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.

The Lovejoy basalt represents the largest eruptive unit identified in California, and its age, volume, and chemistry indicate a genetic affinity with the Columbia River Basalt Group and its associated mantle-plume activity. Recent field mapping, geochemical analyses, and radiometric dating suggest that the Lovejoy basalt erupted during the mid-Miocene from a fissure at Thompson Peak, south of Susanville, California. The Lovejoy flowed through a paleovalley across the northern end of the Sierra Nevada to the Sacramento Valley, a distance of 240 km. Approximately 150 km 3 of basalt were erupted over a span of only a few centuries. Our age dates for the Lovejoy basalt cluster are near 15.4 Ma and suggest that it is coeval with the 16.1–15.0 Ma Imnaha and Grande Ronde flows of the Columbia River Basalt Group. Our new mapping and age dating support the interpretation that the Lovejoy basalt erupted in a forearc position relative to the ancestral Cascades arc, in contrast with the Columbia River Basalt Group, which erupted in a backarc position. The arc front shifted trenchward into the Sierran block after 15.4 Ma. However, the Lovejoy basalt appears to be unrelated to volcanism of the predominantly calc-alkaline Cascade arc; instead, the Lovejoy is broadly tholeiitic, with trace-element characteristics similar to the Columbia River Basalt Group. Association of the Lovejoy basalt with mid-Miocene flood basalt volcanism has considerable implications for North American plume dynamics and strengthens the thermal “point source” explanation, as provided by the mantle-plume hypothesis. Alternatives to the plume hypothesis usually call upon lithosphere-scale cracks to control magmatic migrations in the Yellowstone–Columbia River basalt region. However, it is difficult to imagine a lithosphere-scale flaw that crosses Precambrian basement and accreted terranes to reach the Sierra microplate, where the Lovejoy is located. Therefore, we propose that the Lovejoy represents a rapid migration of plume-head material, at ~20 cm/yr to the southwest, a direction not previously recognized.

Upper Jurassic strike-slip intra-arc basins formed along the axis of earlier Lower to Middle Jurassic extensional intra-arc basins in Arizona. These strike-slip basins developed along the Sawmill Canyon fault zone, which may represent an inboard strand of the Mojave-Sonora megashear system that did not necessarily produce large-scale translations. Subsidence in the Lower to Middle Jurassic extensional arc was uniformly fast and continuous, whereas at least parts of the Upper Jurassic arc experienced rapidly alternating uplift and subsidence, producing numerous large-scale intrabasinal unconformities. Volcanism occurred only at releasing bends or stepovers in the Upper Jurassic arc, producing more episodic and localized eruptions than in the earlier extensional arc. Sediment sources in the Upper Jurassic strike-slip arc were also more localized, with restraining bends shedding sediment into nearby releasing bends. Normal fault scarps were rapidly buried by voluminous pyroclastic debris in the Lower to Middle Jurassic extensional arc, so epiclastic sedimentary deposits are rare, whereas pop-up structures in the Upper Jurassic strike-slip arc shed abundant epiclastic sediment into the basins. Three Upper Jurassic calderas formed along the Sawmill Canyon fault zone where strands of the fault progressively stepped westward in a releasing geometry relative to paleo-Pacific–North America plate motion. We hypothesize that strike-slip basins in the Upper Jurassic arc formed in response to changing plate motions that induced northward drift of North America, causing sinistral deformation of the paleo-Pacific margin. Drift out of the northern horse latitudes into northern temperate latitudes brought about wetter climatic conditions, with eolianites replaced by fluvial, debris-flow, and lacustrine sediments. “Dry” eruptions of welded ignimbrite were replaced by “wet” eruptions of nonwelded, easily reworked ignimbrite and phreatoplinian fall. This Late Jurassic transition from hyperarid to more temperate climatic conditions may thus form a superregional “time line” that ties the Cordilleran plate margin to events in the interior of the continent.

In the western Bisbee Basin of southern Arizona, detailed mapping and sequence analysis of the Glance Conglomerate along the largest basin-bounding fault, the Sawmill Canyon fault zone, reveals interbedded clastic, volcanic, and volcaniclastic lithofacies and their relationship to intrabasinal faulting, unconformities, and basin-bounding faults. The basin fill is dominated by small polygenetic, multivent volcanic complexes ranging in composition from rhyolite to andesite typical of continental arc volcanism. Syndepositional basin-bounding faults, the Sawmill Canyon and Gringo Gulch fault zones, controlled subsidence within the basin and plumbed small batches of magma to the surface. Small intrabasinal faults show stratigraphically limited offsets that alternate between normal and reverse separation. Eight unconformable surfaces occur within the basin. Five are asymmetrical, with one very steep wall and one gradually sloping wall. They show extreme vertical relief (460–910 m) with very high paleoslope gradients (40°–71°) that dip away from the master fault. We interpret these as uplifted fault scarps or paleoslide scars. The other three unconformities are symmetrical, V-shaped surfaces that have less steep walls, with vertical relief of 200–600 m and paleoslope gradients of 20°–25°. We interpret the symmetrical surfaces to be walls of deep paleocanyons cut during basin uplift events or following large ignimbrite eruptions. Analysis of the unconformably bound stratigraphic sequences shows deposition to be related to subsidence along large basin-bounding faults modified by intrabasinal, high-angle, syndepositional normal and reverse faults. Erosion of the sequence-bounding unconformities took place during uplift associated with basin inversion. Alternation of uplift and subsidence and the juxtaposition of intrabasinal reverse and normal faults is typical of strike-slip basins. We interpret the Glance Conglomerate in the Santa Rita Mountains as the fill of an intra-arc strike-slip basin where strike-slip deformation was concentrated along the thermally weakened arc axis. We suggest a model for the Bisbee Basin of a strain-partitioned, obliquely convergent continental arc with backarc extension-transtension.

Abstract The Bald Mountain deposit, a medium-sized (30 Mt) volcanic-hosted massive sulfide (VHMS) deposit of Early Ordovician age in northern Maine, was selected for detailed study because it is one of the best-preserved such deposits in the world. The massive sulfide lies within a 5-km-thick stratigraphic section that forms the Bald Mountain sequence. This study focuses on the volcanic and sedimentary evolution of the Bald Mountain sequence, with the goal of understanding the controls of deep-water volcanotectonic processes on the generation of massive sulfide mineralization.

Abstract The Bald Mountain volcanogenic massive sulfide (VMS) deposit of Early Ordovician age in northern Maine contains 30 million metric tons (Mt) of Cu-Zn-Au-Ag sulfides. It is exceptionally well preserved, lacking penetrative deformation, and having experienced only prehnite-pumpellyite–grade regional metamorphism. The deposit occurs within a homoclinal west-dipping volcanic sequence that consists of, from bottom to top, basalt and basaltic andesite, crystal-poor rhyolite ignimbrite, massive sulfide and related units, crystal-rich rhyolite ignimbrite and intercalated andesite, carbonaceous argillite, and rhyolitic volcaniclastic rocks. Basalts stratigraphically below the massive sulfide are intruded by an elongate body of tonalite-plagiogranite; gabbros intrude rocks both above and below the massive sulfides. The basal contact of the host volcanic sequence is believed to be a thrust with underlying Middle Ordovician clastic sedimentary rocks; the upper contact is depositional with the Middle to Upper Ordovician Winterville Formation and, in places, with Silurian conglomerates. Ordovician synvolcanic faults that predominantly strike 025°, 050° to 060°, 325° to 335°, and 350° formed a small (320 × 275 m) synvolcanic graben in which as much as 215 m of massive sulfide accumulated. Hydrothermal solutions utilized these faults as fluid conduits, causing structurally controlled epidote and silica alteration in the deep footwall. Structurally controlled alteration is also indicated by the presence of magnetic low areas in mafic rocks up to 1 km below the deposit. Movement of zinc- and copper-rich fluids was controlled by the location of the Ordovician faults. Zinc-rich fluids were concentrated along faults that bound the northern, western, and southern sides of the small graben; copper-rich fluids moved along faults that define the eastern side of the graben. Rocks overlying the massive sulfide body show little evidence of the growth faulting that occurred within and below the deposit, indicating that most extensional deformation ceased shortly after exhalative sulfide deposition. Synvolcanic Ordovician faulting and graben formation are the principal causes for the small lateral dimensions of the Bald Mountain deposit relative to those of most VMS deposits of comparable tonnage. Postsulfide deformational events occurred in the Late Ordovician to Early Silurian when rocks hosting the Bald Mountain deposit were thrust over Ordovician clastic sedimentary rocks and in the Early Devonian when Acadian faulting and folding segmented the deposit and tilted it to the west.