New single-grain detrital zircon U-Pb age data from sandstone lenses in the Upper Jurassic Mariposa Formation of the Sierra Nevada foothills metamorphic belt indicate that: (1) the earliest phase of clastic sedimentation mainly involved material derived from the Bragdon and Baird Formations of the Eastern Klamaths and the Paleozoic miogeocline of Nevada ± sources farther to the east, with modest input from the Sierra Nevada arc; (2) the arc became the dominant sediment source for the upper turbidite interval in the Mariposa Formation; and (3) the youngest zircon ages constrain the onset of clastic deposition at 152 ± 1 Ma. Zircon age data also suggest that the local drainage divide migrated westward, resulting in a higher proportion of detritus derived from the Sierra Nevada arc over time. New geologic mapping in the central Sierra Nevada foothills shows that the Mariposa Formation thickens eastward, and that the number of coarse-grained sandstone bodies increases up section. These observations indicate that a topographically low Sierran volcanic arc gradually began to rise, providing increasing amounts of clastic debris to the Mariposa depositional basin. The Mariposa Formation was deposited in a volcanically active deep-water forearc basin and was subsequently disrupted by Nevadan orogenesis during the Late Jurassic. Inasmuch as it was located in the forearc inboard from the Middle Jurassic Coast Range ophiolite, Nevadan deformation cannot have resulted from arc-continent collision in the Sierra Nevada foothills but instead must have been related to tectonism along the plate margin.

Improved depositional age constraints and stratigraphic description of rocks in San Diego require designation of a new Upper Jurassic formation, herein named the Peñasquitos Formation after its exposures in Los Peñasquitos Canyon Preserve of the city of San Diego. The strata are dark-gray mudstone with interbedded first-cycle volcanogenic sandstone and conglomerate-breccia and contain the Tithonian marine pelecypod Buchia piochii. Laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon 206* Pb/ 238 U ages of 147.9 ± 3.2 Ma, 145.6 ± 5.3 Ma, and 144.5 ± 3.0 Ma measured on volcaniclastic samples from Los Peñasquitos and Rancho Valencia Canyons are interpreted as magmatic crystallization ages and are consistent with the Tithonian depositional age indicated by fossils. Whole-rock geochemistry is consistent with an island-arc volcanic source for most of the rocks. The strata of the Peñasquitos Formation have been assigned to the Santiago Peak volcanics by many workers, but there are major differences. The Peñasquitos Formation is marine; older (150–141 Ma); deformed everywhere and overturned in places; and locally is altered to pyrophyllite. In contrast, the Santiago Peak volcanics are nonmarine and contain paleosols in places; younger (128–110 Ma); undeformed and nearly flat lying in many places; and not altered to pyrophyllite. The Peñasquitos Formation rocks have also been assigned to the Bedford Canyon Formation by previous workers, but the Bedford Canyon is distinctly less volcanogenic and contains chert, pebbly mudstones, and limestone olistoliths(?) with Bajocian- to Callovian-age fossils. Here, we interpret the Peñasquitos Formation as deep-water marine forearc basin sedimentary and volcanic strata deposited outboard of the Peninsular Ranges magmatic arc. The Upper Jurassic Mariposa Formation of the western Sierra Nevada Foothills is a good analog. Results of detrital zircon U/Pb dating from an exposure of continentally derived sandstone at Lusardi Creek are consistent with a mixed volcanic-continental provenance for the Peñasquitos Formation. A weighted mean U/Pb age of 144.9 ± 2.8 Ma from the youngest cluster of detrital grain ages is interpreted as the likely depositional age. Pre-Cordilleran arc zircon age distributions (>285 Ma) are similar to Jurassic deposits from the Colorado Plateau, with dominant Appalachian-derived Paleozoic (300–480 Ma), Pan African (531–641 Ma), and Grenville (950–1335 Ma) grains, consistent with derivation either directly, or through sediment recycling, from the Colorado Plateau Mesozoic basins and related fluvial transport systems. Appalachian- and Ouachita-like detrital zircon age distributions are characteristic of Jurassic Cordilleran forearc basins from northeast Oregon to west-central Baja California, indicating deposition within the same continent-fringing west-facing arc system.

Abstract “The Jurassic of California,” according to J. P. Smith, “has been better studied than any other of the older formations, because of its association with the gold-bearing veins.” 1 In spite of all the study that has been devoted to it, however, hardly any of the major problems connected with the Jurassic system have been satisfactorily solved. The scarcity of fossils in most of the beds, the high degree of metamorphism that has affected many of them, and the extraordinarily complex structure have combined to make progress slow and difficult. In this chapter only enough discussion can be allotted to the system to serve as a basis for the discussion of the post-Jurassic rocks, which are of greater interest to most of the present generation of California geologists. Along with the rocks definitely known to be Jurassic in age, some others, especially the Franciscan series or formation, and a whole series of igneous rocks, will be briefly described. The known Jurassic sedimentary rocks of California are found chiefly in the northern Sierra Nevada and eastern Klamath Mountains. They have been described under such names as Modin and Potem formations (Redding district), Mormon, Bicknell, Hinchman, and Thompson formations (Taylorsville region), and Mariposa formation (Mother Lode district). The stages represented are Lower, Middle, and Upper Jurassic; the beds are dark shale, tuffaceous sandstone, and other types. 2 Lower Jurassic rocks are found at Redding and Taylorsville, Middle Jurassic at Taylorsville, and Upper Jurassic, along with remnants belonging to other stages, in the Gold

Irwin's (1972) terrane concept fathered tectonostratigraphy and has done much to decipher complex geology throughout the Circum-Pacific region, Caribbean, and elsewhere in the world. The present chapter analyzes the tectonostratigraphy of seemingly unrelated Jurassic terranes utilizing an accurate chronostratigraphic framework. Faunal paleolatitudinal data, paleomagnetic data (where available), and microfacies analysis have been utilized in the tectonostratigraphic interpretations. Four terranes/subterranes are analyzed herein: (1) the Izee terrane (Blue Mountains, northeastern Oregon), (2) the Rogue Valley subterrane (southwestern Oregon), (3) the Smith River subterrane (northwestern California), and (4) the San Pedro del Gallo terrane (east-central and central Mexico, western Cuba). Paleolatitudinal data indicate that all of these terranes/subterranes have undergone tectonic transport. The Izee terrane originated at low paleolatitudes (Central Tethyan province) during the Late Triassic and migrated to higher paleolatitudes (Southern Boreal province: 30° N) by the late Bathonian (Middle Jurassic). Northward movement is postulated to have occurred along a megashear analogous to the present-day San Andreas fault system. The Rogue Valley and Smith River subterranes both originated at low paleolatitudes (Central Tethyan province) during the Callovian and were transported to higher paleolatitudes (Southern Boreal province) by the the middle Oxfordian (early Late Jurassic). The Huayacocotla remnant of the San Pedro del Gallo terrane originated at high Southern Boreal paleolatitudes (30–40° N) during the late Bathonian or early Callovian and, subsequently was transported from northwest to southeast along the west side of the Walper megashear. By the latest Tithonian (Late Jurassic), the Huayacocotla remnant had been transported to low paleolatitudes (Central Tethyan province). Unconformities in the Bathonian to Callovian interval in the Izee terrane(?) and in the Huayacocotla remnant of the San Pedro del Gallo terrane are believed to reflect the breakup of Pangea. As demonstrated by the author in previous reports, each remnant of the San Pedro del Gallo terrane shows the same paleobathymetric fingerprint: (1) marine deposition at inner neritic depths during the Callovian to early Oxfordian (Middle to early Late Jurassic), (2) marine deposition at outer neritic depths during the late Oxfordian (Late Jurassic), and (3) sudden deepening to upper abyssal depths from the early Kimmeridgian (Late Jurassic) until the end of the Cretaceous. The sudden deepening event (outer neritic to upper abyssal) during the early Kimmeridgian is associated with a disconformity and hiatus in all San Pedro del Gallo remnants (early Kimmeridgian strata overlie middle Oxfordian strata). This event reflects the opening of the Gulf of Mexico. Combined faunal and floral data indicate that the San Pedro del Gallo terrane was in a back-arc position at approximately the same latitude as the Foothills terrane of the Sierra Nevada during the middle Oxfordian. Given the faunal data as well as some paleomagnetic data, it is probable that these San Pedro del Gallo remnants might represent some of the missing Upper Jurassic back-arc deposits from farther north (i.e., western Nevada?). In the Smith River subterrane, the disconformable contact between the volcanopelagic facies and Galice Formation sensu lato reflects a sudden influx of siliciclastic turbidite from a Jurassic volcanoplutonic arc source area and from older rocks of the accreted continental margin that lay inboard of the Nevadan island-arc complex. The same event is represented by the deposition of the siliciclastic turbidite of the Monte del Oro and Mariposa Formations (Foothills terrane, western Sierra Nevada) and by that of the Galice Formation sensu stricto (Rogue Valley subterrane).

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.

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.