This paper is an in-depth review of the architecture and evolution of the Western Sierra Nevada metamorphic province. Firsthand field observations in a number of key areas provide new information about the province and the nature and timing of the Nevadan orogeny. Major units include the Northern Sierra terrane, Calaveras Complex, Feather River ultramafic belt, phyllite-greenschist belt, mélanges, and Foothills terrane. Important changes occur in all belts across the Placerville–Highway 50 corridor, which may separate a major culmination to the south from a structural depression to the north. North of the corridor, the Northern Sierra terrane consists of the Shoo Fly Complex and overlying Devonian to Jurassic–Cretaceous cover, and it represents a Jurassic continental margin arc. The western and lowest part of the Shoo Fly Complex contains numerous tectonic slivers, which, along with the Downieville fault, comprise a zone of west-vergent thrust imbrication. No structural evidence exists in this region for Permian–Triassic continental truncation, but the presence of slices from the Klamath Mountains province requires Triassic sinistral faulting prior to Jurassic thrusting. The Feather River ultramafic belt is an imbricate zone of slices of ultra-mafic rocks, Paleozoic amphibolite, and Triassic–Jurassic blueschist, with blueschist interleaved structurally between east-dipping serpentinite units. The Downieville fault and Feather River ultramafic belt are viewed as elements of a Triassic–Jurassic subduction complex, within which elements of the eastern Klamath subprovince were accreted to the western edge of the Northern Sierra terrane. Pre–Late Jurassic ties between the continental margin and the Foothills island arc are lacking. A Late Jurassic suture is marked by the faults between the Feather River ultramafic belt and the phyllite-greenschist belt. The phyllite-greenschist belt, an important tectonic unit along the length of the Western Sierra Nevada metamorphic province, mélanges, and the Foothills island arc terrane to the west were subducted beneath the Feather River ultramafic belt during the Late Jurassic Nevadan orogeny. South of the Placerville–Highway 50 corridor, the Northern Sierra terrane consists of the Shoo Fly Complex, which possibly contains structures related to Permian–Triassic continental truncation. The Shoo Fly was underthrust by the Calaveras Complex, a Triassic–Jurassic subduction complex. The Late Jurassic suture is marked by the Sonora fault between the Calaveras and the phyllite-greenschist belt (Don Pedro terrane). As to the north, the phyllite-greenschist belt and Foothills island arc terrane were imbricated within a subduction zone during the terminal Nevadan collision. The Don Pedro and Foothills terranes constitute a large-magnitude, west-vergent fold-and-thrust belt in which an entire primitive island-arc system was stacked, imbricated, folded, and underthrust beneath the continental margin during the Nevadan orogeny. The best age constraint on timing of Nevadan deformation is set by the 151–153 Ma Guadelupe pluton, which postdates and intruded a large-scale megafold and cleavage within the Mariposa Formation. Detailed structure throughout the Western Sierra Nevada metamorphic province shows that all Late Jurassic deformation relates to east-dipping, west-vergent thrusts and rules out Jurassic transpressive, strike-slip deformation. Early Cretaceous brittle faulting and development of gold-bearing quartz vein systems are viewed as a transpressive response to northward displacement of the entire Western Sierra Nevada metamorphic province along the Mojave–Snow Lake fault. The preferred model for Jurassic tectonic evolution presented herein is a new, detailed version of the long-debated arc-arc collision model (Molucca Sea–type) that accounts for previously enigmatic relations of various mélanges and fossiliferous blocks in the Western Sierra Nevada metamorphic province. The kinematics of west-vergent, east-dipping Jurassic thrusts, and the overwhelming structural evidence for Jurassic thrusting and shortening in the Western Sierra Nevada metamorphic province allow the depiction of key elements of Jurassic evolution via a series of two-dimensional cross sections.

The Jurassic Smartville arc sequence in the northern Sierra Nevada foothills is bounded both in the north and east by ophiolitic and marine rocks of a disrupted oceanic terrane. The Jarbo Gap ophiolite in the north consists of serpentinized harzburgite and dunite intruded by gabbro and diorite and overlain by volcanic and sedimentary rocks, all of which are metamorphosed to upper greenschist-amphibolite facies. Contacts between the ophiolitic subunits, which commonly have lenticular shapes, are generally tectonic and discontinuous along-strike. Ophiolitic rocks display a penetrative fabric of a west-northwest–striking and steeply to moderately north-northeast–dipping foliation. Locally, mafic to intermediate volcanic rocks within the ophiolite are cut by fault-bounded and/or diapiric intrusions of serpentinized peridotites. Contact relations and the rock types indicate that the Jarbo Gap ophiolite is part of a multiply deformed, heterogeneous oceanic crust. Marine rocks exposed south of and structurally beneath the Jarbo Gap ophiolite form a chert-argillite unit that is composed mainly of blocks of sedimentary, volcanic, plutonic, and ultramafic rocks in an argillite matrix. The matrix displays a northwest-trending, northeast-dipping penetrative foliation, whereas some of the blocks in the matrix show little or no deformation. Olistostromal origin of these undeformed blocks suggests that gravity debris sliding in addition to tectonic mixing was a significant agent for development of the generally chaotic internal structure of the chert-argillite unit. Ophiolitic blocks and detritus in the matrix indicate an older ophiolitic provenance, whereas blocks and clasts of relatively undeformed and fresh volcanic rocks, which occur as olistoliths in a mudstone-sandstone sequence, suggest volcanic activity adjacent to the depocenter(s) of the chert-argillite unit. Age relations, reconstructions of stratigraphic, structural, and contact relations, and regional correlations suggest that the Jarbo Gap ophiolite may have been originally overlain by the chert-argillite unit. Thus the Jarbo Gap ophiolite and the overlying chert-argillite unit are interpreted to have constituted a late Paleozoic–early Mesozoic oceanic basement. The existence of similar structural and stratigraphic relations between stratigraphically and chronologically correlative ophiolitic and marine rocks exposed in the Central Belt east of the Smartville complex suggests that much of the Central Belt is composed of this late Paleozoic–early Mesozoic oceanic basement. Both the Jarbo Gap ophiolite and the chert-argillite unit are intruded by dikes of a basaltic andesite composition that display low FeO and high MgO contents and enrichment in compatible as well as some LIL elements. These dikes, which are Lower Jurassic in age and chemically unrelated to the ophiolitic sequence, show petrographic and petrologic characteristics typical of boninitic affinity and are interpreted as products of primary melts generated by high degrees of melting of a metasomatically enriched source during early stages of subduction. We infer that this subduction during Early Jurassic time may have under-plated the preexisting oceanic basement and produced a submarine volcanic arc terrane. Part of the Smartville complex and the entire Slate Creek complex are interpreted as fragments of this arc terrane, which was built on and across the preexisting oceanic basement. This model suggests that the Western Belt and much of the Central Belt in the northern Sierra Nevada are part of a single ensimatic and polygenetic arc terrane that evolved in early Mesozoic time. This interpretation precludes tectonic scenarios suggesting the existence in the northern Sierra Nevada metamorphic belt of a number of discrete, exotic island-arc systems that were presumably accreted into the North American continental margin during early Mesozoic time. The current structural grain defined by northwest-striking and steeply northeast-dipping faults and associated structures is a relatively late-stage feature that modified the primary contact relations during a period of regional contraction in Late Jurassic to Early Cretaceous time.

The Central and Feather River peridotite belts of the northern Sierra Nevada metamorphic belt constitute a major suture zone between Paleozoic–early Mesozoic continental-margin rocks (Shoo Fly Complex and superjacent strata) of the Eastern belt and Jurassic arc and ophiolitic rocks (Smartville Complex) of the Western belt. This suture zone is structurally complex and has previously been described as mélange. Our data suggest that six major fault-bounded rock assemblages are present across this zone. The faults are isoclinally folded and transposed along steep hinge planes, but have shallowly dipping enveloping surfaces. Rocks of the Eastern belt occupy the highest of five east-dipping thrust sheets which are technically overlain by a sixth, west-dipping thrust sheet. The Western belt rocks are built into a basement composed of the last thrust sheet and postdate the thrust faults. All these rocks and structures are cut by faults of the Late Jurassic Foothills fault system which bound the lithotectonic “belts” (Eastern belt, etc.). The Feather River peridotite belt consists of the Feather River peridotite and the Red Ant Schist, and is extended to include the newly recognized Devils Gate ophiolite. The Feather River peridotite is correlative with or intruded by the Devils Gate ophiolite, and together they constitute a nearly complete cogenetic or polygenetic Paleozoic ophiolite. The Red Ant Schist contains metasedimentary and metavolcanic rocks with local blueschist parageneses; blueschist facies metamorphism is early Mesozoic or older. Several small outliers of Shoo Fly (Eastern belt) sandstone are present in the Feather River peridotite belt. The Central belt consists of the Calaveras Complex, the Fiddle Creek Complex, and the Slate Creek Complex. The late Paleozoic–early Mesozoic Calaveras Complex is an assemblage of phyllite-diamictite, chert, and minor volcanic rocks and marble in the eastern part of the Central belt. The Fiddle Creek Complex lies west of the Calaveras Complex and contains an intact stratigraphic succession, which includes, in ascending stratigraphic order, late Paleozoic ophiolitic mélange, pillow basalt with minor felsic tuff, Middle Triassic–Early Jurassic volcaniclastic and hemipelagic sedimentary rocks, and Middle–Late Jurassic(?) quartzose clastic rocks. The Slate Creek Complex is an Early Jurassic pseudostratigraphic sequence that contains a basal serpentinite-matrix mélange overlain by plutonic and volcanic rocks. The Western belt consists of the Middle–Late Jurassic (160 Ma) volcanic and intrusive rocks of the Smartville Complex and, at one locality, older tonalitic basement equivalent to the Slate Creek Complex. The Slate Creek Complex was juxtaposed against the other rock assemblages before formation of the Smartville Complex, so the Smartville Complex formed in situ. Crosscutting relations define three generations of macroscopic structures. The earliest structures include major mapped and cryptic, west-vergent (east-dipping) thrust faults that juxtapose, in descending structural order, the Shoo Fly Complex (Eastern belt), Feather River–Devils Gate Ophiolite, Red Ant Schist, Calaveras Complex, and Fiddle Creek Complex. These faults predate the east-vergent “Slate Creek thrust,” which carries the Slate Creek Complex over the Fiddle Creek Complex, Calaveras Complex, and Red Ant Schist. The Slate Creek thrust is cut by a pluton dated at about 165 Ma, which places an upper age constraint on assembly of the nappe pile. This nappe pile is cut and overprinted by the steep throughgoing faults (Foothills fault system), folds, and cleavages that dominate the structure of the area; this youngest set of structures makes up the Late Jurassic Nevadan orogeny. Eastward overthrusting of the Slate Complex along the Slate Creek thrust occurred after amalgamation of the other units. Eastward overthrusting was followed by rifting and arc magmatism represented by the Smartville Complex, then by Nevadan faulting, folding, and penetrative deformation. These data preclude the existence of a Late Jurassic suture or collision, but the Fiddle Creek and Slate Creek Complexes, both interpreted as remnants of Early Jurassic arcs, could have collided in the late Early Jurassic or early Middle Jurassic.