Introduction: The 36-18 Ma southern Great Basin, USA, ignimbrite province and flareup: Swarms of subduction-related supervolcanoes

Abstract

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...

Geology and evolution of the McDermitt caldera, northern Nevada and southeastern Oregon, western USA

“…Comparisons are made with strongly peralkaline calderas, for which erupted tuffs have peralkalinity indices >1.1 to 2.0 (Mahood, 1984), and with calderas of the ignimbrite flareup of western North America. The ignimbrite flareup formed a semicontinuous belt of ignimbrites and source calderas from the Great Basin of the United States to the southern end of the Sierra Madre Occidental in Mexico (Swanson and McDowell, 1984;Ferrari et al, 2007;McDowell and McIntosh, 2012;Henry et al, 2012b;Best et al, 2013aBest et al, , 2016. Overall understanding of the geometry, evolution, and geotectonic setting of calderas is based heavily on study of the ignimbrite flareup, particularly in the United States (Table 7; Lipman, 1984Lipman, , 2007Best et al, 2013aBest et al, , 2013bBest et al, , 2016Henry and John, 2013;Lipman and Bachmann, 2015).…”

Quickening the Pulse: Fractal Tempos in Continental Arc Magmatism

“…Among the best documented are those of the Altiplano-Puna Volcanic Complex (APVC) of the Central Andes (de Silva et al 2006), and the various middle Cenozoic volcanic fields of the North American cordillera (e.g. McIntosh et al 1992;Bryan et al 2007;Lipman 2007;Best et al 2013). The primary feature of the time-volume pattern of these ignimbrite flare-ups is that the eruptive history is episodic and in three distinct stages: first, a waxing period of relatively low eruptive flux; second, a climactic stage (the peak of the flare-up) when the largest eruptions happen and the eruptive flux is at its highest; third, a waning stage that returns the arc to a steady state (de Silva and Gosnold 2007;Lipman 2007).…”

“…The primary pulse, which is the high-volume event or flare-up, reflects the largest space/time scale of an excursion in mantle-power input from steady-state magmatism, possibly coupled to deformation patterns in the upper plate (DeCelles et al 2009). This primary 40-60 My mantle signal may occur over a large area: examples include the entire Mesozoic Californian arc, in which flare-ups were driven by delamination events (Ducea 2001); or the middle Cenozoic of the western United States, where the flare-up appears to have been triggered by a regional transition from low-angle plate convergence to an increasingly extensional regime and where peak volcanism largely preceded the bulk of the extension (Lipman 1972;Best et al 2013). …”

“…By examining the space-timevolume record of volcanic activity, the episodic nature and variability of intensity of eruptions can be identified. Particularly in young arcs, or in environments where erosion is minimal and the volcanic record is well preserved, relatively robust extrusive fluxes (if the area through which a magma flows is known) can be delineated through mapping, stratigraphic correlation, and extensive geochronology (McIntosh et al 1992;de Silva and Gosnold 2007;Lipman 2007;Best et al 2013). Magmatic addition rates, or fluxes, are determined by assuming the commonly quoted volcanic-to-plutonic ratios of 3:1, 5:1, and 10:1.…”

Eruptive history, geochronology, and post-eruption structural evolution of the late Eocene Hall Creek Caldera, Toiyabe Range, Nevada

“…2 and 24). These erosional patterns, together with structural relations in Paleozoic rocks, argue against a Tonopah-Austin paleodivide being the pronounced edge of a major orogenic plateau controlled by crustal-scale reverse faults and enhanced by erosion and isostatic uplift (Best et al, 2013). Such a structure would be much more extensive along strike, and any northtrending, west-dipping thrust faults would be associated with north-south trending folds and faults, not localized east-west trending domes like the one in figure 2.…”