposed. The western arc was an east-facing island-arc complex that evolved through a series of steps including formation of a remnant arc and interarc basin. The eastern arc was a west-facing marginal arc that was constructed on the edge of North America. Both arc-subduction complexes consumed intervening oceanic lithosphere and collided during the Late Jurassic Nevadan orogeny. Generation of magmas in both arcs apparently ceased at about this time, and renewed subduction was initiated west of the island arc in latest Jurassic time, giving birth to the Franciscan-Sierran arc-trench complex. Fault zones and mélanges in the western Sierra Nevada reflect the complex suturing at the collision boundary. Pre-Tithonian ophiolite at the base of the Great Valley sequence in the Coast Ranges originated in a back-arc or marginal basin setting with respect to the coeval Sierran foothills arc.

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