ABSTRACT In this study, we determined the timing of burial and subsequent exhumation of Barrovian metamorphic rocks from the Chloride Cliff area of the Funeral Mountains in southeastern California by constraining the ages of different portions of a pressure-temperature ( P-T ) path. Using a split-stream laser-ablation inductively coupled plasma–mass spectrometry (ICP-MS) system, we analyzed 192 domains from 35 grains of monazite within five samples with a spot size of 8 µm to determine U-Pb ages and trace-element abundances from the same samples (same polished sections) that were analyzed to produce the P-T paths. Changes that took place within individual monazite grains reflect localized equilibrium and captured the changes in heavy rare earth element (HREE) abundances in the matrix reservoir that occurred as garnet grew, resorbed, and then regrew, thus constraining ages on different portions of the P-T path. The results show that garnet began growing ca. 168 Ma, began resorbing ca. 160 Ma, began retrograde regrowth ca. 157 Ma, and continued to regrow at least through ca. 143 Ma. The early garnet growth corresponds to a period of pressure increase along the P-T path. The subsequent partial resorption corresponds to the prograde crossing of a garnet-consuming reaction during decompression, and the retrograde garnet regrowth occurred when this same reaction was recrossed in the retrograde sense during further decompression. These results are consistent with previously determined ages, which include a Lu-Hf garnet age of 167.3 ± 0.72 Ma for the early pressure-increase portion of the P-T path, and 40 Ar/ 39 Ar muscovite cooling ages of 153 and 146 Ma in the lower-grade Indian Pass area 10 km southeast of Chloride Cliff. The 40 Ar/ 39 Ar muscovite ages document cooling at the same time as retrograde garnet regrowth was taking place at Chloride Cliff. The oldest monazite age obtained in this study, 176 ± 5 Ma, suggests that southeast-directed thrusting within the Jurassic retroarc was ongoing by this time along the California portion of the western North American plate margin, as a consequence of east-dipping subduction and/or arc collision. The Funeral Mountains were likely located on the east side of the northern Sierra Nevada range in the Jurassic, taking into account dextral strike-slip displacement along the Cretaceous Mojave–Snow Lake fault. The Late Jurassic timing of burial in the Funeral Mountains and its Jurassic location suggest burial was associated with the East Sierran thrust system. The timing of prograde garnet resorption during exhumation (160–157 Ma) corresponds to a change from regional dextral transpression to sinistral transtension along the Jurassic plate margin inferred to have occurred ca. 157 Ma. The recorded exhumation was concurrent with intrusion of the 148 Ma Independence dike swarm in the eastern Sierra Nevada and Mojave regions, which developed within a regime of northeast-southwest extension.

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.

Cretaceous plutonic rocks of the southern Sierra Nevada batholith between latitudes 35.5°N and 36°N lie in a strategic position that physically links shallow, subvolcanic levels of the batholith to lower-crustal (~35 km deep) batholithic rocks. This region preserves an oblique crustal section through the southern Sierra Nevada batholith. Prior studies have produced large U/Pb zircon data sets for an aerially extensive region of the batholith to the north of this area and for the lower-crustal rocks of the Tehachapi complex to the south. We present a large set of new U/Pb zircon age data that ties together the temporal relations of pluton emplacement and intra-arc ductile deformation for the region. We define five informal intrusive suites in the area based on petrography, structural setting, U/Pb zircon ages, and patterns in initial 87 Sr/ 86 Sr (Sr i ). Two regionally extensive intrusive suites, the 105–98 Ma Bear Valley suite and 95–84 Ma Domelands suite, underlie the entire southwestern and eastern regions of the study area, respectively, and extend beyond the limits of the study area. A third regionally extensive suite (101–95 Ma Needles suite) cuts out the northern end of the Bear Valley suite and extends for an unknown distance to the north of the study area. The Bear Valley and Needles suites are tectonically separated from the Domelands suite by the proto–Kern Canyon fault, which is a regional Late Cretaceous ductile shear zone that runs along the axis of the southern Sierra Nevada batholith. The 105–102 Ma Kern River suite also lies west of the proto–Kern Canyon fault and constitutes the subvolcanic plutonic complex for the 105–102 Ma Erskine Canyon sequence, an ~2-km-thick silicic ignimbrite-hypabyssal complex. The 100–94 Ma South Fork suite lies east of the proto–Kern Canyon fault. It records temporal and structural relations of high-magnitude ductile strain and migmatization in its host metamorphic pendant rocks commensurate with magmatic emplacement. Integration of the U/Pb age data with structural and isotopic data provides insights into a number of fundamental issues concerning composite batholith primary structure, pluton emplacement mechanisms, compositional variations in plutons, and the chronology and kinematics of regional intra-arc ductile deformation. Most fundamentally, the popular view that Sierran batholithic plutons rise to mid-crustal levels (~20–15 km) and spread out above a high-grade metamorphic substrate is rendered obsolete. Age and structural data of the study area and the Tehachapi complex to the south, corroborated by seismic studies across the shallow-level Sierra Nevada batholith to the north, indicate that felsic batholithic rocks are continuous down to at least ~35 km paleodepths and that the shallower-level plutons, when and if they spread out, do so above steeply dipping primary structures of deeper-level batholith. This steep structure reflects incremental assembly of the lower crust by multiple magma pulses. Smaller pulses at deeper structural levels appear to be more susceptible to having source isotopic and compositional signatures modified by assimilation of partial melt products from metamorphic framework rocks as well as previously plated-out intrusives. Higher-volume magma pulses appear to rise to higher crustal levels without substantial compositional modifications and are more likely to reflect source regime characteristics. There are abundant age, petrographic, and structural data to indicate that the more areally extensive intrusive suites of the study area were assembled incrementally over 5–10 m.y. time scales. Incremental assembly involved the emplacement of several large magma batches in each (~50 km 2 -scale) of the larger plutons, and progressively greater numbers of smaller batches down to a myriad of meter-scale plutons, and smaller dikes. The total flux of batholithic magma emplaced in the study area during the Late Cretaceous is about four times that modeled for oceanic-island arcs. Integration of the U/Pb zircon age data with detailed structural and stratigraphic studies along the proto–Kern Canyon fault indicates that east-side-up reverse-sense ductile shear along the zone was operating by ca. 95 Ma. Dextral-sense ductile shear, including a small reverse component, commenced at ca. 90 Ma and was in its waning phases by ca. 83 Ma. Because ~50% of the southern Sierra Nevada batholith was magmatically emplaced during this time interval, primarily within the east wall of the proto–Kern Canyon fault, the total displacement history of the shear zone is poorly constrained. Stratigraphic relations of the Erskine Canyon sequence and its relationship with the proto–Kern Canyon fault suggest that it was ponded within a 102–105 Ma volcano-tectonic depression that formed along the early traces of the shear zone. Such structures are common in active arcs above zones of oblique convergence. If such is the case for the Erskine Canyon sequence, this window into the early history of the “proto–Kern Canyon fault” could preserve a remnant or branch of the Mojave–Snow Lake fault, a heretofore cryptic hypothetical fault that is thought to have undergone large-magnitude dextral slip in Early Cretaceous time. The changing kinematic patterns of the proto–Kern Canyon fault are consistent with age and deformational relations of ductile shear zones present within the shallow-level central Sierra Nevada batholith, and with those of the deep-level exposures in the Tehachapi complex. This deformational regime correlates with flat-slab segment subduction beneath the southern California region batholithic belt and resultant tilting and unroofing of the southern Sierra Nevada batholith oblique crustal section. These events may be correlated to the earliest phases of the Laramide orogeny.

The Franciscan Complex, Great Valley Group, and Sierra Nevada batholith have long been considered to represent a Cretaceous convergent margin assemblage. This subduction complex, forearc basin, and magmatic arc triad has also been considered to have formed essentially in place with little or no Cretaceous-age translation between any of the three parts. Below we explore the possibility that the Great Valley Group accumulated in a basin that was translated parallel to the convergent margin as a forearc sliver during the Late Jurassic–Early Cretaceous. There are three different scales of evidence that lead to this hypothesis. The first comes from the processes operating at modern convergent plate boundaries. The second line of evidence is based on analysis of the geologic relations where the Coast Ranges meet the Klamath Mountains province in northern California. Thirdly, we explore published and some new detrital zircon age data in the context of a translational model for the Great Valley forearc basin. We conclude that the Great Valley forearc basin is bounded on its eastern and northern sides by a strike-slip fault that accommodated several hundreds of kilometers of dextral offset in the Late Jurassic–Early Cretaceous. This boundary is now a highly modified fault separating the Klamath Mountains province and the Coast Ranges, across which are juxtaposed two fundamentally different parts of the Great Valley Group. The boundary continues to the south between the Sierra Nevada and the Coast Ranges, where it is buried beneath younger sediments of the Sacramento Valley and/or perhaps includes structures in the Sierran foothills such as the Melones fault. Detrital zircon data suggest to us that the most likely original location of the Coast Ranges Great Valley Group, prior to strike-slip offset, was offshore of the continental arc in the southwest Cordillera (southeast California to northwest Mexico). In addition, we discuss evidence that the boundary between the Franciscan subduction complex and Great Valley forearc basin experienced significant dextral displacement. Finally, we suggest that these plate-boundary-parallel faults are part of an even larger system of Early Cretaceous dextral strike-slip faults in the U.S. Cordillera, including the Mojave–Snow Lake fault, western Nevada shear zone, and Idaho shear zone.

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.