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.

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.

Abstract Petrologic, structural, geochronologic, and geochemical data from rocks exposed in the western foothills of the Sierra Nevada batholith just south of the western Foothills metamorphic belt (the Stokes Mountain region) provide new insight regarding several poorly understood aspects of the development of the compound Sierra Nevada arc. Exposures of three different metasedimentary packages together document transitions in the depositional environments present along the outboard edge of this arc segment throughout the Mesozoic Era, culminating in the emergence of the Cretaceous continental margin arc. Exposures of mafic cumulates and associated differentiates of the Early Cretaceous batholith permit investigation into the earliest stages of differentiation of depleted-mantle–derived arc magmas. Together with the surrounding Kings-Kaweah ophiolite belt, these Mesozoic plutonic and metasedimentary rocks are proposed to form an analog for the crystalline basement underlying much of the eastern half of the Great Valley forearc basin.

1. ABSTRACT The geology of the northern Sierra Nevada of California records >400 million years of active plate margin tectonic events as a part of the North American Cordilleran orogenic belt. This field-trip guide provides geologic background and description of field-trip stops for a two-day field trip of the 2023 Geological Society of America Cordilleran Section Meeting based in Reno, Nevada. In two days, we cannot sample the complete geologic record of the northern Sierra Nevada, so this guide does not provide an exhaustive review of this geology. We will focus on certain aspects of the geology that have been the subject of recent research and present some previously unpublished observations and interpretations including: (1) distinguishing between subduction complexes and deformed assemblages that overlay subduction zones; (2) evidence for subduction initiation, recorded in high-pressure (P), high-temperature (T) amphibolites and possibly greenschist facies rocks structurally beneath them; (3) finding of high- P , high- T amphibolite blocks in mélange zones in subduction complex units accreted structurally beneath intact high- P , high- T amphibolite horizons; (4) differences in stream profiles between southern Cascade and northern Sierra drainages, suggesting different forcing mechanisms for stream erosion in those regions; and (5) complex relationships between stream incision, volcanic deposition, and Late Cenozoic faulting. • DEDICATION • This field–trip guide is dedicated to Eldridge Moores (1938–2018) and Jason Saleeby (1948–2023) who were giants in Sierra Nevada geologic research. Eldridge passed away in October 2018 while leading a field trip in this area on which the leaders of the current field trip (JW, DS) were participants. Jason passed away during the writing of this guide. As will be clear from reading this guide, the saying “standing on the shoulders of giants” applies.