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.

Pre-Middle Jurassic stratigraphic assemblages in the northern Sierra Nevada are contained in tightly folded but macroscopically shallowly dipping nappes. Four shallowly east-dipping f 1 faults juxtapose, in descending structural order, the Northern Sierra terrane, the Feather River peridotite/Devils Gate ophiolite, the Red Ant Schist, the Calaveras Complex, and the Fiddle Creek Complex. The f 2 Slate Creek thrust dips shallowly west; it places the Slate Creek Complex over the Red Ant Schist, Calaveras Complex, and Fiddle Creek Complex, and thus cuts two f 1 faults. As many as three Early Jurassic magmatic arcs may be represented by the stratigraphic assemblages in the f 1 –f 2 nappe pile. A continental-margin arc is represented by Late Triassic–Middle Jurassic volcanogenic rocks in the upper part of the Northern Sierra terrane stratigraphy and by coeval plutons in the eastern part of the Sierra Nevada batholith; the associated subduction complex is represented by blueschist-facies metamorphism in the Red Ant Schist and melange structure in oceanic rocks of the Calaveras Complex. An intraoceanic arc is represented by the Fiddle Creek Complex, which contains ophiolitic basement overlain and intruded by Early Jurassic volcanic and plutonic rocks; the volcanic strata are intercalated with cherty deposits, olistostromes, and volcaniclastic rocks devoid of continental detritus. A more westerly Early Jurassic intraoceanic arc is represented by the Slate Creek Complex, which has ophiolitic structure. These units were juxtaposed along the f 1 and f 2 faults before Callovian-Kimmeridgian (Middle to Late Jurassic) deposition of volcanic and epiclastic strata and coeval intrusion of mafic-intermediate plutons; this postamalgamation arc was extensional in its western part, as indicated by sheeted dikes in the Smartville Complex. The Callovian-Kimmeridgian rocks and their basement of f 1 –f 2 nappes are cut by two sets of oppositely dipping f 3 faults, a shallowly west-dipping set (Taylorsville and related thrust faults) and a steeply east-dipping set (Foothills fault system); f 3 faults are the only major Late Jurassic (Nevadan) or younger faults in the northern Sierra Nevada. This structural chronology rules out models for Late Jurassic (Nevadan) collisions, but allows collision(s) of the Fiddle Creek and Slate Creek arcs in the Early or Middle Jurassic.

New field mapping and bulk-rock geochemical investigations in the southern Klamath Mountains and central Sierran Foothills combined with previous structural, petrochemical, and geochronologic studies allow the distinction between three Triassic–Jurassic basaltic arcs built along the continental edge versus two roughly coeval basaltic complexes that formed farther off the Californian margin. The three Klamath Mountains arcs are: (1) The Hayfork Summit–Salmon River segment of the southern North Fork terrane, formed offshore as a sequence of interlayered chert, volcaniclastic strata and shale, and ocean island basalt (OIB), deposited on a mélanged and serpentinized basement containing blocks of 310- to 265-Ma mid-ocean ridge basalt (MORB). (2) Northward, the Sawyers Bar sector of the central North Fork terrane formed closer to the continental margin; this mafic arc originated at ca. 200–170 Ma as a stack of interdigitated island-arc tholeiites (IAT) and minor OIBs interstratified with, and largely overlying, distal turbidites derived from eastern Klamath terranes. (3) The currently farther outboard Rattlesnake Creek terrane consists of continent-sourced, Lower Jurassic metasedimentary quartzose strata interbedded with island-arc volcanic rocks; this near-shore section was laid down on older ophiolitic basement consisting of tectonized serpentinite and MORB blocks. The remaining two arcs are in the Sierran Foothills: (4) The offshore Peñon Blanco arc consists of cherty and volcaniclastic sedimentary strata interlayered with 200-Ma mafic volcanic-plutonic arc rocks, all resting on a 300-Ma ophiolitic basement; suturing against the structurally higher Mariposa Formation took place after deformation of the latter at ca. 150 Ma. (5) The Slate Creek complex, and possibly the Lake Combie, Owl Gulch, and Sullivan Creek entities, formed along the margin of North America; superjacent units consist chiefly of 207- to 170-Ma volcaniclastic, sedimentary, and arc volcanic rocks deposited on an ophiolitic mélange basement. Metamorphic belts of the central Klamath Mountains and Sierran Foothills evidently contain both near-shore and offshore oceanic arcs.

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.