Abstract

Scattered igneous activity marks the Triassic margin of California, but an Andean volcanic-plutonic arc began to form in the Sierra Nevada–White-Inyo Range and Klamath Mountains at ca. 170 Ma during underflow of the oceanic lithosphere. This calc-alkaline belt supplied volcanogenic erosional debris to the Mariposa-Galice proximal overlap sequence by ca. 165–160 Ma. Transpression also generated high-pressure–low-temperature basaltic eclogites, garnet-blueschists, and amphibolites along the Middle and Late Jurassic convergent plate junction at ca. 170–155 Ma; most of these returned surfaceward as tectonic blocks only during the mid-Cretaceous. The high-grade metamafic (metamorphosed mafic) blocks that formed during construction of the emergent arc and derived Mariposa-Galice strata predate the onset of Franciscan trench deposition. Near the end of Jurassic time, the Klamath salient evidently migrated ∼200 km westward. An earliest Cretaceous westward stepout of the convergent junction apparently formed directly offshore from the Klamath imbricate orogen, but to the south, preexisting oceanic crust–capped lithosphere was trapped landward as the Coast Range ophiolite. Andean arc detritus began to accumulate on this ophiolitic basement within the Great Valley forearc and outboard Franciscan oceanic trench at ca. 145–140 Ma. Because earliest Cretaceous, relatively continuous Great Valley strata were deposited on the stable North American plate, protected from both surface and subcrustal erosion, forearc terrigenous sedimentation also signaled coeval deposition in the outboard trench ∼10–25 m.y. after Middle and Late Jurassic initiation of the continental margin calc-alkaline arc. Voluminous sedimentation and the accretion of Franciscan and San Joaquin sections of Great Valley rocks took place during mid- and Late Cretaceous time after a flare-up in igneous arc activity at ∼125–120 Ma. The youngest Sierran granites have ca. 85 Ma ages, reflecting extinction of the magmagenic zone beneath northern and central California, and presumably reflecting subhorizontal oceanic plate underflow attending the Laramide orogeny. Feebly zeolitized Upper Cretaceous and Lower Tertiary Franciscan Coastal belt rocks, derived from the now-inactive arc, underwent only low-pressure burial metamorphism. Reflecting shallow offloading and/or large-scale transpression, they were not appreciably subducted. Although Coastal belt units are members of the Franciscan lithotectonic assemblage, their origin contrasts with the rest of the subduction complex.

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