The Lunar Crater volcanic field (LCVF) in central Nevada (USA) is dominated by monogenetic mafic volcanoes spanning the late Miocene to Pleistocene. There are as many as 161 volcanoes (there is some uncertainty due to erosion and burial of older centers); the volumes of individual eruptions were typically ∼0.1 km3 and smaller. The volcanic field is underlain by a seismically slow asthenospheric domain that likely reflects compositional variability relative to surrounding material, such as relatively higher abundances of hydrous phases. Although we do not speculate about why the domain is in its current location, its presence likely explains the unusual location of the LCVF within the interior of the Basin and Range Province. Volcanism in the LCVF occurred in 4 magmatic episodes, based upon geochemistry and ages of 35 eruptive units: episode 1 between ca. 6 and 5 Ma, episode 2 from ca. 4.7 to 3 Ma, episode 3 between ca. 1.1 and 0.4 Ma, and episode 4, ca. 300 to 35 ka. Each successive episode shifted northward but partly overlapped the area of its predecessor. Compositions of the eruptive products include basalts, tephrites, basanites, and trachybasalts, with very minor volumes of trachyandesite and trachyte (episode 2 only). Geochemical and petrologic data indicate that magmas originated in asthenospheric mantle throughout the lifetime of the volcanic field, but that the products of the episodes were derived from unique source types and therefore reflect upper mantle compositional variability on spatial scales of tens of kilometers. All analyzed products of the volcanic field have characteristics consistent with small degrees of partial melting of ocean island basalt sources, with additional variability related to subduction-related enrichment processes in the mantle, including contributions from recycled ocean crust (HIMU source; high-µ, where µ = 238U/204Pb) and from hydrous fluids derived from subducted oceanic crust (enriched mantle, EM source). Geochemical evidence indicates subtle source heterogeneity at scales of hundreds of meters to kilometers within each episode-scale area of activity, and temporary ponding of magmas near the crust-mantle boundary. Episode 1 magmas may have assimilated Paleozoic carbonate rocks, but the other episodes had little if any chemical interaction with the crust. Thermodynamic modeling and the presence of amphibole support dissolved water contents to ∼5–7 wt% in some of the erupted magmas. The LCVF exhibits clustering in the form of overlapping and colocated monogenetic volcanoes that were separated by variable amounts of time to as much as several hundred thousand years, but without sustained crustal reservoirs between the episodes. The persistence of clusters through different episodes and their association with fault zones are consistent with shear-assisted mobilization of magmas ponded near the crust-mantle boundary, as crustal faults and underlying ductile deformation persist for hundreds of thousands of years or more (longer than individual episodes). Volcanoes were fed at depth by dikes that occur in en echelon sets and that preserve evidence of multiple pulses of magma. The dikes locally flared in the upper ∼10 m of the crust to form shallow conduits that fed eruptions. The most common volcanic landforms are scoria cones, agglomerate ramparts, and 'a'ā lava fields. Eruptive styles were dominantly Strombolian to Hawaiian; the latter produced tephra fallout blankets, along with effusive activity, although many lavas were likely clastogenic and associated with lava fountains. Eroded scoria cones reveal complex plumbing structures, including radial dikes that fed magma to bocas and lava flows on the cone flanks. Phreatomagmatic maar volcanoes compose a small percentage of the landform types. We are unable to identify any clear hydrologic or climatic drivers for the phreatomagmatic activity; this suggests that intrinsic factors such as magma flux played an important role. Eruptive styles and volumes appear to have been similar throughout the 6 m.y. history of the volcanic field and across all 4 magmatic episodes. The total volume and time-volume behavior of the LCVF cannot be precisely determined by surface observations due to erosion and burial by basin-fill sediments and subsequent eruptive products. However, previous estimates of a total volume of 100 km3 are likely too high by a factor of ∼5, suggesting an average long-term eruptive flux of ∼3–5 km3/m.y.
Abstract Recent volcanism in the Black Rock Desert and Markagunt Plateau volcanic fields in south-central Utah is mainly subalkaline, which is uncommon elsewhere in the Basin and Range Province. The Black Rock Desert volcanic field (BRD) is composed of five subfields and spans about 170 km along the eastern margin of the Basin and Range Province in Utah. The subfields are: (1) Cove Fort, (2) Twin Peaks, (3) Beaver Ridge, (4) Ice Springs, and (5) Fumarole Butte. Although volcanism in BRD began in the late Miocene, about 6 Ma, most activity has occurred in the past 2.5 million years. Black Rock Desert is composed of a tholeiitic suite ranging in composition from basalt to dacite, and calc-alkaline andesite, dacite, and rhyolite. During the 6 million year life of the field, alkali basalts erupted only rarely and comprise no more than three flows. The Markagunt Plateau volcanic field (MP) is located east of Cedar City at the south end of the High Plateaus physiographic province (a structural and stratigraphic transition zone between the Basin and Range Province to the west and the Colorado Plateau to the east) and has been active since the earliest Pliocene, with the most recent eruptions occurring in latest Pleistocene or possibly Holocene time. It contains over 25 cinder cones and associated flows. Flows vary in composition from calc-alkaline basalt, basaltic andesite, and andesite, to olivine tholeiite and alkali basalt. One of the most puzzling attributes of MP and BRD, and of volcanic fields in western Utah in general, is the presence of abundant subalkaline volcanic units. In most other volcanic fields in an intraplate tectonic setting, and in particular in the Basin and Range Province, alkali basalt is the dominant eruptive product with little, if any, calc-alkaline intermediate or silicic rocks. The chemical characteristics of both BRD and MP, while unlike typical intra-plate volcanism, are similar to those of continental rift zones like the Rio Grande Rift on the east side of the Colorado Plateau. We suggest that recent subalkaline volcanism at the western margin of the Basin and Range Province may be signaling the initiation of a rift along the western margin of the Colorado Plateau, similar to the Rio Grande Rift. Alternatively, these chemical signatures could simply be a characteristic of volcanism at, or near, the margins of the Colorado Plateau.
Abstract Although volcanism in the southwestern United States has been studied extensively, its origin remains controversial. Various mechanisms such as mantle plumes, upwelling in response to slab sinking, and small‐scale convective processes have been proposed, but have not been evaluated within the context of rapidly shearing asthenosphere that is thought to underlie this region. Using geodynamic models that include this shear, we here explore spatiotemporal patterns of mantle melting and volcanism near the Colorado Plateau. We show that the presence of viscosity heterogeneity within an environment of asthenospheric shearing can give rise to decompression melting along the margins of the Colorado Plateau. Our models indicate that eastward shear flow can advect pockets of anomalously low viscosity toward the edges of thickened lithosphere beneath the plateau, where they can induce decompression melting in two ways. First, the arrival of the pockets critically changes the effective viscosity near the plateau to trigger small‐scale edge‐driven convection. Second, they can excite shear‐driven upwelling (SDU), in which horizontal shear flow becomes redirected upward as it is focused within the low‐viscosity pocket. We find that a combination of “triggered” edge‐driven convection and SDU can explain volcanism along the margins of the Colorado Plateau, its encroachment toward the plateau's southwestern edge, and the association of volcanism with slow seismic anomalies in the asthenosphere. Geographic patterns of intraplate volcanism in regions of vigorous asthenospheric shearing may thus directly mirror viscosity heterogeneity of the sublithospheric mantle.
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