Terranes of the Western Sierra Nevada Foothills Metamorphic Belt, California: A Critical Review

Abstract

Previous classifications of the western Sierra Nevada Foothills belt have been modified into five geologic packages: the Northern Sierra terrane, Feather River terrane, Calaveras Complex, Jura-Triassic arc belt, and Middle—Late Jurassic arc sequence. The Northern Sierra terrane comprises the Shoo Fly Complex and an overlying Paleozoic succession of arc volcanics. The Feather River terrane is predominantly serpentinized ultramafics containing tectonic blocks of amphibolite, plagiogranite, and mafic schists. The Permian-Late Triassic Calaveras Complex is a subduction mélange comprising psuedostratigraphic sequences of both chert and argillite with incorporated blocks of limestone, amphibolite, greenschist, phyllite, and basalt. The Red Ant schist is also included in this terrane even though it is of higher metamorphic grade. The Jura-Triassic arc belt consists of a Paleozoic basement of disrupted ophiolite, serpentinite mélange, and ultramafic rocks overlain by uppermost Triassic—Early Jurassic arc volcanics and coeval 200 Ma intrusive rocks. This belt may comprise several small terranes. The Middle—Late Jurassic arc sequence comprises 165-155 Ma volcanic arc rocks, greenstones, and metasedimentary rocks of the Mariposa Formation, and rocks of the Smartville Complex. Coeval intrusive rocks are included within this sequence because they are probably comagmatic with the arc volcanics. The Northern Sierra terrane and Feather River terrane are bounded by pre-Nevadan structures that may represent original sutures or subduction zones and have geologic histories that are relatively decoupled from adjacent rocks; they are therefore designated as terranes. The Calaveras Complex formed either adjacent to the Cordilleran margin as a result of east-dipping subduction, adjacent to arc rocks of the Jura-Triassic arc belt from west-dipping subduction, or a combination thereof. Therefore, the Calaveras Complex has a common geologic history with adjacent rocks and is not designated as a terrane. The Jura-Triassic arc belt comprises several ophiolitic microterranes, which may have been either exotic or fringing relative to North America, and are therefore grouped as a belt instead of individual terranes. The Middle-Late Jurassic arc sequence unconformably overlies rocks of the Jura-Triassic arc belt and is therefore a cover sequence instead of a separate terrane. The accretionary history of the Mesozoic Cordilleran margin is complex. After emplacement of the Northern Sierra terrane and Feather River terrane, the system was dominated by arc volcanism and accretion. The entire Sierra Nevada Foothills metamorphic belt was likely accreted to the margin by the Late Jurassic. The Sierra Nevada Foothills comprise units that appear to be correlative with those in the Klamath Mountains. Possible correlations are drawn between the Northern Sierra terrane and Eastern Klamath terrane, Calaveras Complex + Red Ant Schist and Stuart Fork + Eastern Hayfork terranes, Jura-Triassic arc belt and Rattlesnake Creek terrane, and Middle-Late Jurassic arc sequence and Western Hayfork terrane + coeval intrusives.