ABSTRACT Lower Mesozoic clastic rocks that unconformably overlie Paleozoic rocks within the Northern Sierra terrane provide clues regarding the evolution of the terrane during a 60 m.y. interval spanning the late Carnian through Bajocian. New detrital-zircon data provide fresh insights into the ages and provenance of these clastic rocks, together with new inferences about the Mesozoic tectonic evolution of the terrane. Previous studies have shown that from the late Carnian to the Sinemurian (~40 m.y.), a 1-km-thick section of subaerial to shallow-marine clastic arc-derived sediment accumulated and shallow-marine carbonate was deposited. At the base of this section, detrital-zircon results suggest the Northern Sierra terrane was located near a source area, possibly the El Paso terrane, containing Permian igneous rocks ranging in age from 270 to 254 Ma. By the earliest Jurassic, the detrital-zircon data suggest the Northern Sierra terrane was located near a source containing latest Triassic–earliest Jurassic igneous rocks spanning 209–186 Ma. The source of this material may have been the Happy Creek volcanic complex of the Black Rock terrane. A deep-marine, anoxic basin developed within the Northern Sierra terrane ca. 187–168 Ma. Approximately 3.5 km of distal turbidites were deposited in this basin. Previously reported geochemical characteristics of these turbidites link the Northern Sierra terrane with arc rock of the Black Rock terrane during this interval, except for a short time in the late Toarcian, when the terrane received an influx of quartz-rich sediment, likely derived from Mesozoic erg deposits now exposed on the Colorado Plateau. Clastic deposition within the Northern Sierra terrane ended in the Bajocian. Eruption of proximal-facies, mafic volcanic rocks and intrusion of hypabyssal rock and 168–163 Ma plutons reflect development of a magmatic arc within the terrane. These igneous rocks represent the first unequivocal evidence that the Northern Sierra terrane was located within a convergent-margin arc during the Triassic and Jurassic. Because detrital-zircon data from Lower Mesozoic strata within the Northern Sierra terrane indicate that it was depositionally linked with differing source areas through time early in the Mesozoic, the terrane may have been mobile along the western margin of Laurentia. There is little evidence from sediment within the Lower Mesozoic section of the terrane that can clearly be tied to the craton or the continental-margin Triassic arc prior to the late Toarcian. The absence of Upper Triassic or Lower Jurassic plutonic rocks within the terrane prior to the mid-Bajocian is also consistent with some form of isolation from the continental-margin arc system. While new detrital-zircon results place the Northern Sierra terrane proximal to the western margin of Laurentia in the late Toarcian, the current location of the terrane likely reflects Early Cretaceous offset along the Mojave–Snow Lake fault.

Abstract This field trip provides an overview of the stratigraphic and structural evolution of the northern Sierra terrane, which forms a significant part of the wall rocks on the western side of the late Mesozoic Sierra Nevada batholith in California. The terrane consists of a pre-Late Devonian subduction complex (Shoo Fly Complex) overlain by submarine arc-related deposits that record the evolution of three separate island-arc systems in the Late Devonian-Early Mississippian, Permian, and Late Triassic-Jurassic. The two Paleozoic arc packages and the underlying Shoo Fly Complex have an important bearing on plate-tectonic processes affecting the convergent margin outboard of the Paleozoic Cordilleran miogeocline, although their original paleogeographic relations to North America are controversial. The third arc package represents an overlap assemblage that ties the terrane to North America by the Late Triassic and helps constrain the nature and timing of Mesozoic orogenesis. Several of the fieldtrip stops examine the record of pre-Late Devonian subduction contained in the Shoo Fly Complex, as well as the paleovolcanology of the overlying Devonian to Jurassic arc rocks. Excellent glaciated exposures provide the opportunity to study a cross section through a tilted Devonian volcano-plutonic association. Additional stops focus on plutonic rocks emplaced during Middle Jurassic arc magmatism in the terrane, and during the main pulse of Cretaceous magmatism in the Sierra Nevada batholith to the east .

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

Biostratigraphic data, based mostly on radiolarian assemblages, establish synchronous deposition in the northern Sierra terrane and the Havallah basin beginning in the Late Devonian and extending into the early Late Permian. Lower Mississippian and mid-Permian arc-derived volcaniclastic debris was deposited in parts of the Havallah basin during episodes of arc volcanism in the northern Sierra terrane. Between these episodes of arc volcanism, from late Early Mississippian to at least Middle Pennsylvanian, the northern Sierra terrane collected siliceous pelagic deposits that correlate with dominantly chert-argillite sections in the Havallah sequence. These intermixed lithic assemblages suggest shared stratigraphic evolution and geographic proximity between the Sierran arc terrane and the Havallah basin during the late Paleozoic. During Late Devonian and Early Mississippian arc volcanism in the northern Sierra terrane, lower Paleozoic rocks of the Roberts Mountains allochthon were thrust over coeval deposits on the North American shelf. Chert-quartz-rich siliciclastic debris, derived from the Antler orogenic belt, is interbedded with Upper Devonian and Lower Mississippian distal volcanic rocks in the northern Sierra terrane and with Kinderhookian volcaniclastic rocks and chert in the Schoonover sequence. These quartzose-clastic deposits not only provide an independent lithologic link between the Sierran arc terrane and the Havallah basin, they also tie the arc terrane and basin to North America at the time of the Antler orogeny. Late Devonian and Early Mississippian arc volcanism in the northern Sierra terrane occurred in an extensional regime. Extensional tectonism began locally in the Havallah basin during the Famennian and continued into the early Meramecian. Contemporaneous extension in the arc and basin during emplacement of the Roberts Mountains allochthon is difficult to reconcile with existing arc-continent collision models for the Antler orogeny.

The Ashgillian (Upper Ordovician) Montgomery Limestone occurs as slide blocks in melange of the Shoo Fly Complex, northern Sierra Nevada, northern California. Brachiopods and sphinctozoan sponges from the Montgomery Limestone have closest biogeographic ties to coeval faunas of the eastern Klamath Mountains (Yreka terrane), and in the case of the brachiopods, to east-central Alaska (Jones Ridge). The latter was part of North America in the Ordovician. A small collection of Montgomery rugose corals yielded one species that is known elsewhere only in the Yreka terrane and in northern Maine. Montgomery tabulate corals have affinities with contemporaneous faunas of the Yreka terrane, northern Europe/Asia, Australia, and eastern North America. The apparent absence of similar tabulate taxa in western North America may be an artifact of incomplete collecting. As a whole, the biogeographic data indicate that the Montgomery Limestone was deposited close enough to Ordovician North America for faunal interchange to occur, and during its deposition was probably relatively near that continent. A comparison of lithologic units of the Shoo Fly Complex with those of the Yreka terrane indicates that some units in each area have no counterparts in the other (e.g., schist of Skookum Gulch in the Yreka terrane), and other units have general similarities in age (where known) and lithology, but differ in detail. The Yreka terrane has been interpreted as the remnants of an Early Cambrian arc and Ordovician-Devonian arc–fore-arc–accretionary prism, and the Shoo Fly Complex as a fragment of a Devonian or older accretionary wedge. Available biogeographic and stratigraphic data can be reasonably explained, as has been done by earlier authors, by a paleogeography in which the Yreka terrane and Shoo Fly Complex were parts of the same arc-trench system but were situated at different points along the strike of the arc. Lateral changes along strike in tectonic conditions and source areas could account for the observed disparities.

The Eastern Klamath and Northern Sierra terranes of northern California consist of Devonian to Jurassic arc-related rocks that structurally and/or stratigraphically overlie Devonian(?) or older complexes that consist of quartzite, quartzofeldspathic sandstone, chert, and mafic and ultramafic rocks. These terranes lie within a regional belt of Paleozoic arc-related rocks that can be recognized from the Sierra Nevada to British Columbia. The Eastern Klamath and Northern Sierra terranes are geographically separate and have paleontologic linkage, but lack direct stratigraphic ties in strata of Late Devonian to Early Permian age. This chapter describes new stratigraphic correlations and structural interpretations of rocks that lie in the northernmost part of the Sierra Nevada. These rocks include the Butt Valley block of the Northern Sierra terrane, and rocks herein interpreted as dismembered Eastern Klamath terrane. The rocks of Eastern Klamath terrane affinity, Soda Ravine block, occur as a tectonic sliver that lies to the west of the Butt Valley block. The Soda Ravine block is about 2 to 4 km by 20 km and includes limestone lenses equivalent to zone A of the McCloud Limestone of the Eastern Klamath terrane, and upper Middle and Upper Triassic limestone, slate, siltstone, and pebble conglomerate. Similar Permian limestone with McCloud faunal affinities and slate-bearing slivers have been noted by previous workers in this area and to the south. The Butt Valley block had previously been interpreted as part of the west limb of a regional anticline, the Almanor anticline. New mapping suggests that the block is not contiguous with rocks that lie to the east. The Butt Valley block includes the Devonian and Mississippian Taylor Formation, which is unconformably overlain by an Upper Triassic (Carnian and Norian) basal conglomerate and sandstone, which in turn is overlain by Upper Triassic limestone. The Upper Triassic limestone is overlain by a sparsely fossiliferous, lithic-volcaniclastic sequence containing poorly preserved Triassic or Early Jurassic, and probable Early Jurassic, ammonites and clams. This sequence is similar to conglomerate, Triassic limestone, and volcaniclastic rocks of the Jurassic Sailor Canyon Formation, which overlie a regional Late Triassic unconformity west of Lake Tahoe, thus expanding the known distribution of the unconformity in the Sierra Nevada. The Triassic unconformity overlies Carboniferous and older rocks around the North Fork American River area to the south, and overlies Devonian and older rocks on the Butt Valley block, but overlies Permian rocks east and southeast of the Butt Valley block. These relations suggest that either the Butt Valley block was part of a highland or uplifted block along the western part of the northern Sierra terrane or that the Butt Valley block previously lay closer to the North Fork American River area and has been translated northward by right-slip displacement. Northern Sierra terrane east of the Butt Valley block consists of the Hough and Genesse blocks, which are separated by the Grizzly Mountain fault zone. The Grizzly Mountain fault zone is here interpreted to be a transpressional right-slip fault.

Upper Paleozoic rocks of the northern Sierra and eastern Klamath terranes provide detailed stratigraphic records of ensimatic arc-related sedimentation and magmatism. Comparison of Paleozoic stratigraphic relations between the two terranes, however, suggests certain contrasts in depositional environments and the nature, volume, and timing of volcanism for given time intervals. Some lithologic and provenance ties indicate a paleogeographic relation. Variations in stratigraphy between the two terranes and within terranes imply differences in geodynamic setting. These and regional geologic relations indicate an early and persistent paleogeographic tie between the two areas, and they further suggest that the eastern Klamath terrane may have lain outboard and trenchward of the northern Sierra terrane during much of their late Paleozoic evolution. Stratigraphic ties for mid-Permian and lower Mesozoic rocks imply a closer relation and more similar geodynamic setting during subsequent evolution.

Middle Jurassic marine conglomeratic and debris-flow deposits in the Mt. Tallac roof pendant are interpreted to be syntectonic fault trough deposits. Similar deposits in similar stratigraphic successions throughout the northern Sierra Nevada demonstrate that the Middle Jurassic continental-margin arc north of latitude 39° was essentially marine, and possibly extensional or transtensional in nature. Recognition of Middle Jurassic syntectonic deposits in the northern Sierra Nevada establishes continuity of structural style between the arc in the northern Sierra Nevada and a portion of the arc south of latitude 39°, which has been interpreted as an ancient analog of the modern extensional or transtensional Central American arc. The Middle Jurassic marine conglomeratic deposits in the northern Sierra Nevada form one of three post–Late Triassic stratigraphic elements shared by the northern Sierra terrane and rocks of probable cratonal affinity in west-central Nevada. Upper Triassic limestone in the northern Sierra Nevada is linked faunally to correlative limestone of west-central Nevada. While correlation is speculative, Lower to Middle(?) Jurassic intervals of quartzose sandstone are common to parts of the Sailor Canyon Formation in the northern Sierra terrane and to rocks of the Auld Lang Syne Group, Boyer Ranch Formation, and Dunlap Formation of west-central Nevada. Middle Jurassic syntectonic conglomeratic deposits, such as in the Dunlap Formation of west-central Nevada, are now recognized in the northern Sierra terrane as well.

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.

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.

The upper Paleozoic Havallah sequence of central Nevada is a folded and thrust-faulted association of greenstone, siliceous marine sedimentary rocks, and deep-water clastic rocks. Microfossil assemblages (radiolarians, sponge spicules, and conodonts) are used as tools to unravel the stratigraphy and to interpret the paleoenvironments of the siliceous sedimentary rocks. Nine radiolarian assemblages (Osagean to Guadalupian) are described and used for delineation and correlation of fault-bounded lithotectonic units. The biostratigraphic zonation reveals that the oldest rocks in each lithotectonic unit are progressively younger from the structurally highest to the lowest units, suggesting progressive west-to-east upsection stepping of the Golconda sole thrust with accretion of each unit. Analyses of the radiolarian and sponge spicule faunas permit lateral and temporal comparisons of depositional environments. The lower structural units are coarsening-upward sequences of hemipelagic slope deposits overlain by sponge spicule-rich turbidites derived from a shallow source. The uppermost structural unit is a coarsening-upward basinal sequence. Permian sponge spicules in turbidites of the slope sequences and redeposited fusulinids in the basin sequence are similar to those in adjacent autochthonous (North American) regions. Permian radiolarians and sponge spicules in hemipelagic siliceous argillite of the slope sequences are similar to those in the Northern Sierra terrane to the west; the Havallah basin and the Northern Sierra arc terrane were overlain, therefore, by a similar water mass and may have been in proximity during the Permian. Clastic dikes and sills containing volcanic, metamorphic, and sedimentary rock clasts are Leonardian or younger.