TECTONOSTRATIGRAPHIC FRAMEWORK OF THE WESTERN METAMORPHIC BELT OF THE SIERRA NEVADA FOOTHILLS, CALIFORNIA

The Western Metamorphic Belt lies west of the Melones-Dogwood Peak faults and extends beneath the eastern margin of Late Cretaceous and younger overlap strata of the Sacramento-San Joaquin Valley. It is composed of Permian-Jurassic metamorphic and plutonic rocks, and is divided into three fault-bounded tectonostratigraphic terranes.

The Slate Creek terrane (Slate Creek Complex, Jarbo Gap ophiolite, French Creek terrane, Shingle Springs Complex, and Early Jurassic components of the Smartville Complex) is made up of mafic to ultramafic arc and ophiolitic rocks with a pervasive ~190 Ma amphibolite-facies metamorphic overprint. The American River terrane (Clipper Gap Formation, Fiddle Creek Complex, Owl Gulch Volcanics, Lake Combie Complex, Cool Quarry terrane, and Mount Ararat terrane) is made up of Permian to Middle Jurassic ophiolite, sea mount, and island arc igneous rocks and associated sedimentary rocks that lack the high-grade metamorphism of the Slate Creek terrane. These terranes were amalgamated and subsequently intruded by Middle Jurassic gabbro and granodiorite plutons, possibly near or along the western North American margin. The combined Yuba River superterrane was locally rapidly uplifted and overlain by the Middle Jurassic Colfax Formation then mostly returned to depth (≥5 km) where it was intruded by a suite of Late Jurassic (~150-162 Ma) intermediate plutons.

The Cosumnes terrane (Tuolumne ophiolite, Jasper Point, Peñon Blanco, Logtown Ridge, Mariposa, Gopher Ridge, Salt Springs, and Copper Hill Formations) is made up of Permian and Triassic ophiolite and chert overlain by Early Jurassic arc volcanics. This terrane lacks both high-grade metamorphism and Middle Jurassic plutons. It instead includes a second island arc composed of Middle to Late Jurassic volcanics and associated marine sedimentary rocks. The terrane has a Late Jurassic (Nevadan) penetrative cleavage that predates intrusion of unfoliated latest Jurassic and earliest Cretaceous (~142-152 Ma) granitic plutons. Granitic plutons of this age intrude all three terranes, suggesting that the Nevadan orogeny represents the accretion of the Cosumnes terrane to the two previously accreted terranes along the western North American margin, followed by a period of regional plutonism at the end of the Jurassic and into the Cretaceous.