The 36–18 Ma Central Nevada ignimbrite field and calderas, Great Basin, USA: Multicyclic super-eruptions

Abstract

One of the greatest global manifestations of explosive silicic volcanism in the terrestrial rock record occurred during the middle Cenozoic over a large part of southwestern North America, from the Great Basin of Nevada and western Utah into Colorado, Arizona, New Mexico, and Mexico. This subduction-related ignimbrite fl areup is the only one known in the world of its magnitude and of Mesozoic or Cenozoic age that is not related to continental breakup. The southern Great Basin ignimbrite province was a major product of the fl areup . Its central and eastern sectors developed on the Great Basin altiplano, a high orogenic plateau of limited relief dating from pulses of late Paleozoic through Mesozoic orogenic contractile deformation. Caldera-forming activity migrated southwestward through time in response to rollback of a once-fl at slab of subducting lithosphere.In the central sector of the southern Great Basin ignimbrite province, 11 partly exposed, mostly overlapping source calderas and one concealed source comprise the 36-18 Ma Central Nevada caldera complex. Calderas have diameters as much as 50 km, to possibly 80 km. Intracaldera tuff and intercalated wall-collapse breccia are at least 2000 m thick.Surrounding outfl ow ignimbrites consist of 17 regional cooling units (>200 km 3 ) that have been correlated over two or more mountain ranges on the basis of stratigraphic position, paleomagnetic direction, chemical and modal composition, and 40 Ar/ 39 Ar age. Many additional smaller cooling units have been recognized. Possibly as many as eight of the ignimbrites resulted from super-eruptions of 1000 km 3 to as much as 4800 km 3 . This Central Nevada ignimbrite fi eld is presently exposed over an area of ~65,000 km 2 in south-central Nevada and had a volume of 25,000 km 3 corrected for post-volcanic crustal extension. Six of the largest eruptions broadcast ash fl ows over an extension-corrected area of greater than 16,000 km 2 and as much as 160 km from their caldera sources. Individual sections of outfl ow tuff include as many as 14 ignimbrite cooling units; aggregate thicknesses locally reach a kilometer, and stacks a few hundred meters thick are common. Sequences are almost everywhere conformable and lack substantial intervening erosional debris and angular discordances that would testify to synvolcanic crustal extension. Beds of fallout ash a few meters thick associated with the largest eruption have been recognized in the mid-continent of the U.S.Six caldera-forming eruptive episodes are separated by fi ve lulls in activity, each lasting from 1.7 to 4.4 m.y., during which time little (<200 km 3 ) or no ignimbrite was deposited. Some of the longer lulls that preceded the most voluminous eruptions likely refl ected the time for accumulation of magma in huge shallow chambers before eruption was triggered. Other long lulls preceded the last two, single eruptions as the arc magma-generating system was waning prior to the transition to non-arc magma production to the south in the Southwestern Nevada volcanic fi eld....

An ignimbrite caldera from the bottom up: Exhumed floor and fill of the resurgent Bonanza caldera, Southern Rocky Mountain volcanic field, Colorado

“…As a result, contacts between caldera floor versus caldera-fill megabreccia can be broadly gradational and locatable only approximately in many places. Such deep levels of caldera structure have rarely been observed on a comparable areal scale elsewhere, where floor rocks are seen mainly in oblique cross sections through structurally disrupted and tilted caldera remnants, as in the Great Basin of the western United States (e.g., Best et al, 2013aBest et al, , 2013bHenry and John, 2013). Caldera-floor levels of greater areal extent are well exposed at several well-documented large ignimbrite calderas of Paleozoic age; e.g., Ordovician Scafell and Glen Coe calderas in Great Britain (Branney and Kokelaar, 1994;Moore and Kokelaar, 1998) and Permian Sesia and Ora cal deras in northern Italy (Quick et al, 2009;Sbisà, 2010;Willcock et al, 2013), but these differ from Bonanza in important aspects, including morphologic preservation, eruptive history, and tectonic setting.…”

“…For the SRMVF ignimbrites that lack associated proximal-fall deposits, distal Plinian tephra may exist among the many ash beds in the Oligocene White River Formation on the High Plains, Colo rado and Wyoming (e.g., Prothero, 1996;Larson and Evanoff, 1998), although no systematic studies have thus far attempted detailed correlations. Similarly, as much as several hundred cubic kilometers of fallout ash in the midcontinent has been interpreted as correlative with individual large dacitic ignimbrites of mid-Tertiary age erupted in the southern Great Basin (Best et al, 2013a(Best et al, , 2013b.…”

“…The asymmetrical distribution of early-versus later-emplaced compositional phases within the outflow of a single ignimbrite sheet appears somewhat analogous to eruptive processes well documented for eruption of the Bishop Tuff from a zoned magma body (Hildreth and Wilson, 2007). Another example of contrasts in composition and distribution between early-and late-erupted ignimbrite is the Lunar Cuesta Tuff in central Nevada (Best et al, 2013b, fig. 59 and text).…”

“…Similarly, no thick or widespread Plinian fall deposits have been recognized beneath other large ignimbrite sheets in the SRMVF, including those with rhyolitic or zoned compositions. Plinian deposits also appear to be rare or absent in association with compositionally diverse ignimbrite eruptions in other calcalka line Cordilleran systems, such as the southern Great Basin ignimbrite field (Best et al, 2013a(Best et al, , 2013b, the central Andes (Sparks et al, 1985;Lindsay et al, 2001;de Silva et al, 2006), or the 74-ka Toba eruption in Indonesia (Chesner, 2012). This contrast with thick Plinian deposits at well-known younger calderas such as Long Valley, Yellowstone, and Taupo raises questions about recent proposals that precursor tephra deposits typically form at inception of rhyolitic ignimbrite eruptions but not during large dacitic eruptions, in response to contrasting mechanisms and timing of caldera subsidence (summarized by Cashman and Giordano, 2014, and references therein).…”

Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source

“…Several large volcanic fields in western North America (figure 1) might have been eruptive sources for the Duchesne River Formation tuff beds. These include the northeast Nevada others, 1995a, 1995b;1995c;Rahl and others, 2002), Tuscarora (Henry and others, 1998;Smith and others, 2017), Absaroka (Chandler, 2006), Challis (Chandler, 2006), central Nevada (Best and others, 2009;2013b), Indian Peak (Best and others, 2013c), and Robinson Mountain (Lund-Snee and others, 2016; Smith and others, 2017). These volcanic fields either pre-date or post-date the Duchesne River Formation tuffs except for the northeast Nevada and Tuscarora fields.…”