Abstract The tectonomagmatic evolution of eastern Chukotka, NE Russia, is important for refining the onset of Pacific plate subduction, understanding the development of the Amerasia Basin, and constraining Arctic tectonic reconstructions. Field mapping and strategic sample collection provide relative age constraints on subduction-related continental arc magmatism in eastern Chukotka. Ion microprobe U–Pb zircon ages provide absolute constraints and identify five magmatic episodes ( c. 134, 122, 105, 94 and 85 Ma) separated by three periods of uplift and erosion ( c. 122–105, 94–85 and post-85 Ma). Volcanic rocks in the region are less contaminated than their plutonic equivalents which record greater crustal assimilation. These data, combined with xenocrystic zircons, reflect the self-assimilation of a continental arc during its evolution. Proto-Pacific subduction initiated by c. 121 Ma and arc development occurred over c. 35–50 myr. Crustal growth was simultaneous with regional exhumation and crustal thinning across the Bering Strait region. Ocean–continent subduction in eastern Chukotka ended at c. 85 Ma. The timing of events in the region is roughly synchronous with the inferred opening of the Amerasia Basin. Simultaneous arc magmatism, extension and development of the Amerasia Basin within a back-arc basin setting best explain these coeval tectonic events. Supplementary material: Includes SIMS U–Pb and geochemistry data tables, detailed geological map and geochemical figures which are available at https://doi.org/10.6084/m9.figshare.c.3784565

The composite Sunrise Butte pluton, in the central part of the Blue Mountains Province, northeastern Oregon, preserves a record of subduction-related magmatism, arc-arc collision, crustal thickening, and deep-crustal anatexis. The earliest phase of the pluton (Desolation Creek unit) was generated in a subduction zone environment, as the oceanic lithosphere between the Wallowa and Olds Ferry island arcs was consumed. Zircons from this unit yielded a 206 Pb/ 238 U age of 160.2 ± 2.1 Ma. A magmatic lull ensued during arc-arc collision, after which partial melting at the base of the thickened Wallowa arc crust produced siliceous magma that was emplaced into metasedimentary rocks and serpentinite of the overthrust forearc complex. This magma crystallized to form the bulk of the Sunrise Butte composite pluton (the Sunrise Butte unit; 145.8 ± 2.2 Ma). The heat necessary for crustal anatexis was supplied by coeval mantle-derived magma (the Onion Gulch unit; 147.9 ± 1.8 Ma). The lull in magmatic activity between 160 and 148 Ma encompasses the timing of arc-arc collision (159–154 Ma), and it is similar to those lulls observed in adjacent areas of the Blue Mountains Province related to the same shortening event. Previous researchers have proposed a tectonic link between the Blue Mountains Province and the Klamath Mountains and northern Sierra Nevada Provinces farther to the south; however, timing of Late Jurassic deformation in the Blue Mountains Province predates the timing of the so-called Nevadan orogeny in the Klamath Mountains. In both the Blue Mountains Province and Klamath Mountains, the onset of deep-crustal partial melting initiated at ca. 148 Ma, suggesting a possible geodynamic link. One possibility is that the Late Jurassic shortening event recorded in the Blue Mountains Province may be a northerly extension of the Nevadan orogeny. Differences in the timing of these events in the Blue Mountains Province and the Klamath–Sierra Nevada Provinces suggest that shortening and deformation were diachronous, progressing from north to south. We envision that Late Jurassic deformation may have collapsed a Gulf of California–style oceanic extensional basin that extended from the Klamath Mountains (e.g., Josephine ophiolite) to the central Blue Mountains Province, and possibly as far north as the North Cascades (i.e., the coeval Ingalls ophiolite).

The composite California Coast Range ophiolite consists of remnants of Middle Jurassic oceanic lithosphere, a Late Jurassic deep-sea volcanopelagic sediment cover, and Late Jurassic intrusive sheets that invade the ophiolite and volcano-pelagic succession. The dismembered Middle Jurassic Coast Range ophiolite remnants (161–168 Ma) were parts of the axial sequence of an oceanic spreading center that consisted of basaltic submarine lava, subvolcanic intrusive sheets, and gabbro, and coeval but off-axis upper lava, dunite-wehrlite mantle transition zone, peridotite restite, and dikes rooted in the mantle transition zone that fed the upper lava. Hydrothermal metamorphism overprints the lavas, subvolcanic sheets, and part of the gabbro. The nearly complete magmatic pseudostratigraphy with minimal syngenetic internal deformation accords with a “hot” thermal structure and robust magma budget, indicative of fast spreading. Upper Jurassic volcanopelagic strata composed of tuffaceous radiolarian mud-stone and chert (volcanopelagic distal facies) overlie the ophiolite lava disconformably and grade up locally into arc-derived deep-marine volcaniclastics (volcanopelagic proximal facies). An ophiolitic breccia unit at northern Coast Range ophiolite localities caps shallow to deep levels of fault-disrupted Middle Jurassic oceanic crust. The Late Jurassic igneous rocks (ca. 152–144 Ma) are mafic to felsic subvolcanic intrusive sheets that invade the Middle Jurassic ophiolite, its Late Jurassic volcanopelagic cover, and locally the ophiolitic breccia unit. Hydrothermal metamorphism of volcanopelagic beds and underlying ophiolite meta-igneous rocks accompanied the Late Jurassic deep-sea magmatic events. The Middle Jurassic ophiolite formed at a spreading ocean ridge (inferred from its Jurassic plate stratigraphy). Intralava sediment and thin volcanopelagic strata atop the Coast Range ophiolite lava record an 11–16 m.y. progression from an open-ocean setting to the distant submarine apron of an active volcanic arc, i.e., the sediments accumulated upon oceanic lithosphere being drawn progressively closer to a subduction zone in front of an ocean-facing arc. Trace-element signatures of Coast Range ophiolite lavas that purportedly link ocean-crust formation to a suprasubduction-zone setting were influenced also by processes controlled by upper-mantle dynamics, especially the mode and depth of melt extraction. The polygenetic geochemical evidence does not decisively determine tectonic setting. Paleomagnetic and biostratigraphic evidence constrains the paleolatitudes of Coast Range ophiolite magmatism and volcanopelagic sedimentation. Primary remanent magnetism in ophiolite lavas at Point Sal and Llanada Coast Range ophiolite remnants records eruption within a few degrees of the Middle Jurassic paleoequator. The volcanopelagic succession at Coast Range ophiolite remnants consistently shows upward progression from Central Tethyan to Southern Boreal radiolarian assemblages, recording Late Jurassic northward plate motion from the warm-water paleo-equatorial realm. Northward seafloor spreading was interrupted by local Late Jurassic rift propagation through the Middle Jurassic oceanic lithosphere. Coast Range ophiolite crust with volcanopelagic soft-sediment cover that lay in the path of propagating rifts hosted rifting-related magmatic intrusions and hydrothermal metamorphism. The advancing broad deformation zone between propagating and failing rifts left paths of pervasive crustal deformation marked now by fault-disrupted ophiolite covered by depression-filling ophiolitic breccias, found at northern Coast Range ophiolite remnants. Coast Range ophiolite lithosphere that lay outside the propagating and failed rift zones lacks those features. The rift-related magmatism and crustal deformation took place at ephemeral spreading-center offsets along a transform fault. Late Jurassic seafloor spreading carried Middle Jurassic oceanic lithosphere northeastward toward a subduction zone in front of the Middle to Late Jurassic arc that fringed southwestern North America. Termination of oblique subduction during the late Kimmeridgian, replaced by dextral transform faulting, left a Coast Range ophiolite plate segment stranded in front (west) of the trench. The trench was then filled and locally bridged by the arc’s submarine sediment apron by the latest Jurassic, allowing coarse volcaniclastic (proximal volcanopelagic) deposits to lap onto earlier, plate-transported tuffaceous radiolarian chert (distal volcanopelagic) deposits. Deep-marine terrigenous muds and sands from southwestern Cordilleran sources then buried the stranded Coast Range ophiolite–volcanopelagic–ophiolitic breccia unit oceanic crust during latest Jurassic northward dextral displacement, which proceeded offshore. Those basal Great Valley Group strata record lower continental-slope and basin-plain marine sedimentation on Jurassic oceanic basement, i.e., the Coast Range ophiolite and adjacent Franciscan oceanic lithosphere (Coast Range serpentinite belt). Forearc basin deposition did not begin until the mid–Early Cretaceous, when the inception of outboard Franciscan subduction lifted and tilted the Coast Range ophiolite–volcanopelagic–ophiolitic breccia unit–basal Great Valley Group succession and Coast Range serpentinite belt to form a basin-bounding forearc ridge. Thereafter, Cretaceous Franciscan subduction and accretionary wedge growth operated in front (west) of the submerged ridge, and Great Valley Group forearc basin terrigenous sediments accumulated behind it.

Geochronology, especially U-Pb zircon geochronology, has made important contributions to our understanding of the Jurassic Coast Range ophiolite of California. However, much of the older work is primitive by modern standards, and even some recent U-Pb work is limited in its precision and accuracy by a range of factors. We apply a new zircon analysis method, chemical abrasion–thermal ionization mass spectrometry (CA-TIMS), to generate high-precision, high-accuracy multistep 206 Pb*/ 238 U plateau ages for zircons from plagiogranites from the Point Sal (Coast Range ophiolite) and San Simeon (Coast Range ophiolite) ophiolite remnants. These remnants have been postulated to have been part of a single, contiguous remnant prior to offset along the San Gregorio–San Simeon–Hosgri fault system. Two fractions of zircon from a Point Sal Coast Range ophiolite plagiogranite, and one fraction of zircon from a San Simeon Coast Range ophiolite plagiogranite yield 206 Pb*/ 238 U plateau ages that are indistinguishable from one another—a mean age for the three determinations is 165.580 ± 0.038 Ma (95% confidence, mean square of weighted deviates [MSWD] = 0.47). The error quoted is an internal precision, which is appropriate for comparison of the ages to one another. The fact that the San Simeon and Point Sal ages are indistinguishable, even with such very small internal precision errors, is a remarkably robust confirmation of the correlation between the San Simeon and Point Sal ophiolite remnants.

The Elder Creek ophiolite, which crops out along the South, Middle, and North Forks of Elder Creek, is the largest exposure of mid-Jurassic Coast Range ophiolite in the northern Coast Ranges of California. The Elder Creek ophiolite contains almost all of the components of a classic ophiolite (mantle tectonites, cumulate ultramafics and gabbro, dike complex, volcanics), although most of the volcanic section has been removed by erosion and redeposited in the overlying Crowfoot Point breccia. It differs from classic ophiolite stratigraphy in that it has substantial volumes (25%–30% of the complex) of felsic plutonic rocks intimately associated with the other lithologies. The felsic lithologies include hornblende diorite, quartz-diorite, tonalite, and trondhjemite, which crop out in four distinct associations: (1) as rare, small pods within the sheeted dike complex, (2) as the felsic matrix of igneous breccias (agmatites), (3) 1–25-m-thick dikes that crosscut cumulate or isotropic gabbro, and (4) sill-like plutons up to 500 m thick and 3 km long that intrude the upper part of the plutonic section. Typical phase assemblages include quartz, plagioclase, hornblende, and pyroxene, in a hypidiomorphic texture. The Elder Creek tonalite-trondhjemite-diorite (TTD) suite spans a wide range in composition: 54%–75% SiO 2 , 3.3%–14.3% FeO*, and 2.7%–6.4% MgO; all are low in K 2 O (<0.7%). The large sill-like plutons are generally higher in silica (average 69% SiO 2 ) than the dikes, pods, and agmatite matrix (average 60% SiO 2 ). Mg#’s range from 65 to 17, with Cr up to 227 ppm at 58% silica. High-Mg diorites with 4%–7% MgO at 53%–58% SiO 2 are common in the dike suite, but relatively high MgO, Mg#, and Cr values are found in the large plutons as well. The major- and trace-element characteristics are consistent with partial melting of subducted, amphibolite-facies oceanic crust at relatively low pressures (5–10 kbar) outside the garnet stability field. Melting of subducted oceanic crust at these pressures can only occur during the collision and subduction of an active spreading center. Subsequent reaction of these melts with the overlying mantle wedge has increased their refractory element concentrations. The occurrence of zircons with inherited Pb isotope characteristics implies the involvement of subducted sediments containing an ancient zircon component. Formation of the Elder Creek TTD suite was a transient event associated with ridge collision-subduction. This is consistent with previous models for the Coast Range ophiolite and other suprasubduction-zone ophiolites; it is not consistent with an ocean-ridge spreading-center origin.

The polygenetic Ingalls ophiolite complex in the central Cascades, Washington, is one of several Middle to Late Jurassic ophiolites of the North American Cordillera. It consists primarily of mantle tectonites. High-temperature mylonitic peridotite, overprinted by serpentinite mélange (Navaho Divide fault zone), separates harzburgite and dunite in the south from lherzolite in the north. Crustal units of the ophiolite occur as steeply dipping, kilometer-scale fault blocks within the Navaho Divide fault zone. These units are the Iron Mountain, Esmeralda Peaks, and Ingalls sedimentary rocks. Volcanic rocks of the Iron Mountain unit have transitional within-plate–enriched mid-ocean-ridge basalt affinities, and a rhyolite yields a U-Pb zircon age of ca. 192 Ma. Minor sedimentary rocks include local oolitic limestones and cherts that contain Lower Jurassic (Pliensbachian) Radiolaria. This unit probably formed as a seamount within close proximity to a spreading ridge. The Esmeralda Peaks unit forms the crustal section of the ophiolite, and it consists of gabbro, diabase, basalt, lesser felsic volcanics, and minor sedimentary rocks. U-Pb zircon indicates that the age of this unit is ca. 161 Ma. The Esmeralda Peaks unit has transitional island-arc–mid-ocean ridge basalt and minor boninitic affinities. A preferred interpretation for this unit is that it formed initially by forearc rifting that evolved into back-arc spreading, and it was subsequently deformed by a fracture zone. The Iron Mountain unit is the rifted basement of the Esmeralda Peaks unit, indicating that the Ingalls ophiolite complex is polygenetic. Ingalls sedimentary rocks consist primarily of argillite with minor graywacke, conglomerate, chert, and ophiolite-derived breccias and olistoliths. Radiolaria from chert give lower Oxfordian ages. The Ingalls ophiolite complex is similar in age and geochemistry to the Josephine ophiolite and its related rift-edge facies and to the Coast Range ophiolite of California and Oregon. The Ingalls and Josephine ophiolites are polygenetic, while the Coast Range ophiolite is not, and sedimentary rocks (Galice Formation) that sit on the Josephine and its rift-edge facies have the same Radiolaria fauna as Ingalls sedimentary rocks. Therefore, we correlate the Ingalls ophiolite complex with the Josephine ophiolite of the Klamath Mountains. Taking known Cretaceous and younger strike-slip faulting into account, this correlation implies that the Josephine ophiolite either continued northward ~440 km—thus increasing the known length of the Josephine basin—or that the Ingalls ophiolite was translated northward ~440 km along the continental margin.

The Ingalls ophiolite complex, central Cascades, Washington, mainly consists of mantle-derived ultramafic tectonite, with crustal rocks consisting of gabbro, diabase, basalt, and sedimentary rocks. The crustal rocks occur as faulted blocks within serpentinite mélange (Navaho Divide fault zone). Mafic rocks in most of these blocks comprise the Late Jurassic Esmeralda Peaks unit. Herein, we define an older, Early Jurassic unit within the Ingalls ophiolite complex, which we call the Iron Mountain unit. This unit occurs along the southern edge of the complex and consists dominantly of mafic volcanic rocks with minor sedimentary rocks. A rhyolite within the Iron Mountain unit yields a ca. 192 Ma U-Pb zircon age, consistent with an Early Jurassic age assignment based on radiolarians in cherts. The presence of volcanic rocks that have within-plate basalt magmatic affinities and oolitic limestone suggests that the Iron Mountain unit formed as a seamount. Magmatic affinities range from within-plate basalt to enriched mid-ocean-ridge basalt (E-MORB), which is compatible with a mantle plume close to a ridge. The Early Jurassic age of the Iron Mountain unit, which is ~30 m.y. older than the Esmeralda Peaks unit, indicates that the Ingalls ophiolite complex is polygenetic. The Iron Mountain unit most likely represents basement that was rifted in a suprasubduction-zone setting in the Late Jurassic during formation of the Esmeralda Peaks unit.

La Désirade Island in the Cenozoic Lesser Antilles forearc region exposes a pre-Tertiary complex of oceanic volcanic, plutonic, and dike rocks. Previous work has established the stratigraphy and structure of the La Désirade igneous complex and also its late Mesozoic age. Dredge hauls from the nearby submerged Désirade fault scarp consist of similar volcanic and dike rocks plus greenstone, diabase, and gabbro. The composite section from island and submarine escarpment resembles upper oceanic crust but of controversial origin, original tectonic setting, and geodynamic significance. More precise ages for the La Désirade igneous complex and its individual members provide important constraints on proposed tectonic models. We reanalyzed Radio-laria from intralava sediments in basaltic pillow lava and zircon from trondhjemite to pinpoint their age. The radiolarian assemblage correlates with those of formations in east-central and west-central Mexico. The Mexican radiolarian faunas are chronostratigraphically calibrated by co-occurring ammonites and Buchia. Abundant Mexican biostratigraphic and chronostratigraphic data (ammonites, Radiolaria, and Buchia) constrain the composite radiolarian assemblage from six localities on La Désirade to zone 4, upper subzone 4β (mid-upper Tithonian). Using the new chemical abrasion (CA) thermal ionization mass spectrometry zircon method of Mattinson, the results from three zircon fractions from trondhjemite provide a 143.74 ± 0.33 Ma U-Pb age for the La Désirade igneous complex. Combined biostratigraphic, chronostratigraphic, and geochronometric data put the geochronologic age for the mid-late Tithonian near 143.74 Ma, a maximum for the latest Jurassic.

Suprasubduction-zone ophiolites have been recognized in the geologic record for over thirty years. These ophiolites are essentially intact structurally and stratigraphically, show evidence for synmagmatic extension, and contain lavas with geochemical characteristics of arc-volcanic rocks. They are now inferred to have formed by hinge retreat in the forearc of nascent or reconfigured island arcs. Emplacement of these forearc assemblages onto the leading edge of partially subducted continental margins is a normal part of their evolution. A recent paper has challenged this interpretation. The authors assert that the “ophiolite conundrum” (seafloor spreading shown by dike complexes versus arc geochemistry) can be resolved by a model called “historical contingency,” which holds that most ophiolites form at mid-ocean ridges that tap upper-mantle sources previously modified by subduction. They support this model with examples of modern mid-ocean ridges where suprasubduction zone–like compositions have been detected (e.g., ridge-trench triple junctions). The historical contingency model is flawed for several reasons: (1) the major- and trace-element compositions of magmatic rocks in suprasubduction-zone ophiolites strongly resemble rocks formed in primitive island-arc settings and exhibit distinct differences from rocks formed at mid-ocean-ridge spreading centers; (2) slab-influenced compositions reported from modern ridge-trench triple junctions and subduction reversals are subtle and/or do not compare favorably with either modern subduction zones or suprasubduction-zone ophiolites; (3) crystallization sequences, hydrous minerals, miarolitic cavities, and reaction textures in suprasubduction-zone ophiolites imply crystallization from magmas with high water activities, rather than mid-ocean-ridge systems; (4) models of whole Earth convection, subduction recycling, and ocean-island basalt isotopic compositions ignore the fact that these components represent the residue of slab melting, not the low field strength element–enriched component found in active arc-volcanic suites and suprasubduction-zone ophiolites; and (5) isotopic components indicative of mantle heterogeneities (related to subduction recycling) are observed in modern mid-ocean-ridge basalts (MORB), but, in contrast to the prediction of the historical contingency model, these basalts do not exhibit suprasubduction zone–like geochemistry. The formation of suprasubduction-zone ophiolites in the upper plate of subduction zones favors intact preservation either by obduction onto a passive continental margin, or by accretionary uplift above a subduction zone. Ophiolites characterized by lavas with MORB geochemistry are typically disrupted and found as fragments in accretionary complexes (e.g., Franciscan), in contrast to suprasubduction-zone ophiolites. This must result from the fact that oceanic crust is unlikely to be obducted for mechanical reasons, but it may be preserved where it is scraped off of the subducting slab.

The tectonic significance of early Paleozoic convergent-margin rocks of the Alexander and Sierran-Klamath terranes is poorly understood. New phengite 40 Ar/ 39 Ar and Rb-Sr results from the schist of Skookum Gulch of the Yreka subterrane in the Klamath Mountains (454 ± 10 Ma) confirm that blueschists are the oldest known subduction-zone rocks of the western North American Cordillera. The blueschists are juxtaposed with kilometer-scale tectonic blocks of ca. 565 Ma tonalite. Detrital zircons from the blueschists require close proximity to a diverse source of cratonal or derivative supracrustal rocks and preclude formation within an isolated intra-oceanic setting. This strong cratonal provenance (mostly 1.0–2.0 Ga, with resolved concentrations of 1.49–1.61 Ga zircon) is also exhibited by adjacent Early Devonian lower greenschist units of the Yreka subterrane (Duzel phyllite and Moffett Creek Formation). Additional results from temporally equivalent arc-derived sedimentary units (Sissel Gulch graywacke and Gazelle Formation) yield strongly unimodal zircon age distributions of early Paleozoic zircon. The results indicate that the Yreka subterrane formed at an Ordovician–Silurian–Early Devonian convergent margin near a Mesoproterozoic-Paleoproterozoic craton and Ediacaran crust. Appreciable 1.49–1.61 Ga zircon within the Yreka subterrane is compatible with a recent biogeographic analysis that indicates a non-Laurentian origin for the eastern Klamath terrane. Additional new data reveal that key early Paleozoic convergent-margin rocks within the northern Sierran-Klamath and Alexander terranes share similar arc and cratonal provenance, including 1.49–1.61 Ga zircon. We hypothesize that the rocks from all three areas are dispersed tectonic fragments that were derived from the same convergent margin and were independently transported to western North America. Of the orogenic source regions indicated by previous paleomagnetic and biogeographic analysis, the detrital zircon provenance favors western Baltica over eastern Australia.

Metamorphosed Triassic and Jurassic volcanic and sedimentary rocks have been mapped, described, and measured in the Singatse, Buckskin, and northern Wassuk Ranges near Yerington, west-central Nevada. Herein, we establish new formation names for these rocks and correlate them regionally with other Triassic-Jurassic rocks, in part by use of fossil and radiometric ages. From oldest to youngest, rocks in the Singatse Range consist of a Middle Triassic or older volcanic sequence (McConnell Canyon volcanics), an Upper Triassic sequence of interbedded fine-grained clastic sedimentary rocks, carbonate rocks, tuffaceous sedimentary rocks, and tuffs (Malachite Mine Formation and tuff of Western Nevada Mine), a thick Upper Triassic limestone (Mason Valley Limestone), an uppermost Triassic and Lower Jurassic siltstone sequence (Gardnerville Formation), an Early and/or Middle Jurassic limestone-gypsum-quartzite sequence (Ludwig Mine Formation), and Middle Jurassic volcanic rocks. The sequence is exposed in septa between two Middle Jurassic batholiths and was folded and metamorphosed during emplacement of the batholiths. The Middle Jurassic volcanic rocks are best exposed in the Buckskin Range to the west, where they consist of a lower andesitic sequence (Artesia Lake volcanics) and an upper sequence of more felsic, porphyritic rocks (Fulstone Spring volcanics). The Triassic and Early Jurassic rocks are also exposed in the Wassuk Range to the east and include a thick section of andesitic and silicic volcanics, which may be in part equivalent to the McConnell Canyon volcanics, the lower part of which is intruded by the possibly cogenetic Middle Triassic Wassuk diorite and associated quartz monzonite and quartz porphyry. The McConnell Canyon volcanics apparently formed as part of an Early to early Late Triassic continental-margin volcanic arc that extended from the Mojave Desert area to northern California and Nevada. Volcanism waned in Late Triassic time, and the volcanic rocks were covered by interbedded volcaniclastic, clastic sedimentary, and carbonate rocks that include the Malachite Mine Formation and tuff of Western Nevada Mine. Late Triassic carbonate sequences, such as the Mason Valley Limestone, succeed the interbedded rocks, but this appears to have taken place earlier to the north, whereas volcanism persisted for a longer time to the south. Fine-grained siliciclastic sedi ments, with minor carbonate and local volcanic-derived strata, were deposited above the more massive carbonates in a wide area during latest Triassic and Early Jurassic deposition of the Gardnerville Formation and correlative rocks. The Ludwig Mine Formation is part of a sequence of quartz-rich sandstone, evaporates, and carbonates that is widespread in western Nevada and lies on top of and ties together diverse older rock sequences of quite different character. In addition to the arc volcanic, carbonate, and clastic sequence of Yerington and surrounding regions, these older rock sequences include thick, lithologically different, basinal turbidite-mudstone sequences of similar Late Triassic to Early Jurassic age to the north, strata of the shelf terrane to the northeast and east, and probably also rocks of the North American continental platform and parts of the Sierra Nevada. The Artesia Lake and Fulstone volcanics comprise a Middle Jurassic volcanic center related to the Yerington batholith and to nearby igneous centers that is part of a volcanic arc that extended from north of the Yerington district southward through the Mojave Desert and Arizona.

Terranes of the Klamath Mountains, California-Oregon, are primarily the products of transpressive-transtensional margin processes that span the mid-Devonian to Middle Jurassic time interval. One of the more enigmatic units of this orogen is the early Mesozoic, north-south–trending, medially situated North Fork terrane. Based on the integration of new and previously published data, ophiolitic rocks in the North Fork terrane are interpreted as the basement to superjacent mafic metavolcanic units and associated metasedimentary strata. We propose that these volcanic and sedimentary rocks accumulated in a forearc position relative to the eastern Klamath fringing arc. New mapping in the southern Klamath Mountains reveals that northwest-southeast–striking structures in the southern North Fork terrane are regionally consistent with those in the rest of the belt, ophiolitic rocks are variably deformed (one expression being a serpentinite-matrix mélange), and the terrane is dominated by mafic volcanic rocks and fine-grained clastic metasedimentary rocks interbedded with chert. Bulk-rock geochemical data for metasedimentary rocks indicate that chert and shale possess a continental component. Detrital zircon U-Pb ages for three sandstone samples (one from the southern and two from the central North Fork) show that (1) sedimentary rocks of both terrane segments had a similar source of Mesozoic zircons, (2) sedimentary rocks of the central North Fork terrane have ages consistent with a source in the nearby eastern Klamath Mountains, and (3) sediment of the southern North Fork was derived primarily from an early Mesozoic source. We conclude that the North Fork terrane represents tectonically disrupted remnants of a forearc basin and its ophiolitic basement that were located seaward of the early to middle Mesozoic fringing arc of the eastern Klamath terrane and landward from a coeval oceanic trench. Slightly coarser-grained sedimentary rocks are preserved in the central part of the terrane, whereas rocks in the south lack continent-derived material coarser than mud.

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.

We integrate new stratigraphic, structural, geochemical, geochronological, and magnetostratigraphic data on Cenozoic volcanic rocks in the central Sierra Nevada to arrive at closely inter-related new models for: (1) the paleogeography of the ancestral Cascades arc, (2) the stratigraphic record of uplift events in the Sierra Nevada, (3) the tectonic controls on volcanic styles and compositions in the arc, and (4) the birth of a new plate margin. Previous workers have assumed that the ancestral Cascades arc consisted of stratovolcanoes, similar to the modern Cascades arc, but we suggest that the arc was composed largely of numerous, very small centers, where magmas frequently leaked up strands of the Sierran frontal fault zone. These small centers erupted to produce andesite lava domes that collapsed to produce block-and-ash flows, which were reworked into paleocanyons as volcanic debris flows and streamflow deposits. Where intrusions rose up through water-saturated paleocanyon fill, they formed peperite complexes that were commonly destabilized to form debris flows. Paleo-canyons that were cut into Cretaceous bedrock and filled with Oligocene to late Miocene strata not only provide a stratigraphic record of the ancestral Cascades arc volcanism, but also deep unconformities within them record tectonic events. Preliminary correlation of newly mapped unconformities and new geochronological, magnetostratigraphic, and structural data allow us to propose three episodes of Cenozoic uplift that may correspond to (1) early Miocene onset of arc magmatism (ca. 15 Ma), (2) middle Miocene onset of Basin and Range faulting (ca. 10 Ma), and (3) late Miocene arrival of the triple junction (ca. 6 Ma), perhaps coinciding with a second episode of rapid extension on the range front. Oligocene ignimbrites, which erupted from calderas in central Nevada and filled Sierran paleocanyons, were deeply eroded during the early Miocene uplift event. The middle Miocene event is recorded by growth faulting and landslides in hanging-wall basins of normal faults. Cessation of andesite volcanism closely followed the late Miocene uplift event. We show that the onset of Basin and Range faulting coincided both spatially and temporally with eruption of distinctive, very widespread, high-K lava flows and ignimbrites from the Little Walker center (Stanislaus Group). Preliminary magnetostratigraphic work on high-K lava flows (Table Mountain Latite, 10.2 Ma) combined with new 40 Ar/ 39 Ar age data allow regional-scale correlation of individual flows and estimates of minimum (28,000 yr) and maximum (230,000 yr) time spans for eruption of the lowermost latite series. This work also verifies the existence of reversed-polarity cryptochron, C5n.2n-1 at ca. 10.2 Ma, which was previously known only from seafloor magnetic anomalies. High-K volcanism continued with eruption of the three members of the Eureka Valley Tuff (9.3–9.15 Ma). In contrast with previous workers in the southern Sierra, who interpret high-K volcanism as a signal of Sierran root delamination, or input of subduction-related fluids, we propose an alternative model for K 2 O-rich volcanism. A regional comparison of central Sierran volcanic rocks reveals their K 2 O levels to be intermediate between Lassen to the north (low in K 2 O) and ultrapotassic volcanics in the southern Sierra. We propose that this shift reflects higher pressures of fractional crystallization to the south, controlled by a southward increase in the thickness of the granitic crust. At high pressures, basaltic magmas precipitate clinopyroxene (over olivine and plagioclase) at their liquidus; experiments and mass-balance calculations show that clinopyroxene fractionation buffers SiO 2 to low values while allowing K 2 O to increase. A thick crust to the south would also explain the sparse volcanic cover in the southern Sierra compared to the extensive volcanic cover to the north. All these data taken together suggest that the “future plate boundary” represented by the transtensional western Walker Lane belt was born in the axis of the ancestral Cascades arc along the present-day central Sierran range front during large-volume eruptions at the Little Walker center.