The relative timing and magnitude of middle Tertiary extension and volcanism in the Great Basin (northern Basin and Range province) of the western United States remain controversial. To constrain the timing, we present 31 stratigraphic sections from the central part of the province, together with data from other studies in the Great Basin. Especially significant in this record of regional paleogeographic and associated tectonic conditions are thick sections of many well‐dated ash flow sheets emplaced during the period of the most voluminous, or peak, volcanic activity about 31–20 Ma. From these data we make the following conclusions: (1) Extension prior to the period of peak volcanism was apparently localized. (2) Extension during peak volcanism (the ignimbrite flareup) was minor and in places possibly related to magmatic processes in the shallow crust, rather than to regional tectonic processes. Angular unconformities and interbedded epiclastic deposits within sequences of volcanic rocks from 31 to about 22–20 Ma that would manifest synvolcanic faulting, tilting, and erosion are limited. (3) In the Great Basin as a whole, major extension and peak volcanism correlate poorly in space as well as time. (4) Essentially dip‐slip faults cutting the entire conformable volcanic sequence are common in the Great Basin and indicate a widespread episode of extension after peak volcanism. Southward sweeping Tertiary volcanism in the Great Basin reflects migration of the mantle magma supply that powered crustal magma systems. We suspect this migration was related to progressive southward foundering and steepening of dip of a subducting oceanic plate (after an earliest Tertiary near‐horizontal configuration beneath the continental lithosphere) and consequent backflow of asthenospheric mantle into the widening wedge between the plates. In the northern Great Basin, where the sweep was rapid, we postulate that relatively small volumes of mantle‐derived magma were inserted as dikes into the lower, locally extending crust which was unusually warm because of Mesozoic compressional thickening; crustal magma systems so powered were repeatedly tapped to feed modest volume eruptions of chiefly intermediate composition lava and minor silicic ash flow tuff. As the sweep stagnated in the central southern Great Basin, copious volumes of mafic magma were inserted into the crust, apparently mostly as extensive horizontal sheets, or sills, in a nonextending, uplifting crust in a state of nearly isotropic horizontal stress. These sills and the high mantle power input optimized crustal magma generation, creating huge volumes of silicic magma that vented as large volume ash flows, chiefly about 31–20 Ma. After about 22–20 Ma, the volcanic‐capped plateau collapsed in a widespread network of north striking extensional faults as plate boundary compressive forces were overcome by spreading forces within the uplift. Eruption of lava again became the dominant mode of volcanism.

Uncertainty surrounds the fate of the orogenic plateau in what is now the Great Basin in western Utah and Nevada, which resulted from the Mesozoic and earliest Cenozoic contractile deformations and crustal thickening. Although there is some consensus regarding the gravitational collapse of the plateau by extensional faulting and consequent crustal thinning, whether or not the plateau existed during the middle Cenozoic Great Basin ignimbrite flareup -one of the grandest expressions of continental volcanism in the geologic record -had remained in doubt. We use compositions of contemporaneous calc-alkaline lava flows as well as configurations of the ignimbrite sheets to show that the Great Basin area during the middle Cenozoic was a relatively smooth plateau underlain by unusually thick crust. We compare analyses of 376 intermediate-composition lava flows in the Great Basin that were extruded at 42 -17 Ma with compositions of .6000 analyses of the late Cenozoic lava flows in continental volcanic arcs that correlate roughly with known crustal thickness. This comparison indicates that the middle Cenozoic Great Basin crust was much thicker than the present ca. 30 km thickness, likely as much as 60 -70 km. If isostatic equilibrium prevailed, this unusually thick continental crust must have supported high topography. This high terrain in SE Nevada and SW Utah was progressively smoothed as successive ignimbrite outflow sheets were emplaced over areas currently as much as tens of thousands of square kilometres to aggregate thicknesses of as much as hundreds of metres. The generally small between-site variations in the palaeomagnetic directions of individual sheets lend further support for a relatively smooth landscape over which the sheets were draped. We conclude that during the middle Cenozoic, especially towards the close of the ignimbrite flareup, this Great Basin area was a relatively flat plateau, and because it was also high in elevation, we refer to it as an Altiplano. It was not unlike the present-day Altiplano-Puna in the tectonically similar central Andes, where an ignimbrite flareup comparable to that in the Great Basin occurred at ca. 9-3 Ma. Outflow ignimbrite sheets that were deposited from 35 to 23 Ma on the progressively smoothed Altiplano in south-eastern Nevada were derived from source calderas to the west. Of the 12 major sheets from seven sources, nine are distributed unevenly east of their sources while the remaining three sheets are spread about as far east as west of their sources. This eccentricity of sources near the western margin of 75% of the sheets indicates the existence of a NS-trending topographic barrier in central Nevada that restricted westward dispersal of ash flows. In a symmetric manner, eastward dispersal of ash flows from sources farther west seemed to have been impeded by this same topographic barrier. The westward dispersal was controlled in part by westward-draining stream valleys incised in the sloping flank of the Great Basin Vol. 51, Nos. 7 -8, July-August 2009, 589-633 A...

Two fundamentally different types of silicic volcanic rocks formed during the Cenozoic of the western Cordillera of the United States. Large volumes of dacite and rhyolite, mostly ignimbrites, erupted in the Oligocene in what is now the Great Basin and contrast with rhyolites erupted along the Snake River Plain during the Late Cenozoic. The Great Basin dacites and rhyolites are generally calc-alkaline, magnesian, oxidized, wet, cool (<850°C), Sr-and Al-rich, and Fe-poor. These silicic rocks are interpreted to have been derived from mafic parent magmas generated by dehydration of oceanic lithosphere and melting in the mantle wedge above a subduction zone. Plagioclase fractionation was minimized by the high water fugacity and oxide precipitation was enhanced by high oxygen fugacity. This resulted in the formation of Si-, Al-, and Sr-rich differentiates with low Fe/Mg ratios, relatively low temperatures, and declining densities. Magma mixing, large proportions of crustal assimilation, and polybaric crystal fractionation were all important processes in generating this Oligocene suite. In contrast, most of the rhyolites of the Snake River Plain are alkaline to calc-alkaline, ferroan, reduced, dry, hot (830-1,050°C), Sr-and Al-poor, and Nb-and Fe-rich. They are part of a distinctly bimodal sequence with tholeiitic basalt. These characteristics were largely imposed by their derivation from parental basalt (with low f H 2 O and low f O 2 ) which formed by partial melting in or above a mantle plume. The differences in intensive parameters caused early precipitation of plagioclase and retarded crystallization of Fe-Ti oxides. Fractionation led to higher density magmas and mid-crustal entrapment. Renewed intrusion of mafic magma caused partial melting of the intrusive complex. Varying degrees of partial melting, fractionation, and minor assimilation of older crust led to the array of rhyolite compositions. Only very small volumes of distinctive rhyolite were derived by fractional crystallization of Fe-rich intermediate magmas like those of the Craters of the Moon-Cedar Butte trend.

During the middle Cenozoic, from 36 to 18 Ma, one of the greatest global expressions of long-lived, explosive silicic volcanism affected a large segment of southwestern North America, including central Nevada and southwestern Utah in the southern Great Basin. The southern Great Basin ignimbrite province, resulting from this fl areup, harbors several tens of thousands of cubic kilometers of ash-flow deposits. They were created by more than two hundred explosive eruptions, at least thirty of which were super-eruptions of more than 1000 km 3. Forty-two exposed calderas are as much as 60 km in diameter. As in other parts of southwestern North America affected by the ignimbrite fl areup, rhyolite ash-fl ow tuffs are widespread throughout the southern Great Basin ignimbrite province. However, the province differs in two significant respects. First, extrusions of contemporaneous andesitic lavas were minimal. Their volume is only about 10% of the ignimbrite volume. Unlike other contemporaneous volcanic fi elds in southwestern North America, only a few major composite (strato-) volcanoes predated and developed during the fl areup. Second, the central sector and especially the eastern sector of the province experienced super-eruptions of relatively uniform, crystal-rich dacite magmas; resulting deposits of these monotonous intermediates measure on the order of 16,000 km 3. Following this 4 m.y. event, very large volumes of unusually hot and dry trachydacitic magmas were erupted. These two types of magmas and their erupted volumes are apparently without parallel in the middle Cenozoic of southwestern North America. A fundamental goal of this themed issue is to present basic stratigraphic, compositional, chronologic, and paleomagnetic data on the unusually plentiful and voluminous ignimbrites in the southern Great Basin ignimbrite province. These data permit rigorous correlations of the vast outfl ow sheets that span between mountain-range exposures across intervening valleys as well as correlation of the sheets with often-dissimilar accumulations of tuff within dismembered source calderas. Well-exposed collar zones of larger calderas reveal complex wall-collapse breccias. Calculated ignimbrite dimensions in concert with precise 40 Ar/ 39 Ar ages provide insights on the growth and longevity of the colossal crustal magma systems. Exactly how these subduction-related magma systems were sustained for millions of years to create multicyclic super-eruptions at a particular focus remains largely unanswered. What factors created eruptive episodes lasting millions of years separated by shorter intervals of inactivity? What might have been the role played by tears in the subducting plate focusing a high rate of mantle magma fl ux into the crust? What role might have been played by an unusually thick and still-warm crust inherited from earlier orogenies? Are the numerous super-eruptions, especially of the unusual monotonous intermediates and succeeding trachydacitic eruptions, during the Great Basin ignimbrite fl areup simply a result...

In continental-margin subduction zones, basalt magmas spawned in the mantle interact with the crust to produce a broad spectrum of volcanic arc associations. A distinct style of very voluminous arc volcanism develops far inland on thick crust over periods of 10-20 m.y. and involves relatively infrequent caldera-forming explosive eruptions of dominantly calc-alkaline rhyolite, dacite, and trachydacite with repose times of 10 4-10 6 yr. Volumes of individual eruptions are large (10 2-10 3 km 3), and nested super-eruptions of thousands of cubic kilometers are common. Calderas are as much as 60-75 km in diameter, and surrounding individual ignimbrite outflow sheets extend outward as much as 150 km, blanketing upwards of 10 5 km 2. Little or no basalt is extruded, whereas andesitic differentiates coeval with silicic ignimbrites range from minor to dominant in relative volume. A common feature in these flareups is essentially nonextending, thick, inland crust overlying a subducting oceanic plate with transverse tears that rolled back to a steeper dip from a previously flat configuration. Lithospheric delamination is locally possible. Large volumes of basalt that provide heat and mass for silicic magma generation in the crust form by fluid fluxing of the growing mantle wedge overlying the steepening dehydrating slab and from asthenospheric decompression. Variations in the mantle input, together with variations in crustal thickness, temperature, and composition, modulate the expression of the flareups. As a consequence of the high flux of mantle-derived magma into the thick crust, geotherms become elevated, and the brittle-ductile transition can rise to depths as shallow as 7 km. At this transition, diapirically rising magmas from a melting, assimilation, storage, and homogenization (MASH) zone are blocked and spread laterally into discoid chambers that grow until a thermomechanical threshold is attained, triggering climactic eruption and caldera collapse. This ignimbrite flareup style of continental arc volcanism is exemplified by the mid-Cenozoic southern Great Basin ignimbrite province; other examples include the contemporaneous Southern Rocky Mountain, Mogollon-Datil, vast Sierra Madre Occidental volcanic fields, and the late Cenozoic Altiplano-Puna volcanic complex in the Central Andes. Rhyolitic and trachydacitic ignimbrites typically have erupted, but where the crust was predominantly felsic, prewarmed, and orogenically thickened, well-developed MASH zones have spawned multiple super-eruptions of phenocryst-rich dacite, or monotonous intermediates, and smaller volumes of calc-alkaline rhyolite ignimbrite. In the Great Basin, eruptions of dry, hot trachydacite magma followed the monotonous intermediates. Partial melting in thinner crust with a major mafic component yielded more alkalic rhyolite and related trachydacite.

The Bingham porphyry Cu-Au-Mo deposit, Utah, may only be world-class because of substantial contributions of sulfur and metals from mafic alkaline magma to an otherwise unremarkable calc-alkaline system. Volcanic mafic alkaline rocks in the district are enriched in Cr, Ni, and Ba as well as Cu, Au, platinum group elements (PGE), and S. The bulk of the volcanic section that is co-magmatic with ore-related porphyries is dacitic to trachytic in composition, but has inherited the geochemical signature of high Cr, Ni, and Ba from magma mixing with the mafic alkaline rocks. The volcanic section that most closely correlates in time with ore-related porphyries is very heterogeneous containing clasts of scoriaceous latite, latitic, and minette, and flows of melanephelinite, shoshonite, and olivine latite in addition to volumetrically dominant dacite/trachyte. Bingham ore-related porphyries show ample evidence of prior mixing with mafic alkaline magmas. Intrusive porphyries that have not been previously well-studied have several chemical and mineralogical indications of magma mixing. These ''mixed'' lithologies include the hybrid quartz monzonite porphyry, biotite porphyry, and minette dikes. Even some of the more silicic latite and monzonite porphyries retain high Cr and Ba contents indicative of mixing and contain trace amounts of sapphire (<1 mm). The heterogeneous block and ash flow deposits also contain sapphire and are permissively correlated with the intrusions based on chemical, mineralogical, and isotopic data. Magma mixing calculations suggest about 10% of the monzonitic/latitic orerelated magma may have been derived from mafic alkaline magma similar to the melanephelinite. If the original S content of the mafic magma was about 2,000-4,000 ppm, comparable with similar magmas, then the mafic magma may have been responsible for contributing more than half of the S and a significant portion of the Cu, Au, and PGE in the Bingham deposit.

Dedicated to J. Hoover Mackin, who initiated study of the Indian Peak-Caliente ignimbrite fi eld with his recognition in the 1950s that the "lava fl ows" near Cedar City are actually widespread ignimbrites, including the unusual trachydacitic Isom-type tuffs and the colossal Needles Range monotonous intermediates.

The Indian Peak volcanic field is representative of the more than 50,000 km3 of ashflow tuff and tens of calderas in the Great Basin that formed during the Oligocene-early