Abstract

The western Sierra Nevada metamorphic belt is subdivided into six tectonic terranes, which are juxtaposed along mostly cryptic, pre-Late Jurassic faults. The Northern Sierra terrane contains a basement of imbricated and locally chaotic lower Paleozoic quartz sandstone, mafic rocks, chert, argillite, and other rocks, overlain unconformably by Upper Devonian-Middle Jurassic volcanic and siliceous strata. The Feather River terrane is a Paleozoic, possibly polygenetic ophiolite. The Red Ant terrane is a small terrane with an early Mesozoic blueschist-facies overprint. The Calaveras terrane is a chaotic assemblage of upper Paleozoic-Triassic argillite, chert, volcanic rock, and limestone contained in two regionally mapped mélange units. The Tuolumne River terrane is a stratigraphic assemblage of disrupted Paleozoic ophiolite overlain by Middle Triassic-Lower Jurassic siliceous and volcanic strata, both intruded by 200 Ma (Lower Jurassic) plutons. The Slate Creek terrane is a 200 Ma pseudostratigraphic sequence of cumulate ultramafic, plutonic, and volcanic rocks. The Northern Sierra and Calaveras terranes contain Permian McCloud fossils, whereas the Tuolumne River terrane contains Permian McCloud and Tethyan fossils in olistoliths within lower Mesozoic deposits.

These terranes are bounded mostly by steeply dipping faults of the Late Jurassic Foothills fault system but locally are juxtaposed along earlier, highly deformed, low-angle faults that are remnants of major terrane-bounding faults. A system of early, mostly cryptic, east-dipping faults juxtapose, in descending structural order, the Northern Sierra, Feather River, Red Ant, Calaveras, and Tuolumne River terranes. Each of the four boundaries that separate these five terranes have three or more independent pre-Late Jurassic upper age brackets, including crosscutting plutons, sedimentary provenance, and shared structural fabrics. The Slate Creek terrane tectonically overlies the Tuolumne River, Calaveras, and Red Ant terranes along an isoclinally folded, west-dipping fault that is cut by a 165 Ma pluton.

Callovian-Kimmeridgian strata and coeval intrusive rocks, including the 155-165 Ma volcanic rocks, sheeted dikes, and zoned mafic plutons of the Smartville Complex, are deposited on and intruded into the older, previously juxtaposed terranes and constitute an overlap sequence. The well-known faults of the Foothills fault system, including the Melones fault, cut Kimmeridgian strata of the overlap sequence and thus cannot include ancient sutures. These faults reactivated many of the earlier terrane-bounding faults. Pre-Late Jurassic amalgamation of the Sierra Nevada metamorphic belt rules out previous models of Late Jurassic arc-continent collision; earlier collisions and/or noncollisional tectonic processes must be invoked to explain strong contractional deformation in the western Sierra Nevada metamorphic belt.

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