The Great Basin Altiplano During the Middle Cenozoic Ignimbrite Flareup

Home , Altiplano, Andesites, Arc Dome, Cerro Panizos, Geology of Nevada, Great Basin

International Geology Review Vol. 51, Nos. 7–8, July–August 2009, 589–633

The Great Basin Altiplano during the middle Cenozoic ignimbrite ﬂareup: insights from volcanic rocks Myron G. Besta, Deborah L. Barra†, Eric H. Christiansena*, Sherman Grommeb, Alan L. Deinoc and David G. Tingeya

aDepartment of Geological Sciences, Brigham Young University, Provo, UT 84602-4606, USA b420 Chaucer Street, Palo Alto, CA 94301, USA; cBerkeley Geochronology Center, Berkeley, CA 94709, USA (Accepted 26 February 2009)

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

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 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

*Corresponding author. Email: [email protected]

ISSN 0020-6814 print/ISSN 1938-2839 online q 2009 Taylor & Francis DOI: 10.1080/00206810902867690 http://www.informaworld.com 590 M.G. Best et al.

Altiplano in western Nevada and adjacent California; at least one of these ash ﬂows travelled as far west as the western foothills of the Sierra Nevada. The nature and origin of the implied topographic barrier are uncertain. It is possible that heavy orographic precipitation on the western slope of the Altiplano and consequent focused denudation and isostatic uplift created a NS-trending topographic high at the crest of the western slope and facing the smoothed Altiplano to the east. The barrier also lies near and essentially parallel to the buried western edge of the Precambrian basement and to a zone of thermal-diapiric domes that were spawned in thickened crust as the basement edge was overrun by late Palaeozoic–Mesozoic thrust sheets. Keywords: arc volcanic rocks; crustal thickness; great basin; ignimbrite ﬂareup; Altiplano; orogenic plateau

Introduction The middle Cenozoic ignimbrite flareup (Coney 1978) in the area that became the Great Basin of the western USA ranks as one of the most voluminous productions of silicic magma in the geologic record. At least a dozen very large volume (.1000 km3), or ‘supervolcanic’ (de Silva 2008; Miller and Wark 2008), eruptions as well as a greater number of large volume (100s of km3) eruptions occurred from the latest Eocene to early Miocene from 36 to 18 Ma (Best et al. 1989a, 1989b, 1993, 1995; Best and Christiansen 1991; Maughan et al. 2002; John et al. 2008). This flareup occurred in a remarkably brief period of time relative to the span of some 200 million years through the Mesozoic and into the middle Cenozoic when subduction of oceanic crust beneath the western North American plate resulted in arc magmatism. In addition to the widespread rhyolite, four of these supervolcanic eruptions were of crystal-rich dacite magma, the monotonous intermediates of Hildreth (1981) and Maughan et al. (2002). Eruptions of these colossal volumes of rhyolitic and hotter dacitic magma, accompanied by extrusion of only very minor more mafic magma – and no true basalt until the waning stages of the ignimbrite flareup – imply unusual magma generation in the volcanic arc, requiring a prodigious amount of thermal energy. To augment the usual heat input from mantle-derived mafic magmas in the arc system, and to provide the necessary volume of silicic source rock, necessitates, in our view, an unusually thick crust that was already at near-solidus temperatures in its deeper part. Is there evidence for such an unusually thick crust in the Great Basin area during the

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 ignimbrite flareup? Tectonic reconstructions and comparisons with active mountain belts together with palaeobotanical and isotopic data suggest that, following Mesozoic–earliest Cenozoic contractile deformations, the Great Basin area was a high orogenic plateau capping unusually thick crust, not unlike the present-day Altiplano-Puna in the central Andes Mountains. But this crustal welt may have collapsed and thinned before the middle Cenozoic when the ignimbrite flareup occurred. The purpose of this paper is to present independent evidence from the middle Cenozoic volcanic rocks in the Great Basin that indicate the continued existence of an unusually thick crust during the flareup and that shows it was similar to the Andean Altiplano. Our plan is to, first, review current thinkings on the nature of the middle Cenozoic crust in the Great Basin and the modern crust in the central Andes, and then present the compositional data that indicates an unusually thick crust in the Great Basin. We then describe pertinent information on the ignimbrite deposits that constrains the topographic character of the Great Basin Altiplano. International Geology Review 591

Previous thoughts on crustal thickness of the Great Basin during the middle Cenozoic In a widely cited paper, Coney and Harms (1984) reconstructed the crustal thickness in the Great Basin area after late Palaeozoic and Mesozoic contractile orogenic deformations, showing the crust to be as much as 50–60 km along a thickened welt, mostly in eastern Nevada (Figure 1). Although admitting ‘circularity’ (p. 552) in their reconstruction, they boldly asserted that the eastern Great Basin ‘ ...was a vast Tibetan or Andean Altiplanolike plateau prior to middle Cenozoic crustal extension’ (p. 553). Comparing the active tectonics of the Himalaya–Tibet region and the Andes to the late Mesozoic–early Cenozoic development of the western USA, Molnar and Lyon-Caen (1988, p. 202) ‘presumed [the existence of a] high plateau in western and central Nevada’ and suspected that ‘the crust reached a thickness of 50–70 km’. Dilek and Moores (1999) considered the western US Cordillera as a mature Tibetan Plateau, which has an average elevation of 5 km underlain by crust of 60–85 km thickness. However, analogies between the western US Cordillera and the Tibetan Plateau are ﬂawed in that the latter lies inboard of a continent– continent collision, whereas the US Cordillera resulted from subduction of oceanic lithosphere studded with island arcs and oceanic plateaus. McQuarrie and Chase (2000) referred to the elevated thick crust in the hinterland of the Sevier fold-thrust belt in western Utah and eastern Nevada (Figure 1) as the ‘Sevier Plateau’. In a review of the Cordilleran thrust belt, (DeCelles 2004, p. 147; see also DeCelles and Coogan 2006) referred to this hinterland as the ‘Nevadaplano’ whose crustal thickness was 50–60 km and palaeo-elevation was more than 3 km.

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37û 60 Arizona New Mexico Las Vegas California Thrust faults and folds Palaeogene crustal 50 thickness (km) Coney and Harms (1984) 250 50 100 150 200 km

Figure 1. Major thrust faults and fold belts in the Great Basin of Nevada and Utah (Oldow et al. 1989; McQuarrie and Chase 2000; see also DeCelles 2004) and hypothetical contours (in km) of early Tertiary crustal thickness (Coney and Harms 1984). 592 M.G. Best et al.

Several geologists have concluded that the thickened crust underpinning the orogenic plateau in the Great Basin area became gravitationally unstable and experienced extensional collapse and consequent thinning. Sonder et al. (1987) modelled these phenomena and concluded that a time delay of as much as 100 million years can occur between the end of compression and initiation of extension, depending on factors such as the thermal regime of the lithosphere and particularly the Moho temperature. For a relatively hot Moho temperature (500–7008C), the time delay is near zero. While there is no doubt that substantial extension and crustal thinning has taken place since after the ignimbrite flareup – resulting in the Basin and Range physiography – we question whether significant extension occurred on a regional basis before this flareup. For example, in the northeastern Great Basin area, the Camilleri et al. (1997) model states that the crust thinning was from 70 to 50 km by extensional unroofing between the Cretaceous and Eocene–Oligocene, whereas Constenius (1996) concluded that extensional basins formed from the middle Eocene to early Miocene. However, on the basis of his study of interbedded ash-flow tuffs and sedimentary deposits in northeastern Nevada, Henry (2008) concluded that Eocene extension was minor. The well-documented extreme extension found locally in core complexes in the eastern Great Basin (e.g. MacCready et al. 1997) does not appear to be of regional character. Hudson et al. (2000) document greater than 100% extension at 24 Ma in the southern Stillwater Range in west-central Nevada (this and other localities referred to below are shown in Figure 2), which they interpret to be a local focusing of regional extension. But 110% extension in the northern Toiyabe and Shoshone Ranges to the northeast occurred 16–10 Ma (Colgan et al. 2008). Humphreys (1995) points out that the ignimbrite flareup followed soon after the demise of the Laramide deformation in the western USA at about 45 Ma with the beginning of north to south rollback of the subducting oceanic lithosphere from beneath the Great Basin area. Removal of the once ‘flat’ subducting slab brought hotter asthenosphere into contact with the overlying continental lithosphere prompting voluminous magma generation in the crust. We concur with this scenario but disagree with Humphrey’s contention that the crust thinned by extension during the ignimbrite flareup. In our survey of stratigraphic sections of outflow ignimbrite sheets in the central and eastern Great Basin, we found only limited local evidence for crustal extension (and consequent thinning) during the flareup (Best and Christiansen 1991). As the flareup waned, after the maximum production rate of silicic magma eruption at approximately 31–26 Ma, the Great Basin experienced widespread and profound EW crustal extension. In some places this began as early as 24 Ma, such as in the Stillwater Range (Hudson et al. 2000) and in the Caliente area of southeastern Nevada (Rowley et al. 1995), but was

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 widespread and profound after about 18 Ma (McQuarrie and Wernicke 2005). Another approach to the determination of the thickness of the middle Cenozoic crust is to compensate for the amount of later extensional thinning, assuming plane strain in the EW direction and using the observed province-wide crustal thickness today of a relatively uniform 30 ^ 5 km (Allmendinger et al. 1987; Mooney and Braile 1989; Gilbert and Sheehan 2004). However, estimates of the amount of whole-province extensional thinning range widely, for example, from as much as 100% (Hamilton 1989) to as little as 20–30% (Stewart 1980). According to the detailed tectonic reconstruction of McQuarrie and Wernicke (2005), the total extension, mostly after the ignimbrite ﬂareup, oriented about N 788W across the Great Basin (between longitude about 1128 and 1208 W and latitude 408 240 and 388 400 N), is 236 km or 50%. This value implies an initial crustal thickness before extension of 45 ^ 7km. However, this plane-strain calculation assumes the crust to be a closed system, which is unlikely for two reasons. First, there have been additions to the crust in the form of mantle-derived magma (Okaya and Thompson 1986; Mayer 1986; Gans 1987; Lachenbruch and Morgan 1990) that provided heat and added mass to the middle Cenozoic silicic International Geology Review 593

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Figure 2. Locations of 376 analysed samples of middle Tertiary lava ﬂows in the Great Basin, which lies between the Sierra Nevada and the Colorado plateaus, are shown by red circles. Because of the small scale of this ﬁgure some sample sites overlap as a single point. Also shown are the outlines of the CCC, Caliente caldera complex; CNCC, central Nevada caldera complex; IPCC, Indian Peak caldera complex; and TQCC, Toquima caldera complex as well as other localities cited in the text, as follows: A, Austin; BM, Battle Mountain; CA, Clan Alpine Mountains; CS, Carson Sink; DD, Diamond Mountains; DM, Dogskin Mountain; DS, Donner Summit; DV, Death Valley; FC, Fish Creek Mountains; HP, Haskell Peak; LV, Las Vegas; NP, New Pass Range; PA, Pahroc Range; PR, Paradise Range; P, Provo, Utah; RR, Reese River Valley; SC, Schell Creek Range; S, Seaman Range; SH, Shoshone Range; ST, Stillwater Range; T, Tonopah; TQ, Toquima Range; TY, Toiyabe Range; W, Wah Wah Mountains; Y, Yerington; and YR, Yuba River drainage.

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 magma systems. The added mafic–ultramafic magmatic rock at the base of the open-system crust could correspond to the anomalously low velocity (7.4 km/s) layer, several kilometres thick, interpreted from a seismic profile in north-central Nevada (Catchings 1992) and the 7.5 km/s lens in western Utah and easternmost Nevada (Smith et al. 1989). Second, Bird (1991; see also Gans 1987) claims that the puzzling flat Moho, despite spatially heterogeneous extension, is the consequence of horizontal ductile flow of the hot lower crust between the more rigid upper crust and mantle in the thickened (55? km), high (2? km) crust that resulted from the Sevier orogeny. McQuarrie and Chase (2000) claimed that eastward flow from the Sevier hinterland thickened and elevated the Colorado Plateau crust. Wernicke et al. (1988) claimed flow towards the south into the highly extended Las Vegas–Death Valley corridor.

Palaeoaltitude of the Great Basin area Palaeoaltitudes provide additional and independent insight into the crustal thickness of the middle Cenozoic Great Basin. But because of an imperfect worldwide correlation between 594 M.G. Best et al.

crustal thickness and topographic elevation, it is not feasible to infer quantitative values of the former from the latter. Despite inherent uncertainties, palaeoaltitudes derived from palaeobotanical studies provide qualitative insight into crustal thickness. Such studies indicate that high altitudes prevailed in the western USA during the Cenozoic (see review by Chase et al. 1998). But, regrettably, there are only limited data for the Great Basin. Wolfe et al. (1997) list a dozen sites in west-central Nevada for which mid-Miocene floras (mostly 15–16 Ma) indicate palaeoelevations of about 3 km above sea level, 1–1.5 km above the present elevation, thus implying thicker crust. Gregory-Wodzicki (1997) determined palaeoaltitude from a late Eocene flora in western Utah of 2.9–3.6 km, compared to the present elevation of 1.7 km. Palaeoaltitudes can also be determined isotopically. Using (U–Th)/He ages of apatites in the Sierra Nevada, House et al. (2001) argue that the range may have occupied a position at the western edge of an orogenic plateau of at least 3 km elevation that spanned the Cordilleran interior during Late Cretaceous time. Deuterium values in Eocene sediment deposits in the Sierra Nevada led Mulch et al. (2006) to basically the same conclusion as those of House et al. (2001). Horton et al. (2004; see also Poage and Chamberlain 2002) tracked changing palaeoelevations in the Great Basin using shifts in oxygen and hydrogen isotope ratios in authigenic minerals precipitated from meteoric waters in the Cenozoic basin deposits. They argue that the rain-shadow effect of a high Sierra Nevada that caused precipitation of isotopically lighter isotopes of oxygen and hydrogen farther inland in the Great Basin area prevailed throughout much of the Cenozoic. Isotopic shifts corresponding to about 2 km of surface uplift of the Great Basin between the middle Eocene and early Oligocene were followed by subsidence from the middle Miocene to Pliocene. A similar pattern of elevation change was claimed by Wolfe et al. (1997) using palaeobotanical data. Crowley et al. (2008) analysed oxygen isotopes in mammalian tooth enamel and concluded that a Sierran rain-shadow had existed since at least 16 Ma. Kent-Corson et al. (2006) conclude there is a growing body of evidence for a migration of high surface elevation from the northern to the southern Great Basin from the late Eocene to the Miocene. They relate this phenomenon to the southward rollback of the subducting slab and upwelling of hotter and expanding asthenosphere beneath the continental lithosphere (Davis et al. 2009). These conclusions are at variance with the long-standing belief that significant uplift of the Sierra Nevada to its present elevation had not occurred until the latest Cenozoic

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 (e.g. since ca. 5 Ma). For example, Wakabayashi and Sawyer (2001) constructed this scenario from an extensive analysis of current topography coupled with thermochrono- logic and geobarometric data. Clark et al. (2005) concluded that the southern Sierra experienced two stages of uplift since 32 Ma that brought the elevation to 4 km that is observed today from the roughly 1.5 km prevailing through the early Cenozoic. A possible indication of the altitude of the Great Basin ignimbrite province before its subsequent extensional collapse and subsidence lies in the altitude of the present Sierra Madre Occidental ignimbrite plateau in Mexico (Swanson et al. 2006). This ignimbrite plateau is readily interpreted to be a southward continuation of the Great Basin ignimbrite province along the western North American continental margin with which it shares many similarities (Best et al. 1989b, Table 1) but has essentially escaped late Cenozoic extensional faulting after the ignimbrite ﬂareup. The Sierra plateau lies mostly between 2.2 and 2.4 km elevation, roughly 0.5 km higher than the present average Great Basin. Peaks in the plateau are as much as 1.8 km above and canyons 1 km below the 2.2–2.4 km elevation. Downloaded By: [Brigham Young University] At: 20:48 15 September 2009

Table 1. Chemical and isotopic data for continental volcanic arcs of known crustal thickness and for middle Cenozoic lavas in the Great Basin.

Min 87 86 87 86 Chemical Sr isotope K2O K2O/Na2O Sr/ Sr Sr/ Sr Thickness analyses analyses (57% (57–63% (,63% (mean crust Region Symbol (n) (n) SiO2) SiO2) SiO2) all) (km) Reference Andes central A 200 110 2.20 0.80 0.7050 0.7070 60 Leeman (1983) Andes north A 54 39 1.50 0.63 0.7038 0.7043 55 Leeman (1983) Andes south A 66 21 1.40 0.7034 0.7042 35 Leeman (1983) Andes San Pedro A 65 1.85 70 O’Callaghan and Francis (1986) Andes Tata Sabaya A 30 4 2.80 0.74 0.7052 0.7060 70 de Silva et al. (1993) Review Geology International Andes Bolivia minor cntrs A 17 16 0.7041 0.7069 70 Davidson and de Silva (1992) Andes Ollague A 28 25 2.20 0.73 0.7063 0.7081 70 Feeley and Davidson (1994) Andes Payachata A 18 18 2.75 0.74 0.7061 0.7066 65 Davidson et al. (1990) Andes Tapungato A 21 11 1.80 0.51 0.7046 0.7047 62 Hildreth and Moorbath (1988) Andes Tapungatito A 15 5 2.27 0.80 0.7049 0.7049 61 Hildreth and Moorbath (1988) Andes Ojos del Salado A 25 4 2.30 0.67 0.7065 0.7067 60 Baker et al. (1987) Andes Cerro Alto A 22 8 1.94 0.55 0.7047 0.7048 60 Hildreth and Moorbath (1988) Andes Marm-San Jose A 26 16 1.93 0.58 0.7048 0.7052 59 Hildreth and Moorbath (1988) Andes Palomo A 9 5 1.71 0.54 0.7042 0.7043 54 Hildreth and Moorbath (1988) Andes Tinguirrica A 17 12 2.36 0.73 0.7039 0.7041 53 Hildreth and Moorbath (1988) Andes Sordo Lucas A 3 3 1.50 0.54 0.7042 0.7042 51 Hildreth and Moorbath (1988) Andes Planch-Pet-Azuf A 83 24 1.80 0.56 0.7039 0.7042 48 Hildreth and Moorbath

(1988) and Tormey et al. 595 (1995) Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 596 Table 1 – continued

Min 87 86 87 86 Chemical Sr isotope K2O K2O/Na2O Sr/ Sr Sr/ Sr Thickness analyses analyses (57% (57–63% (,63% (mean crust Region Symbol (n) (n) SiO2) SiO2) SiO2) all) (km) Reference Andes Descabez-Grand A 6 6 1.62 0.46 0.7038 0.7039 45 Hildreth and Moorbath (1988) Andes Cerro Azul A 43 25 1.70 0.46 0.0736 0.7039 45 Hildreth and Moorbath (1988) Andes Tatara-San Pedro A 22 1.60 0.54 41 Ferguson et al. (1992) Andes Laguna del Maule A 14 9 2.00 0.62 0.7039 0.7041 41 Frey et al. (1984) Andes San Pedro-Pellado A 23 9 1.70 0.61 0.7037 0.7039 41 Davidson et al. (1988)

Andes Cordon El Guadal A 34 8 1.90 0.56 0.7039 0.7040 41 Feeley et al. (1998) Best M.G. Andes Nev Longavi A 11 2 1.02 0.7039 40 Hildreth and Moorbath (1988) Andes Antuco A 22 6 1.34 0.7037 40 Hildreth and Moorbath

(1988) al. et Andes Chillan A 24 1.29 37 Hildreth and Moorbath (1988) Andes Puyehue A 64 32 1.40 0.37 0.7038 0.7040 32 Gerlach et al. (1988) Andes Mocho-Choshuenco A 20 20 1.00 0.7040 0.7041 30 McMillan et al. (1989) Andes Calbuco A 22 9 0.70 0.18 0.7037 0.7040 30 Lopez-Escobar et al. (1995) Aleutians Katmai K 36 16 1.10 0.35 0.7030 0.7033 33 Hildreth et al. (2004) East Aleutians K 255 24 1.20 0.44 0.7028 0.7032 38 Leeman (1983) Cascades C 190 1.30 0.43 0.7026 40 Leeman (1983) Cascades Mt Hood C 190 1.20 41 Leeman (1983) Cascades Crater Lake C 57 1.10 0.28 41 Bacon and Druitt (1988) Cascades Medicine Lake C 35 1.40 0.32 39 Mertzman (1977) Cascades Mt St Helens C 32 9 1.30 0.27 0.7030 0.7036 41 Halliday et al. (1983) Cascades Mt Shasta C 112 32 1.00 0.31 0.7028 0.7034 38 Grove et al. (2005) San Juan precaldera S 56 14 2.50 0.82 0.7046 0.7051 48 Colucci et al. (1991) Conejos Downloaded By: [Brigham Young University] At: 20:48 15 September 2009

Table 1 – continued

Min 87 86 87 86 Chemical Sr isotope K2O K2O/Na2O Sr/ Sr Sr/ Sr Thickness analyses analyses (57% (57–63% (,63% (mean crust Region Symbol (n) (n) SiO2) SiO2) SiO2) all) (km) Reference San Juan postcaldera S 20 7 2.30 0.68 0.7043 0.7047 48 Parat et al. (2005) Huerto San Juan syncaldera S 18 0.7045 0.7063 48 Riciputi et al. (1995) Mexico P 261 49 1.50 0.7032 0.7039 40 Leeman (1983)

Mexico, Valley of P 73 1.50 0.40 43 Wallace and Carmichael Review Geology International (1999) Mexico NW Mex. volc. P 16 16 1.55 0.41 0.7037 0.7040 30 Verma and Nelson belt (1989) Mexico Sanganguey P 14 1.50 0.47 30 Nelson and Livieres (1986) Mexico Paricutin P 27 12 1.30 0.40 0.7037 0.7040 33 Wilcox (1954) and McBirney et al. (1987) Central America M 337 40 1.50 0.45 0.7031 0.7040 33 Leeman (1983) Guatemala A M 87 1.50 0.53 50 Carr et al. (2003) Guatemala B M 330 6 1.40 0.42 0.7038 0.7039 46 Carr et al. (2003) Guatemala-El Salvador M 116 22 1.60 0.46 0.7032 0.7036 38 Carr et al. (2003) Honduras, Yohoa M 13 11 0.7029 0.7030 37 Carr et al. (2003) El Salvador, Hond, M 246 69 1.50 0.48 0.7036 0.7040 34 Carr et al. (2003) Nicaragua Costa Rica Rincon M 19 1.60 0.52 36 Carr et al. (2003) Costa Rica other M 76 7 1.30 0.44 0.7038 0.7038 37 Carr et al. (2003) Costa Rica Platanar M 50 1.70 0.62 0.7036 0.7037 39 Carr et al. (2003) Costa Rica Poas M 21 4 1.80 0.57 0.7037 0.7037 41 Carr et al. (2003) Costa Rica Barba M 21 3 2.60 0.77 0.7036 0.7037 43 Carr et al. (2003) Costa Rica Irazu M 50 2.20 45 Carr et al. (2003) Lesser Antilles Dominica L 280 23 0.90 0.7041 0.7045 33 Leeman (1983) Lesser Antilles Grenada L 264 52 1.20 0.7039 0.7049 33 Leeman (1983) Lesser Antilles Saba L 42 1.10 0.35 33 Defant et al. (2001) 597 Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 598 Table 1 – continued

Min 87 86 87 86 Chemical Sr isotope K2O K2O/Na2O Sr/ Sr Sr/ Sr Thickness analyses analyses (57% (57–63% (,63% (mean crust Region Symbol (n) (n) SiO2) SiO2) SiO2) all) (km) Reference Lesser Antilles St Kitts L 173 14 0.60 0.7036 0.7037 33 Leeman (1983) Kamchatka H 330 12 1.30 0.37 0.7031 0.7033 35 Leeman (1983) Turkey-Iran T 139 2.30 0.84 0.7036 0.7050 50 Leeman (1983) New Zealand Z 234 16 1.50 0.42 0.7041 0.7052 35 Leeman (1983) Sumatra Java U 262 108 1.70 0.7039 0.7047 30 Leeman (1983) Japan Kyushu J 541 ? 1.50 0.7037 0.7050 28 Leeman (1983) Japan Honshu J 595 78 1.00 0.7026 0.7040 30 Leeman (1983)

Japan Hokkaido J 125 46 1.10 0.7026 0.7037 28 Leeman (1983) Best M.G. Nevada Egan Range 26 21 3.20 1.30 0.7084 0.7125 Feeley and Grunder (1991) and Grunder (1992)

Central Nevada caldera 7 2 2.30 1.26 0.7093 0.7101 Askren (1992) al. et complex Indian Peak caldera 10 7 2.90 1.19 0.7071 0.7091 Askren (1992) and Hart complex et al. (1998) All Great Basin 376 2.90 1.01 Barr (1993) International Geology Review 599

Geology of the central Andean plateau in brief Because the tectonic setting of the modern central Andean continental margin appears to be an appropriate model for the middle Cenozoic Great Basin area, it is useful to examine pertinent aspects of Andean geology (Allmendinger et al. 1997; de Silva et al. 2006). With an average elevation of 4 km and volcano peaks of more than 6 km, the relatively arid 350–400 by 1800 km central Andean plateau is the highest region in the world associated with voluminous arc volcanism. The internally draining Altiplano, at an average elevation of 3.8 km, lies in the northern two-thirds of the plateau. Attesting to its low relief as an enclosed basin during the Pleistocene, much of the Altiplano was covered by Lake Ballivian whose remnants include Lakes Titicaca and Poopo and several salars (playas). In the southern plateau, in northwest Argentina and a small part of adjoining Chile, Puna has an average elevation of 5 km. The 60–70 km thick crust of the Altiplano-Puna, which lies above the 308-east-dipping Nazca plate, is predominantly felsic (Beck and Zandt 2002), but the lower part may be mafic. A decrease in crustal thickness in the topographically higher Puna implies a relatively low-density uppermost mantle, perhaps resulting from recent lithospheric delaminations and invasion of hotter asthenosphere beneath the crust. Although delamination may have contributed to the plateau elevation, workers generally agree that most of the uplift resulted from tectonic shortening of the crust through the Palaeogene to essentially its present state by the mid-Miocene (Hartley et al. 2007). After a long period of thrust faulting, the southern and northern margins of the central Andean plateau are now experiencing NS crustal extension with development of stretching faults perpendicular to the mountain belt. Tibet is also currently experiencing similar extension. This extension appears to be a result of gravitational buoyancy forces exceeding compressive tectonic forces in the elevated plateaus (Molnar and Lyon-Caen 1988; Dilek and Moores 1999, p. 932). Rare, but widespread, EW-striking dikes and local grabens in the Great Basin indicate a similar state of stress during the middle Cenozoic (Best 1988; Best et al. 1998). The central Andean plateau experienced an ignimbrite flareup from the late Miocene through the Pliocene that apparently accompanied a steepening of the dip of the subducting Nazca plate. The increased volume of the asthenospheric wedge promoted melting in the crust, thereby generating copious volumes of silicic magmas that created the largest young ignimbrite province on Earth, covering more than 500,000 km2 (Allmendinger et al. 1997, p. 157). The focus in space and time of this flareup lies in the Altiplano-Puna volcanic Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 complex (de Silva 1989), near the common borders of Argentina, Bolivia, and Chile, where the ash-flows were erupted mostly from 9 to 3 Ma (Salisbury et al. 2008). During this flareup about a dozen known calderas developed (e.g. Ort et al. 1996; Lindsay et al. 2001; Soler et al. 2006), the largest of which measures 65 £ 35 km. The ignimbrite deposits cover an area of about 70,000 km2 with an aggregate volume of nearly 15,000 km3 (de Silva et al. 2006; de Silva and Gosnold 2007). Seven of the individual deposits exceed 1000 km3,thus qualifying as supervolcanic, and typically are of crystal-rich, calc-alkaline high-K dacite. Another of this genre, but located 200 km south of the Altiplano-Puna volcanic complex, is the Cerro Galan ignimbrite (Francis et al. 1989); its outflow sheet is upwards of 200 m thick and extends as much as 100 km in all directions from its 50 £ 30 km resurgent caldera source. Essentially coextensive with the Altiplano-Puna volcanic complex is an underlying subhorizontal slab of magma 1–2 km thick at a depth of 17–19 km (Zandt et al. 2003; de Silva et al. 2006), which may be a remnant of the magma-generating activity during the ignimbrite flareup. 600 M.G. Best et al.

The tectonic setting and ignimbrite flareup of the central Andean plateau during the late Cenozoic bear significant similarities to the middle Cenozoic Great Basin area (Best and Christiansen 1991, pp. 13,522–13,525, Figure 14; Maughan et al. 2002, pp. 150–154, Figure 18a). Especially noteworthy is the occurrence in both of several thousands of cubic kilometres of crystal-rich, calc-alkaline high-K dacite ignimbrite–the monotonous intermediates of Hildreth (1981) and Maughan et al. (2002) – together with lesser, but still prodigious, volumes of rhyolite ash-flow tuff. Such large scale, supervolcano melting events require unusually large inputs of mantle-derived magma into the continental crust to power the silicic magma-generating systems. But a compounding, even necessary, factor in such large-scale melting is an unusually thick and pre-warmed crust (de Silva et al. 2006).

Chemical composition of middle Cenozoic Great Basin lava ﬂows: implications for crustal thickness The tectonic and geologic similarities between the Andean orogenic plateau underlain by 60–70 km-thick crust and the middle Cenozoic Great Basin area suggest that the latter was also a high plateau underlain by unusually thick crust. However, the inherent uncertainties in palaeo-elevation determinations and the complexity of the tectonic and erosional history of the Great Basin and its inferred crustal thickness, as summarized above, provide little unambiguous support for this suggestion. Obviously, there is a need for an independent determination of the thickness of the Great Basin crust during the middle Cenozoic. Did thick crust prevail through the ignimbrite ﬂareup, or not? We seek an answer to this question in the composition of the volcanic rocks.

Rationale and previous work It is well known that the composition of calc-alkaline magmas extruded in subduction- related or arc settings reﬂects the type and thickness of the crust through which they ascend (e.g. Gill 1981; Leeman 1983). Generally, the greater the path length through relatively less dense and thicker felsic continental crust the greater is the opportunity for ascending mantle-derived magma to be modiﬁed by differentiation processes, yielding more evolved felsic magma compositions to be extruded. Longer paths allow more opportunity for

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 fractional crystallization and open-system assimilation of felsic continental rock, as well as mixing with incompatible-element-enriched partial melts, especially in lower levels of thick and therefore hotter crusts. Consequently, extruded magmas that experienced a longer ascent path through continental crust will have greater concentrations of elements such as K, Rb, and Ba, and a greater initial 87Sr/86Sr. A smaller proportion of more primitive low-silica, basaltic magma is expected to be extruded. But because of differences in age of the continental crust and its composition around the world and because of variable rates of ascent and amounts of open-system differentiation of the primary mantle-derived magmas there can be only a crude correspondence between the composition of extruded magmas and the thickness of the crust at continental margins. An additional factor governing the composition of arc volcanic rocks stems from subcrustal factors, such as where the primitive basaltic magmas are generated and contamination with sediment dragged down on the subducting oceanic plate. Nonetheless, variations in the mantle component in ascending magmas can be swamped by the thickness-related crustal contribution (Hildreth and Moorbath 1988). International Geology Review 601

Leeman (1983) inventoried whole-rock chemical and isotopic parameters of volcanic rocks in mostly active arcs around the world, where the thickness of the crust has been documented. Hildreth and Moorbath (1988) examined analyses of 15 andesite–dacite composite volcano suites in the Andes that are sited on crust ranging in thickness from about 30 to 60 km. We have expanded and updated these two compilations to include locales, where the thickness of the crust is constrained to within approximately 3 km. Our resulting ‘global’ data base in Table 1 is based on more than 6000 individual analyses comprising 72 suites, including 26 individual volcanic suites in the Andes, suites in 10 areas of western North America, 11 suites in segments of the central America arc, 4 suites from the Caribbean, and 6 suites from the western Pacific. The only few analyses included from the Mediterranean arc are from Turkey–Iran, because of the complexity of plates and plate interactions and the common occurrence of highly potassic volcanic rocks, such as in the Eolian Islands. Minor proportions of alkaline rocks occur together with the more prominent and typical calc-alkaline compositions in some arc suites listed in our global data base. These alkaline rocks, in the active Mexican volcanic belt and the Oligocene San Juan volcanic field of Colorado, have not been included in the global data base. In the northwestern Mexican volcanic belt the alkaline rocks appear to be associated with local rifting and the more mafic compositions have an oceanic-island-like source (Verma and Nelson 1989); alkaline basalts in the Valley of Mexico have a complex ancestry (Wallace and Carmichael 1999). In the San Juan field, where we assume the present crustal thickness of about 48 km also prevailed during volcanism, extrusion of minor amounts of Rock Creek alkaline lavas of the Conejos Formation preceded caldera-forming ash-flow eruptions while trachyandesitic lavas of the Huerto Andesite followed. These water-poor, mostly high-Zr (to as much as 580 ppm) alkaline rocks are believed to have resulted from crystal fractionation of mantle-derived alkalic basalt parent magmas, modified by assimilation of prior sub-volcanic intrusions (Parat et al. 2005), in contrast to the more open-system, wetter and more oxidizing conditions that dominate the evolution of normal arc calc-alkaline magmas. Our global data base in Table 1 lists essentially the same compositional parameters as used by Leeman (1983) in his inventory, viz.: (1) The average K2O/Na2O ratio for all analyses of andesites (57–63 wt. % SiO2) within a suite. Leeman (1983) also used the average K2O/Na2O in basaltic andesites, but these more mafic lavas are uncommon in the Great Basin and their ratios were not used in our investigation. (2) The mean value of K2O at 57.5 wt. % silica in a K2O–SiO2 variation diagram for the suite. (3) The minimum, Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 mean, and maximum initial 87Sr/86Sr ratio for the rocks in the suite. Additional data include the crustal thickness determined by geophysical methods as cited in published works. For the Andean locales, crustal thicknesses are from Hildreth and Moorbath (1988) and Allmendinger et al. (1997), and for the western US, from Mooney and Braile (1989), Prodehl and Lipman (1989), and Gilbert and Sheehan (2004). Nd and Pb isotopes are not listed in Table 1 as such data are not available in many publications, especially older ones. Neither are Mg and Na concentrations that have been correlated with crustal thickness in oceanic arc settings. Leeman (1983) used the percentage of andesite, dacite, and rhyolite in suites, but because of a limited number of samples in some individual suites or because of sampling biases, we found considerable scatter in this parameter; thus, it was not used.

Great Basin data set We collected, and analysed by X-ray ﬂuorescence spectrometry, 268 samples of essentially unaltered middle Cenozoic lava ﬂows in the Great Basin that were extruded 602 M.G. Best et al.

6 SHOSHONITIC

4 HIGH K O (wt%)

2 3 K

2 MEDIUM K

1 LOW K Basalt Basaltic Andesite Dacite andesite 0 45 50 55 60 65 70

SiO2 (wt%)

Figure 3. K2O–SiO2 diagram for middle Cenozoic lava ﬂows from the Great Basin. Field boundaries and rock-suite labels from Le Maitre (1989) except shoshonitic from Ewart (1982).

42–17 Ma. An additional 108 high-quality whole-rock analyses, by a variety of methods, of rocks of the same time period were selected from reports published by other workers, yielding a total of 376 analyses that form the Great Basin data set used here (Barr 1993; see supplementary data at www.informaworld.com/tigr). The analyses are of lavas bracketing the Great Basin ignimbrite flareup in time (36–18 Ma) and space (Figure 2). Many of the samples have been dated by various isotopic methods (e.g. McKee et al. 1993). For others, an approximate age of varying uncertainty is provided from bracketing dated ash-flow sheets and from latitudinal proximity to other isotopically dated lava flows. The latitudinal proximity basis for age approximation follows from the general southward sweep of inception of volcanism through the Great Basin during the Cenozoic (Best et al. 1989b, Figure 3; Best and Christiansen 1991, Figure 2). Though imperfect, the pattern of inception provides a useful approximation, in most cases, for the age of an individual lava sample, where no other data are available. Rhyolites and quartz-rich high-silica dacites were excluded from our Great Basin data set, because the intent of the sampling programme was to inventory the mantle contribution in arc volcanic rocks, however much that might be. Even though we made a

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 special effort to collect the most mafic lava samples in the Great Basin, only 14% (53) of our samples have less than 57 wt. % silica (Figures 3 and 4). Only five true basalts, according to the IUGS chemical classification (Le Maitre 1989), were found and these are 17–20 Ma. Hence, the greater than 17 Ma cut-off effectively excludes basalts from our data set. ‘Fundamentally basaltic’ rocks in the central and eastern Great Basin related to extensional tectonism (Christiansen and Lipman 1972) are mostly less than 13 Ma and have been dealt with in regional studies (e.g. Best et al. 1980; Fitton et al. 1991; Nelson and Tingey 1997). Eight of our samples with less than 52 wt. % silica are alkaline and all but one of these are 17–23 Ma, the other being 38 Ma (McKee et al. 1993). Alkaline samples seen in Figure 4 constitute only 6% (24) of our data set; they have mostly less than 57 wt. % silica and lack the high-Zr content of the alkaline rocks of the San Juan field described above. Of the remainder, 4.5% (16) are tholeiitic (ferroan) and the rest are calc- alkalic (magnesian), according to the criteria of Miyashiro (1974). Subduction-related lavas were extruded throughout the 42–17 Ma period in the Great Basin. However, a fundamental transition in the composition of volcanic rocks occurred International Geology Review 603

12 ALKALINE SUBALKALINE 10 Latite Trachydacite 8 Basanite Shoshonite O (wt %) 2 6 Trachy- basalt O + K 2 4 Na

2 Basalt Basaltic Andesite Dacite andesite 0 45 50 55 60 65 70

SiO2 (wt%)

Figure 4. Total alkalies–silica diagram for middle Cenozoic lava ﬂows from the Great Basin. Field boundaries and rock-type labels from Le Maitre (1989). Dotted line (from Le Bas et al. 1992) separates 23 alkaline (silica undersaturated) samples above from 353 subalkaline below.

about 21–17 Ma when the sweeping volcanic activity reached and stagnated in the southern part of the province (Best and Christiansen 1991, Figure 2). Three of the more mafic lava flows in our data set extruded after about 21 Ma possess higher Nb and Ti concentrations, manifesting waning generation of arc magmas typified by negative Nb and Ti anomalies in normalized element variation diagrams. Increasing proportions of true basalt lacking an arc chemical signature were extruded after 20 Ma, along with high-silica rhyolite, heralding the initiation of a bimodal volcanic association accompanying crustal extension (Christiansen and Lipman 1972). Our Great Basin data set is essentially a unimodal spectrum of intermediate- composition arc lavas that are mostly high-K but with lesser medium-K and shoshonitic compositions (Figures 3 and 4). Most samples are andesite with lesser dacite, trachydacite, basaltic andesite and K-rich latite and shoshonite. The most widespread phenocryst assemblage in the lavas includes plagioclase, two pyroxenes, and Fe–Ti oxides. In rocks that contain greater than 55 wt. % silica, amphibole, biotite, sanidine, and quartz are Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 common whereas olivine is essentially restricted to rocks having less than 55 wt. % silica.

Comparative determination of crustal thickness Values of compositional parameters in the global data base (Table 1) are plotted against crustal thickness in Figures 5–12. Linear best-ﬁt lines (not shown) have R2 values of 0.47–0.50. Parameter values for the four sets of data for the Great Basin (‘Nevada ...’ and ‘All Great Basin’ bottom of Table 1) are shown as arrows.

Average K2O/Na2O ratio in andesites The global data show a positive correlation between the average ratios and crustal thickness up to about 52 km, beyond which the ratios, all for the Andes, appear to ﬂatten out (Figure 5). The four Great Basin ratios, shown by arrows in the diagram, all lie above the global ratios, suggesting the Great Basin crust was greater than about 50 km thick. In 604 M.G. Best et al. )

2 1.4

Egan Range Central Nevada 1.2 Indian Peak and Great Basin >30 Ma

1.0 Great Basin <30 Ma

T 0.8 S AA M A A A S A 0.6 MA A M A A A A A A M M A P M O (wt%) in andesites (57-63 wt% SiO M M A 2 MKC 0.4 P P Z P M AKL H CC O/Na

2 C

0.2 A

Average K Average 0.0 20 30 40 50 60 70 80 Thickness of crust (km)

Figure 5. Thickness of the crust plotted against average K2O/Na2O in andesites for the suites of lava samples listed in Table 1. Arrows on left side of diagram are values of middle Cenozoic lava suites in the Great Basin.

3.5

Egan Range

3.0 Indian Peak Great Basin A A 2 M 2.5 S Central A S T AA Nevada M A A

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 2.0 A A AA M A A A UMA A A MM A A 1.5 J P MMMZ PP A A A A C M PHMA CAC L K C J KL C

O (wt.%) at 57.5 wt.% SiO 1.0 AJ CA 2 L K A L 0.5

0.0 20 30 40 50 60 70 80 Thickness of crust (km)

Figure 6. Thickness of crust plotted against K2O at 57.5 wt% SiO2 on variation diagrams for the suites of lava samples listed in Table 1. Arrows on left side of diagram are values of middle Cenozoic lava suites in the Great Basin. International Geology Review 605

0.7090 Central Nevada

Egan Range 0.7080

Sr 0.7070 Indian Peak 86 A Sr/ A 87 0.7060 A

A 0.7050 A A A A S A S

Minimum initial A A 0.7040 A L Z A U AL M AAA AMA A A J AP PLM MAMAM T A MP 0.7030 MK H C MCK JJ C

0.7020 20 30 40 50 60 70 80 Thickness of crust (km)

Figure 7. Thickness of crust plotted against the minimum initial 87Sr/86Sr in lavas with ,63 wt. % SiO2 for the suites of samples listed in Table 1. Arrows on left side of diagram are values of middle Cenozoic lava suites in the Great Basin.

0.7140

Egan Range 0.7120

0.7100 Central Nevada Sr

86 Indian Peak Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 Sr/ A 87 0.7080

A A A A S 0.7060 A

Z S A J L T A U S A A Average initial L A AAAA APJ AAMPM A A 0.7040 M PA AM J L MMMCM C KH K M

0.7020 20 30 40 50 60 70 80 Thickness of crust (km)

Figure 8. Thickness of crust plotted against the average initial 87Sr/86Sr in the suites of lavas listed in Table 1. Arrows on left side of diagram are values of middle Cenozoic lava suites in the Great Basin. 606 M.G. Best et al.

2000 Great Basin lavas Other western hemisphere 7 6 (except Central America) 6 1600 Crustal thickness 7 3 30-39 km 7 4 7 40-49 km 6 5 50-59 km 6 6 6 6 6 6 60-69 km 6 3 6 6 3 7 70-79 km 7 66 7 1200 6 7 7 6 3 7 6 3 6 43 3 7 7 3 3 7 3 4 4 4 4 3 3 4 4 66 4 3 4 4 44 4 7 44 34 3 33443 37 774 44 4 Ba (ppm) 4 4 7 3 73 4 7 4 44 4 44 67767774444674 4 4 4 44 800 4 4 7 4 6 77 6 7 77 3 33 4 6 3 7 7 7 67 4 3 4333 3 646 464 47 6 6 66 44 33 4 4 6 6 6 6 7 6 664 44 455 3 34334443333 5 6 7 3 6 6 6 6 6 4 6 3 4 43 343 3 7 666 4 6 4 6 6 44 3 34 4343 4 4 4 6 334 6 4353 44 43434544 4 3 64 6 4 63 44 44454 4444 44 4 3 4 4 4 43 3 4 44544553344 3 4 4 4 46 6 3 4 544444444 35 5 3 6 4 664 4545 5 5433545344545 44434 44 44 4 44 434 4 433 4444 45434 3544 3 34335 5 4 6 4 44 4 4453 54344445434354434343444353444 3 4 3 400 447434 4444 4 43 435 54 445 443435344 344 4 4 4 444 4444747434343344434433434 33 43 34344 4 4 3444454444434444454434443434 3 33 3 4 434444444444444444333334343 3 333 4 4 3 43 444434433434 444434 3 3 353 4 33343 3 44444334343333 34 3333 3 3 3433 344334 3 3 3 33 3 43333334333333 3 33 3 333 33333 3333 4 3 3 3 3 3 6 0 40 45 50 55 60 65 70 75

SiO2 (wt%)

Figure 9. Ba versus SiO2 in Great Basin lavas compared to lavas from other locales listed in Table 1. ‘Main trend’ extends from about 50 wt. % SiO2 and200ppmBato75wt.%SiO2 and 900 ppm Ba.

the Great Basin data set, the average K2O/Na2O in andesites older than 30 Ma is clearly greater than in younger andesites (not distinguished in Figure 5). The implied thinning of the crust with time is consistent with removal of lower crustal rock by outward ductile ﬂow, or by extension, or both, as discussed above. The average K2O/Na2O is slightly greater in lavas east of Longitude 1178 W (not distinguished in the Figure 5), suggesting a slightly thicker crust to the east on the Precambrian basement (see below).

2000 Central America Crustal thickness 30 - 39 km 1600 40 - 49 km 50 - 59 km Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 1200

Ba (ppm) 800

400

0 40 45 5055 60 65 70 75

SiO2 (wt%)

Figure 10. Ba versus SiO2 in lavas from Central America listed in Table 1 (see Carr et al. 2003). Line is ‘main trend’ from previous ﬁgure. International Geology Review 607

240 Great Basin lavas Other western hemisphere (except Central America) 200 Crustal thickness 30-39 km 40-49 km 50-59 km 160 60-69 km 70-79 km

120 Rb (ppm)

0 40 45 50 55 60 65 70 75

SiO2 (wt%)

Figure 11. Rb versus SiO2 in Great Basin lavas compared to lavas from other locales listed in Table 1. ‘Main trend’ extends from about 50 wt. % SiO2 and 10 ppm Ba to 75 wt. % SiO2 and 70 ppm Ba.

240

Central America Crustal thickness 200 30 - 39 km 40 - 49 km 50 - 59 km 160

120 Rb (ppm)

80 Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 40

0 40 45 50 55 60 65 70 75

SiO2 (wt%)

Figure 12. Rb versus SiO2 in lava samples from Central America listed in Table 1 (see Carr et al. 2003).

In the Paradise Range, at about 1178 500 W (Figure 2), an older suite (about 26–24 Ma) of andesite lavas has a ratio of about 0.8 whereas a younger suite (20–16 Ma) has a ratio of 0.6 (John 1992, 2001). These ratios suggest thinner crust than that of our data set, but the lavas are relatively young, lie in western Nevada, and some are altered; nonetheless, they imply crustal thinning through time. 608 M.G. Best et al.

K2O at 57.5% SiO2 This diagram is similar to the previous one except the positive correlation of values continues to thickest crust (Figure 6). Again, there is a slight tendency for the K2O parameter to ﬂatten above 42 km, but this tendency is a result of only two points, viz. M and S in crust 40–50 km thick. The Great Basin K2O parameters correspond to a crust thicker than about 45 km. However, if the parameter for the only seven central Nevada analyses is excluded in preference to the parameters for the more abundant (26) Egan Range analyses and for all (376) the Great Basin analyses, a thickness of more than 70 km is suggested. No difference in the parameter can be seen as a function of age or longitude of the lavas. In the Paradise Range (see above), K2O at 57.5% SiO2 in the older lava suite is 2.9 but 2.0 in the younger, again implying crustal thinning through time.

Minimum and average initial 87Sr/86Sr (Maximum 87Sr/86Sr ratios are not shown here because of a wide scatter.) In the global compilation of Leeman (1983), the mean 87Sr/86Sr ratio has a relatively strong positive correlation with crustal thickness whereas the minimum ratios are more scattered. Leeman (1983) notes that ‘ ... these correlations are surprisingly strong considering that the age of continental crust and its composition differ considerably for these arcs’. In our expanded global data base of 87Sr/86Sr ratios, the correlation with thickness is equally strong and, as in Leeman’s compilation, the correlation coefficient is smaller for the minimum ratios than for the average ratios, which have the highest R2 (0.50) value for a best-fit straight line of any plotted parameter. Figure 7 indicates the crust beneath the area of the Indian Peak caldera complex, based on seven samples (Hart et al. 1998), was 60–70 km in thickness whereas Figure 8 suggests a greater thickness. The 20 87Sr/86Sr ratios for the Egan Range lavas cited by Grunder (1992) and shown in Figures 7 and 8 are much higher than those from the Indian Peak area, which would suggest a much greater crustal thickness. However, the crust in this area may be significantly older (Archean) and more radiogenic (Wooden et al. 1999) than other circum-Pacific regions in our global data base. We have no definitive explanation for what would seem to be unrealistic thicknesses of 70–80 km, or more, that are implied for the Great Basin crust in Figures 5–8. Such extreme thicknesses are known, today, only in the Himalayan region, where collision of

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 the Indian and Asian continental crusts has stacked them atop one another. However, this tectonic setting is unlike the continental arc regime of the Great Basin area from the Mesozoic through middle Cenozoic. In the Great Basin, sparse exposures in horsts reveal several kilometres of weakly metamorphosed Neoproterozoic and lower Cambrian miogeoclinal sedimentary rocks that were deposited on the thinned older crust along the passive rifted margin of the continent (Stewart 1980; Hintze and Kowallis 2009). Contamination of magma with the pelitic rocks within this dominantly siliciclastic sequence could have contributed to the high 87Sr/86Sr ratios and to the extreme implied crustal thicknesses in Figures 5–8. But whether this underpinning of the Great Basin differs signiﬁcantly from the crust underlying arcs in the global data base is not readily answered; extensive blankets of volcanic rocks hide direct clues to the nature of the underlying crust. Some compositional parameters are probably asymptotic with respect to crustal thickness. Strontium isotope ratios might correlate more or less linearly with respect to crustal thickness until some particular thickness is attained, beyond which the magma International Geology Review 609

composition would then depend more on the proportion of crust assimilated and its thermal regime. For example, a 100% crustal melt would have the composition of the fusible portion of the crust, regardless of thickness. In the case of a continental margin with a thick section of miogeoclinal rocks, 87Sr/86Sr ratios would then be governed more by the age of the crust and its Rb/Sr ratio. Thus, it is that strongly peraluminous two-mica granites of Cretaceous age with initial 87Sr/86Sr ratios more than 0.720 in the central Great Basin appear to be partial melts of the miogeoclinal wedge (Best et al. 1974; Lee and Christiansen 1983).

Ba and Rb In unusually thick, thermally equilibrated continental crusts the temperature in the deeper parts likely exceeds the solidus temperatures of felsic rocks (e.g. Sonder et al. 1987). Parcels of partial melt too small in concentration to form buoyant diapirs can nonetheless mix with ascending mantle-derived magmas. Because of the small partition coefficients of Ba and Rb for the common plagioclase-dominated source rocks in deep continental crust, these two incompatible elements are enriched in granitic partial melts and in any mantle- derived magmas mixing with them. Therefore, relatively high Ba and Rb in extruded lavas indicate long crustal path length (i.e. thick underlying crust). Moreover, these are two of the most commonly analysed trace elements in the global data set, thus facilitating comparisons. Because Ba and Rb concentrations in calc-alkaline arc volcanic rocks are a function of silica content, we present plots against this oxide (Figures 9–12). In the Ba plot (Figure 9), ‘Other western hemisphere’ individual rocks (not averages of suites as in previous variation diagrams) are mostly situated on crust 30–59 km thick, defining a ‘main trend’ that is believed to have resulted from crystal fractionation with minimal crustal contamination and mixing. Most of the analyses of rocks sited on crust 50–70 km thick lie above the main trend, thus reflecting the enrichment in Ba from mixing with Ba-rich anatectic partial melts. Significantly, virtually the entire population of Great Basin rocks also lies above the main trend; the most Ba-rich rocks in the Great Basin coincide with similarly enriched rocks in the Andes, where the crust is 60–70 km thick. It should be noted, however, there is a small proportion of rocks sited on 30–59 km-thick crust that also lie above the main trend. These are rocks from the San Juan field, where the crust is probably (see below) 48 km thick, and a few, particularly those with greater than 65 wt. % SiO2, from Mexico, where the crust is 30–43 km thick. How are we to account Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 for these anomalous rocks? Do they weaken the thickness correlation? Not plotted in Figure 9 are the Ba contents of rocks in the central American arc (Carr et al. 2003), which are shown separately, for the sake of clarity, in Figure 10. Comparing the two figures it is immediately obvious that virtually all of the central America rocks lie above the main trend defined by other western hemisphere rocks. Rocks sited on the thinnest crust, 30–39 km thick, have the highest Ba contents. This anomaly may be explained by an unusually high input of Ba-rich sediment into the magma- generating systems in the central America subduction zone (Plank and Langmuir 1993; Patino et al. 2000). Ba is presumably removed from the sediments in the shallower parts of the subduction zone, thus accounting for the negative correlation of Ba content and crustal thickness. Or, the Ba-rich sediments are located where the crust is 30 km thick. The anomalously high Ba content of the Mexican rocks may also be the result of contamination by Ba-rich sediment. However, it is unlikely that the relatively high Ba contents of the San Juan lavas can be similarly explained because these rocks lie so far inland, about 500 km east of the eastern 610 M.G. Best et al.

margin of the Great Basin. One explanation is suggested by the fact that the San Juan lavas plotted (S) in Figures 5–8 consistently lie slightly above the other global data points. This shift could mean that the assigned crustal thickness of 48 km at the time of Oligocene volcanism in the San Juan region, based on current geophysical information (Prodehl and Lipman 1989), is too thin. If true, the Ba content of these lavas is less anomalous. In the plot of Rb (Figure 11), ‘Other western hemisphere’ arc rocks again deﬁne a main differentiation trend and rocks sited on 50–69 km crust lie above it. However, Andean rocks on the thickest crust (70 km) are not the highest in Rb and many analyses for rocks sited on crust 40–49 km thick also lie well above this trend. Because Rb is not a highly concentrated element in subducted sediment, the anomalous Rb points cannot be accounted for in the same way as for Ba. The considerable scatter of Rb values is probably a result of the large variations of Rb in crustal rocks. For example, accreted ocean ﬂoor or island arc terranes have less Rb (and other incompatible elements) than highly processed upper continental crust of Proterozoic age (e.g. Taylor and McLennan 1985).

Summary Compositions of middle Cenozoic lava ﬂows indicate the Great Basin crust at this time was at least 50 km thick and likely 60–70 km. The crust was probably somewhat thinner in the west than in the east and thinner in the waning stages of the ignimbrite ﬂareup when crustal extension began.

Sr/Y ratios and depth of magma equilibration If magmas are generated at depths sufﬁcient to stabilize garnet rather than plagioclase in the residue, then the Sr/Y ratio of the magma will be relatively high. The minimum depth for this condition depends on the composition of the source rock and the temperature but is likely satisﬁed at depths of perhaps greater than 50 km. A Sr/Y . 40 and Y , 18 ppm are commonly used to distinguish adakitic magmas that appear to have equilibrated with garnet (e.g. Castillo et al. 1999). Less than 10% of the intermediate composition rocks we have analysed from the Great Basin have Sr/Y . 40 and only seven samples have Y , 18 ppm. The average Sr/Y ratio is 25 ^ 11 and the average Y concentration is 29 ^ 6 ppm. Consequently, there is little chemical evidence that the middle Cenozoic lavas equilibrated with or fractionated garnet at high pressure in thick continental crust. However, the lack of a garnet signature cannot be used as an argument against an Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 unusually thick continental crust because regardless of where the middle Cenozoic lavas were initially generated they now possess a middle to shallow crustal imprint. Their high initial Sr ratios (Table 1) suggest contamination with middle to upper crustal materials, while their position on pseudoternary phase diagrams of Baker (1987) suggest crystallization pressures between 2 and 5 kb, or 7–17 km deep.

Spatial aspects of ash-flow sheets: Implications for an Altiplano The unusually thick crust, together with the assumption of isostatic adjustment, implies the existence of some sort of elevated terrain in the Great Basin area during the ignimbrite flareup. Because thick crust can underlie rugged mountain belts as well as flat highlands, we now seek evidence for the character of the topographic relief of the Great Basin area during the middle Cenozoic ignimbrite flareup. Ash flows spread outward from their sources as mobile avalanches whose lateral dispersal is governed by the dynamics of the eruption as well as by the surface topography International Geology Review 611

in the area of dispersal. Subsequent erosive and tectonic processes can modify the primary shape of a sheet. Examination of large ignimbrite outﬂow sheets in the Great Basin provides insights into the nature of the landscape on which the sheets were deposited.

Progressively smoothed terrain By the late Eocene, the orogenic plateau in what is now the Great Basin that resulted from contractile deformation during earlier orogenies had been affected by erosion (e.g. DeCelles 2004). Valleys cut into the terrane of mostly Palaeozoic rocks are marked by local sediment deposits overlain by the earliest ignimbrites. In the southern Wah Wah Mountains of southwestern Utah, Abbott et al. (1983) mapped such a palaeovalley filled with about 700 m lava flows and ash-flow tuffs ranging in age from at least 32 to about 30 Ma. In the same area and westward to the state line, the thickness and distribution of slightly older ignimbrites suggests EW-trending palaeovalleys that are nearly as deep. In east-central and central Nevada, Hagstrum and Gans (1989) and Radke (1992) found evidence for smoothing of palaeotopography by the earliest ash-flow tuffs. In the northern Toquima Range (Figure 2), McKee (1976a) found that the 35 Ma Pancake Summit Tuff is 200 m thick but within about 6.5 km to the north, west, and south it pinches out completely. In northeastern Nevada, Henry (2008) documented palaeovalleys as much as 10 km wide and at least 500 m deep, perhaps as much as 1.6 km. Continued deposition of ash flows filled palaeovalleys, progressively smoothing the landscape. As an example, one can compare the substantial dispersion of palaeomagnetic directions for the Kalamazoo tuff with the tight cluster of directions for the thin tuff of Clipper Gap (Gromme et al. 1972, Figure 8). The Kalamazoo was deposited at 35 Ma near the initiation of the flareup on an irregular topography carved into Palaeozoic rocks over an area of about 120 km2 in the Schell Creek Range (Figure 2; Hagstrum and Gans 1989), whereas the Clipper Gap was deposited near the end of the flareup, partly in the area of the Kalamazoo, but in a sheet less than 20 m thick. The aggregate thickness of the ignimbrite outflow sheets surrounding the Indian Peak, Caliente, and central Nevada caldera complexes (Figure 2) is of the order of several hundred metres (Best and Christiansen 1991, Figures 5 and 6), thus effectively eliminating any relief less than this. Anderson and Rowley (1975, p. 9) noted that ‘The region of the high plateaus [of central Utah] was not physiographically distinct from the adjacent Colorado plateaus and

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 Great Basin during deposition of the lower Cenozoic sequence’. The ignimbrites in this sequence comprise the three widespread and very large volume monotonous intermediates emplaced from 31 to 29 Ma. In the earliest investigation of ignimbrites in the southwest Utah sector of the Great Basin, Mackin (1960, pp. 89, 97), observed that sheets emplaced about 24–22 Ma ‘ ...were spread over a surface of low relief ...’andas ‘ ...flattish sheets over a tolerably level plain ...’. However, compilation of thicknesses of outflow sheets from published geologic maps reveals that one of these ignimbrites, the 24(?) Ma Swett Tuff Member of the Condor Canyon Formation, seems to be distributed in two northeast-trending palaeo-valleys on either side of its eruptive source area. The apparent absence of palaeovalleys in southwestern Utah, as manifest by ignimbrite deposits, from about 30 Ma (see above) to about 24 Ma may be the result of flooding of the landscape by ash flows from the Indian Peak caldera complex to the northwest during its peak eruptive activity. Three features of all but the oldest outflow tuff sheets in the central and eastern Great Basin support the existence of a smoothed depositional landscape. First, thicknesses of 612 M.G. Best et al.

outflow sheets beyond their source calderas generally vary in a systematic manner such that these variations can be contoured on small scale maps (e.g. Best et al. 1989a, 1995; Sweetkind and du Bray 2008). Thicknesses of sheets diminish more or less systematically outwards from their sources, but lobate and eccentric outflow patterns from sources are common. Second, relatively thin ash-flow sheets such as the 25 Ma Clipper Gap and Nine Hill tuffs (Best et al. 1989b, pp. 105–110; Deino 1989), crop out over present areas of .10,000 km2 in eastern Nevada. Third, the generally small between-site variations in palaeomagnetic direction in individual younger ignimbrite sheets indicates an absence of substantial topography over which the ignimbrite was draped (e.g. Gromme et al. 1972, unpublished data 1990–1998; Best et al. 1995). On a hilly depositional surface, non- horizontal compaction foliations would have increased the between-site variations and adversely affected the success of palaeomagnetic correlations. Evidence for this fortunate circumstance consists of the positive results of the palaeomagnetic tilt tests – commonly known as fold tests–for nearly all of the regionally extensive ignimbrites surrounding the central Nevada caldera complex (Figure 2; see also Figure 14) with sufficient sites, as well as for the Nine Hill Tuff, for the most part east of the Carson Sink. The palaeomagnetic data are consistent with a mostly smooth landscape near the end of the flareup extending from approximately longitude 1178 W eastward across central and eastern Nevada and into the high plateaus of south-central Utah, a present area greater than 120,000 km2, equivalent to more than 80,000 km2 prior to Basin and Range extension. Only approximately 12% of this area was occupied by caldera complexes and single calderas, which possessed significant topographic relief. Locally, this smoothed ignimbrite terrain was interrupted by numerous effusions of lava that today are generally less than 1 km thick and range in composition from rhyolite to andesite (Best et al. 1989a, Fig. 3). Few major composite volcanoes on the scale of the modern Cascades formed in the smoothed terrain during the ignimbrite flareup. In the compilation of 34–17 Ma volcanic rocks of Nevada, Stewart and Carlson (1976) show three andesitic volcanoes that developed on the margin of the ignimbrite province. Within the province, remnants of a small dacitic composite volcano now about 10 km in diameter and 0.3 km high formed in the Seaman Range in southeastern Nevada at about 27 Ma (du Bray and Hurtubise 1994). In the nearby Pahroc Range, an unusually large pile of andesitic lava and volcanic debris flows emplaced at about 28 Ma is as much as 1.3 km thick and extends along the range for about 15 km (Swadley et al. 1995). Major composite volcanoes did develop in the Marysvale area of central Utah just to the east of the Great

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 Basin during the middle Cenozoic (Steven et al. 1979). The scarcity of large composite volcanoes, and accompanying mountainous relief, in the middle Cenozoic ignimbrite province contrasts with the character of the contemporaneous Southern Rocky Mountains volcanic field in southwestern Colorado, where the volume of lava and volcanic debris flows in such edifices exceeds that of ash-flow tuffs (Lipman and McIntosh 2008).

Summary statement: existence of the Great Basin Altiplano We conclude that during, and especially in the latter stages of, the ignimbrite flareup, a substantial part of the Great Basin area was a relatively smooth plain capping a sequence several hundreds of metres thick of essentially conformable ignimbrite outflow sheets. Locally, in the ‘outflow alley’ between calderas, the plain was dotted by piles of lava. Together, with the evidence indicated above for an unusually thick crust, which very likely resulted in an isostatically high elevation, we feel justified in concluding that this part of the middle Cenozoic Great Basin area was a relatively flat, high plateau, i.e. an Altiplano. International Geology Review 613

This orogenic plateau apparently had not experienced significant regional tectonic extension and crustal thinning by the time of the ignimbrite flareup. Moreover, it did not experience such extension concurrent with the ignimbrite flareup, in contrast to the interpretation of Gans et al. (1989). Had there been significant concurrent extensional faulting the success of palaeomagnetic correlations of ash-flow sheets and wide dispersion from their sources described above would have been greatly hindered. The general absence of sediments between the conformable outflow sheets (Best and Christiansen 1991, Figures 5 and 6) that would have been shed off fault-related uplifts into adjacent basins is also inconsistent with contemporaneous extension. South and southwest of the Caliente caldera complex (Figure 2), lacustrine limestone is intercalated within the lower part of the ignimbrite sequence and lower in that sequence conglomerate of Palaeozoic- rock clasts becomes more abundant than limestone (Scott et al. 1992; Anderson and Hintze 1993); these stratigraphic relations imply progressive filling by tuff of a depositional basin on the southern margin of the middle Cenozoic ignimbrite field and not synvolcanic extensional faulting.

Eccentricity of source calderas within their corresponding outflow sheets Rarely do outflow sheets possess anything close to axial symmetry surrounding their source caldera. In principle, it seems unlikely that pyroclasts would vent uniformly around the perimeter of the ring-fracture to create a radially uniform outflow sheet. Among others, Gromme et al. (1997) have documented lobate emplacement of ash flows vented from adjacent sectors of a caldera system. Topographic influences on ash-flow dispersal as well as differential post-emplacement erosion of the outflow sheet can also create an eccentric source-outflow sheet relationship. The manner in which large to very large volume (100s to 1000s of km3) outflow tuff cooling units in the central Nevada field were distributed from about 35 to 23 Ma around their sources in the central Nevada caldera complex (Best et al. 1993, unpublished data 1990–2008) furnish significant insight into the topography of the Great Basin Altiplano. The location of the source from which two large outflow cooling-unit members of the 35 Ma Stone Cabin Formation were derived has not been ascertained so we cannot tell how these sheets are distributed around their source. Of the remaining 12 sheets for which the locations of seven sources are known, nine are distributed asymmetrically, more to the east than the west of their sources; that is, their eruptive sources are

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 eccentrically offset toward the west within their corresponding outﬂow sheets (Figure 13). Three sheets are distributed essentially symmetrically east and west of their sources, namely, one of four members of the 26 Ma Shingle Pass Formation and two of three members of the 27 Ma Monotony Tuff. One of these two symmetrically distributed Monotony tuffs occurs west of about 1178 W Longitude and is the only one of the 14 ignimbrite sheets known to occur west of this meridian. Because the occurrence of this one Monotony sheet so far west is anomalous we made a special effort to be certain of the correlation of the Monotony Tuff to the east with the tuff of Miller Mountain (Robinson and Stewart 1984) that is found about 75 km due west of Tonopah. The latter has an identical precise 40Ar/39Ar age on sanidine as the average (n ¼ 10) sanidine age of the Monotony and has a modal and whole-rock chemical composition (30 elements) consistent with the Monotony. Microprobe analyses of biotite and hornblende phenocrysts in the Miller Mountain and Monotony are indistinguishable and are distinct from the compositions of these phenocrysts in other tuffs in the central Nevada ﬁeld. 614 M.G. Best et al.

(a) Tuffs younger than 27 Ma 117° 115° 113° 40°

Austin Clipper Gap 25 Ely * Shingle 39° Pass 26 Lunar LC Cuesta 25 SP Pahranagat 38° Tonopah P 23

Caliente

Utah ° Nevada 37

250 50 100 150 200 km

(b) Tuffs older than 27 Ma 117° 115° 113° 40°

Austin PS Ely Pancake Summit Stone 39° 35 Cabin 35 Windous WB Butte 31 M ° Tonopah 38 Monotony 27

Caliente Utah

Nevada 37° 250 50 100 150 200 km Downloaded By: [Brigham Young University] At: 20:48 15 September 2009

Figure 13. Distributions of major outﬂow tuff sheets and their source calderas in the central Nevada volcanic ﬁeld that surrounds the central Nevada caldera complex, based on our unpublished data (see Best et al. 1989b, 1995 for preliminary data). Calderas labelled with abbreviations of the name of the erupted tuff and with age in Ma. (a) ,27 Ma. Asterisk southeast of Austin is our inferred location of the source of the tuff of Clipper Gap; no fault-bounded caldera source has been found. (b) .27 Ma.

It is unlikely that eruptive mechanisms alone – with the one possible exception of the Monotony sheet just described – would have allowed 75% of the ash ﬂows to spread more to the east than to the west. Thus, topography and/or erosion are left as factors governing the eccentricity of most of the tuffs. Erosion of the western parts of the outﬂow sheets is an unlikely explanation, because ignimbrite sheets of much the same, but particularly younger, age range are widely exposed to the west (e.g. Whitebread and Hardyman 1987; John 1992) and they too should have been eroded away. Therefore, we believe that central International Geology Review 615

120° 117° 114°

NEVADA

0.706 Line 41° Toyabe Uplift

Austin

Ely AD * T MJ

Tonopah

CALIFORNIA Caliente UTAH 37° Ignimbrites from CNCC ARIZONA

250 50 100 150 200 km

Figure 14. Western edge of the Precambrian basement corresponding to the ISr ¼ 0.706 line (modiﬁed from Wooden et al. 1998) and ‘possible distribution of Toyabe uplift zone’ (darker grey) of (Speed et al. 1988, Figure 22-11). Calderas embedded eccentrically in the western part of the maximum distribution of outﬂow sheets from Figure 13. Calderas west of the Toquima Range from John (1992) include Arc Dome (AD, 25 Ma) and Toiyabe (T, 22 Ma). Mt Jefferson caldera (MJ, 27 Ma) in the Toquima caldera complex from Henry et al. (1996).

Nevada ash flows could not always surmount a NS-trending topographic barrier positioned just east of Tonopah and Austin (Figure 2). In a symmetric manner, little or none of the outflow from sources west of the barrier (Figure 14) is known for certain east of it. The 22 Ma outflow tuff of Toiyabe (John 1992), whose caldera source is located about 20 km south of the south end of the Reese River

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 Valley, extends at least 200 km to the west near the Nevada-California state line. However, Shawe and associates (Shawe 1998; Shawe and Byers 1999; Shawe et al. 2000) found none of this distinctive titanite-bearing tuff in their detailed mapping of the Toquima Range and we have found none farther east in the central Nevada volcanic ﬁeld. Nonetheless, the 24 Ma tuff of Arc Dome, whose source is apparently in the southernmost Reese River Valley, may crop out in the western Toquima Range, where Shawe (1998) called it the Diamond King Formation. About 50 km farther to the southeast, we found a thin (10 m) tuff of similar composition as the Arc Dome, except for the presence in the former of titanite, that has analytically the same 40Ar/39Ar age on sanidine. Without further data we can only make a tentative correlation with the Arc Dome. None of the other ignimbrites described by John (1992) west of the Toquima Range have been found in our investigation of the tuffs in the central Nevada ﬁeld. The reconnaissance work of Garside et al. (2002, 2005; also C.D. Henry unpublished data 2008) indicates some of the tuffs that have sources in the Toquima Range occur far to the west, to at least Yerington. Thus, source calderas of the Toquima, Toiyabe, 616 M.G. Best et al.

and possibly the Arc Dome tuffs are eccentrically positioned to the east within their corresponding westward extending outflow sheets. Most of their westward dispersal appears to have been governed by ancient drainage ways on the western sloping margin of the Great Basin Altiplano (see below). But to the east their dispersal seems to have been impeded by the same topographic barrier as the one controlling dispersal of ash flows from calderas to the east in the central Nevada field. The mirror-image eccentricity of ignimbrite sheets and sources on either side of the topographic barrier is a fundamental feature of the middle Cenozoic Great Basin ignimbrite province. Although our current understanding of the correlation and distribution of ignimbrite sheets on which the existence of this topographic barrier is based is not perfect, there is, in our opinion, sufficient evidence for it. This barrier was actually recognized at least as early as the 1980s by Bart Ekren and Jack Stewart, who suggested its existence at that time to the senior author (MGB). The location of the barrier is constrained to lie essentially in Monitor Valley between the Monitor and the Toquima Ranges but to the south swings west roughly through Tonopah (Figures 2, 13, and 14). Lunar Cuesta and Big Ten Peak ash flows erupted about 25.6 Ma were dispersed eastward from their sources at the south end of the Monitor Range and 35 Ma Pancake Summit ash flows eastward from their source at the north end of the range. Ash flows from sources farther to the east were also dispersed eastward. Ash flows erupted about 29–26 Ma from sources in the Toquima Range travelled westward. The barrier was not strictly impassable and its effectiveness may have diminished with time. We have noted that one cooling unit of the 27 Ma Monotony Tuff transgressed the apparent barrier just north of Tonopah. The 25 Ma tuff of Arc Dome that erupted from a source west of the barrier is tentatively correlated with tuff to the east of it. Comparison of the distribution of tuffs that are less than 27 Ma with that of older tuffs (Figure 13) suggests that with time the ash flows from eastern sources advanced farther west (one cooling unit of the Monotony Tuff excepted), partially surmounting the topographic barrier. Although no exposed source for the 24 Ma Clipper Gap ash-flow tuff has been found, thickest sections of the tuff in the Toquima and Monitor Ranges suggest the source may lie concealed beneath northern Monitor Valley. A thinner lobe extends almost to Utah, implying a topographic barrier west of the Toquima Range or special eruption dynamics to create the asymmetric tuff distribution. An apparently more serious difficulty with the implied topographic barrier is the fact that the 25 Ma Nine Hill Tuff, whose concealed source may be in the Carson Sink area,

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 crops out from the western foothills of the Sierra Nevada across most of Nevada to a little west of Ely, in many places as a relatively thin, densely welded sheet (Figure 2; Deino 1985, 1989; Best et al. 1989b, Figure R3). Ash flows would not likely have been dispersed so far to the east with the eruptive source apparently lying well down the western slope of the Altiplano (see below), unless by this time the effectiveness of the barrier or the slope reduced in some way. Possibly, crustal extension as documented in the Stillwater Range just to the east of the apparent source area at about the time of eruption of the Nine Hill (Hudson et al. 2000) may have modified the topography so as to allow dispersal of the ash flows eastward. The unusual chemical composition of the Nine Hill magma (Deino 1985) may have resulted in unusual eruption processes that allowed the ash flow to be so widely dispersed. The origin of the barrier is less certain than is its location and apparent effectiveness in blocking the dispersal of most ash flows. A study of the topography of the Tibetan and Andean Altiplano continental plateau margins by Masek et al. (1994) may furnish an explanation for the apparent ash-flow barrier in the Great Basin Altiplano. They point out International Geology Review 617

(a) 6 Tibetan plateau Andean plateau 4

2 Elevation (km)

0 0 100 200 300 400 500 600 Distance (km)

(b) 6

Final topography 4 Initial topography

Denudation 2 Elevation (km) Isostatic uplift 0 0 100 200 300 400 500 600 Distance (km)

Figure 15. Topographic profiles across the edges of high orogenic plateaus from Masek et al. (1994). (a) Average profiles, based on 100-km-wide swaths (Masek et al. 1994, Figure 3), adjusted to the same horizontal scale, across the Tibetan (solid line) and Andean (dashed lines) plateaus and oriented so that storms approach from the left as they did for the western margin of the Great Basin Altiplano. The upper Andean profile is for the northeast (Beni) part of the Altiplano and the lower for the southeastern (Pilcomayo) part. Note the topographic high on the lip of the plateau between its interior on the right and its sloping margin. (b) Results of numerical modelling show a representative initial topography and the final topography that results from the interaction of heavy orographic precipitation on the initial plateau slope and consequent erosional denudation and isostatic uplift.

that the margin of a high continental plateau that faces prevailing storms experiences

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 substantial orographic precipitation and consequent high erosion, whereas the plateau interior receives much less precipitation so that there is accordingly little denudation. (For the Great Basin Altiplano its western margin was exposed to storms coming inland from the Pacific.) Because of the focused denudation along the margin and consequent isostatic uplift, a topographic high develops at the lip or break in slope between the plateau interior and the sloping margin (Figure 15(a)). Numerical modelling of precipitation, erosion, and isostatic uplift by Masek et al. (1994) simulated development of the topographic profile observed in the Tibetan and Andean plateau margins (Figure 15(b)). If such a ridge had evolved on the western margin of the Great Basin Altiplano, its position would have been near that of our postulated topographic barrier to ash flow dispersal. The topographic barrier lies near and parallels the western edge of the Precambrian basement as well as the Toiyabe uplift zone (Figure 14) that is postulated by Speed et al. (1988) to consist of a chain of domical uplifts, at least some of which are thermal and diapiric. North of Austin, the zone is manifest by three domical uplifts that expose deep- seated rocks of moderate metamorphic grade; the dome at Austin is cored by a Jurassic 618 M.G. Best et al.

granitic pluton. However, Speed et al. (1988) do not specify how the zone is expressed south of Austin and only note that the uplift zone as a whole lies along the Precambrian basement edge that was overrun by Palaeozoic–Mesozoic thrust sheets (Figure 1).

Character of the western margin of the Altiplano In western Nevada, west of the topographic barrier that impeded the distribution of ash flows, the dispersal of ash-flow tuffs contrasts with that to the east and appears to have been, at least in part, controlled by palaeovalleys, or ancient drainages, incised in the western sloping margin of the Altiplano. The 34 Ma tuff of Cove Mine is draped over a roughly NS-trending topographic high in the northern Fish Creek Mts. and the southern and eastern parts of Battle Mountain (Figure 2; Doebrich 1995, Figure 19; S. Gromme, H.C. Palmer, and W.D. MacDonald unpublished data 1968–2006; John et al. 2008). To the east, the tuff of Cove Mine, which may have erupted from the Caetano caldera to the south, evidently filled a broad NS palaeovalley. Part of the outflow from the caldera, the Caetano Tuff itself, travelled westward through an evident gap in the Toiyabe uplift at least as far as the west side of what is now the Tobin Range (Burke 1977). In the Toiyabe Range, the 25 Ma Nine Hill Tuff (Deino 1989) can be observed draped over an erosional surface, which retained appreciable topographic relief, on the Jurassic granitic pluton exposed east of Austin along US Highway 50 (McKee 1976b). Garside et al. (2005; see also Henry 2008) summarize evidence for the existence of Eocene-Oligocene palaeovalleys that extended across the area of the present Sierra Nevada and were headed in a highland in west-central Nevada. As one example of such palaeovalleys, they (p. 217) cite the occurrence of the Guild Mine Member of the Mickey Pass Tuff, which was first described in the Yerington district by Proffett and Proffett (1976), in the ancestral Yuba River drainage in the Sierra Nevada and indicate its correlation with the intracaldera lower tuff of Mt Jefferson in the Toquima Range (Figures 2 and 14). The out-flow length in this palaeovalley is about 210 km, after compensating for subsequent crustal extension (C.D. Henry Written Communication, December 2008). Some of these ancient drainage ways were major canyons; for example, in the Yerington district, Proffett and Proffett (1976) document a palaeovalley 1.6 km deep filled with Oligocene tuffs and floored by conglomerate. In describing the mid-Cenozoic topography west of the Precambrian continental

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 margin and especially west of the Stillwater Range, we can do no better than to quote Larry Garside: It is clear that the pre-tuff erosional surface had some relief ...and there was a well-developed system of westward-flowing streams in western Nevada, which headed in a central Nevada highland ... These streams apparently flowed in broad palaeocanyons and palaeovalleys developed on the basement. Locally, stream deposits are preserved in the central parts of these valleys below the rhyolitic ash-flow tuffs ... Deposition of these tuffs has preserved these palaeovalleys. The source calderas of the outflow ash-flow tuffs ...are believed to have been to the east, in western or central Nevada ... The tuffs filled the channels and may have covered the surrounding higher grounds as well. They were thicker in the channels and thus became more strongly welded there, possibly producing slightly lower surfaces directly above the channels and allowing subsequently developed stream channels to follow the original, pre- tuff drainages rather closely. (Garside et al. 2003) Garside’s comment about channel overflow applies especially to the wide distribution of remnants of the Nine Hill Tuff on the western slope of the Sierra Nevada between 388 and 398 450 N (Deino 1985). International Geology Review 619

An EW-trending belt of complex middle Cenozoic volcanic activity that is approximately 40 km wide extends across four mountain ranges from the east margin of the Shoshone Range to the west margin of the Stillwater Range, a present distance of about 100 km. Mapping by Riehle et al. (1972) in the southern Clan Alpine Mountains and by John (1995) and Hudson et al. (2000) in the southern Stillwater Range showed that this belt, delineated on Sheet 2 of Stewart and Carlson (1976), is characterized in its western part by faulted and tilted calderas, subjacent coeval plutons, and ash-flow tuffs, mostly intracaldera. North of the belt and separated from it by a wide swath of presently exposed Mesozoic rocks, which may have been a topographic high during middle Cenozoic time, a wide band of a thick uninterrupted sequence of welded ash-flow tuffs extends from the Shoshone Range at least to the western margin of the Stillwater Range. This area is characterized by the complete absence of calderas but is dominated by outflow tuff sheets. In the New Pass Range and western Shoshone Mountains it is about 57 km wide and consists of up to 19 separate cooling units with an aggregate thickness of about 600 m in which intercalated sedimentary rocks are essentially absent (McKee and Stewart 1971). To the west in the Clan Alpine Mountains the zone of ash-flow tuffs is about 30 km wide and consists of at least 12 separate tuffs, having a total thickness of 250 m or more (Riehle et al. 1972; Hudson and Geissman 1991). Farther west in the Stillwater Range the same zone is about 20 km wide and is abruptly terminated by the western frontal fault of the Stillwater Range facing the Carson Sink (Hudson and Geissman 1991). Moreover, this zone of outflow tuffs can be traced farther westward in progressively narrowing palaeovalleys through Dogskin Mountain and the Diamond Mountains and into the Sierra Nevada proper at Haskell Peak (Stewart and Carlson 1976, Sheet 2; Brooks et al. 2003; Henry et al. 2004; Faulds et al. 2005a, 2005b). At Haskell Peak, a stratigraphic section of nine ash-flow tuffs has been uniquely preserved within an ancient erosional channel cut into bedrock (Brooks et al. 2003). At least three of these tuff units can be unambiguously correlated throughout the area just described, which extends as far east as the western Shoshone Range and spans a present-day EW distance of about 275 km. Notable among these three tuffs is the Nine Hill, which near the present crest of the Sierra Nevada, occupies at least two broad channels (Deino 1985). Exposures of the Nine Hill Tuff lie tens of kilometres farther southwest in the foothills of the Sierra Nevada. The fact that several mid-Cenozoic ash-flow tuffs spread into the Sierra Nevada from sources farther east raises the question of the elevation of the east margin of the Sierra at that time. It is well known that the ancient Pacific shoreline in Late Cretaceous through

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 Miocene time coincided approximately with the present eastern margin of the Sacramento and San Joaquin valleys (i.e. the western margin of exposed Sierra Nevada bedrock). In Figure 16, we show two schematics depicting the implications of two different models for the history of the Sierra Nevada structural block (mostly a Cretaceous batholith). In the traditional model (Figure 16(a)), the main uplift occurred in two stages. The ﬁrst stage occurred between about 85 Ma to about 50 Ma, represented by exhumation of the batholithic rocks and terminating at the time of deposition of the Eocene Auriferous Gravels. Negligible additional uplift occurred until after about 5 Ma when the present elevation would have been obtained. This model dates back at least to the time of Waldemar Lindgren (1911) and is based almost entirely on geomorphologic evidence, reinforced lately by potassium-argon ages of overlying late Cenozoic volcanic rocks (Wakabayashi and Sawyer 2001, and comprehensive references therein). Part A of Figure 16 is an attempt to show how this particular uplift history might connect with the western part of the Great Basin Altiplano; the ﬁgure involves a gross (more than sixfold) linear extrapolation of the slopes of Eocene Auriferous Gravel calculated and projected by Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 620 (a) Reconstructed elevation (km) (b) Elevation (km) 0 1 2 3 4 0 1 2 3 4 Reconstructed distancefromwest marginofSierraNevada(km) 0 0 Base oftuff Reconstructed elevationsanddistances inmid-Cenozoictime Present elevationsanddistances at Donner Bedrock Bedrock Summit Distance eastfromwestmarginofSierraNevada(km) crests crests Present elevations;distancesinmid-Cenozoictime Present elevationsanddistances 100 100 Donner Summit 4.5 +/Ð0.5m/km(Yeend,1974) 7.8 ±0.5m/km

Base oftuffatDonnerSummit Nine Hill

Mulch et al. (2006) ..Best M.G.

21-22 m/km 200 200 Fireball Ridge Mopung Hills

Stillwater Range tal. et 300 300

Hypothetical elevation Paradaise Range Slope ofgravelalone Clan Alpine Mts 11.8 +/Ð0.5m/km 11.8 +/Ð0.5m/km Slope ofgravel+

(Yeend, 1974) New Pass Range bedrock crests of altiplano 400 400 500 500 0 5 10 0 5 10

Elevation (1000 feet) Elevation (1000 feet) International Geology Review 621

Yeend (1974). Shifting the longitudes of seven of the thick sections of welded ash-flow tuffs mentioned above to compensate for post-Oligocene extension brings the present elevations closer to the extrapolated gradient, but the extrapolated gradient would only intersect the original (Oligocene) longitude of the New Pass Range at about 2.5 km palaeoelevation, which is at least 0.5 km less than what we envisage for the Great Basin Altiplano. A more recent version of the Sierra Nevada uplift history has been derived from palaeoelevations inferred from measurements of oxygen and hydrogen isotopes in Cenozoic authigenic minerals, both within the Auriferous Gravels of the Sierra Nevada (Mulch et al. 2006) and in Miocene and Pliocene basins to the east (Poage and Chamberlain 2002). These workers concluded that nearly all the uplift of the Sierra Nevada occurred between the solidification of the youngest Late Cretaceous batholithic rocks (85–80 Ma) and the deposition of the Eocene Auriferous Gravels (about 50–40 Ma). Many (U–Th)/He cooling ages measured in apatite from the granitic rocks by House et al. (2001) also imply early uplift. Mulch et al. (2006) concluded that Eocene elevations in the northern Sierra Nevada were 1.7–1.8 km, with highest bedrock peaks up to 2.2 km. In constructing Part B of Figure 16, we have taken the 1.7–1.8 km elevation inferred by Mulch et al. (2006) and have applied it to Donner Summit, again using a linear extrapolation from ancient sea level. This leaves about 300–400 m between the Mulch et al. (2006) estimate and the present elevations at Donner Summit to represent post-Eocene uplift coupled with an unknown amount of erosion of the bedrock crests. Quoting Mulch et al. (2006, p. 88): ‘Because relative surface displacements between the western Basin and Range province and the northen Sierra Nevada are of Miocene and younger age ..., we speculate that the Eocene Sierra Nevada formed the western edge of a high-elevation landscape that characterized large areas of the western United States ...’. Following their speculation, we have taken two-dimensional liberties with the elevations as well as the original longitudes of the sections of ash-flow tuffs described above. First, we moved them proportionally westward according to 50% post-Oligocene extension (McQuarrie and Wernicke 2005), and then increased their elevations so as to represent an even slope from the Sierra Nevada eastward through the Clan Alpine Mountains to the

Figure 16. Schematic proﬁles of the western slope of Great Basin Altiplano projected eastward to

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 the Clan Alpine Mountains and New Pass Range in the western Great Basin. Points represent the bases of Oligocene–Miocene ash-flow tuff sections projected onto a WE profile at the latitude of Donner Summit. The two elevations shown for the Clan Alpine Mountains correspond to the CCA and NCA sections of Hudson and Geissman (1991). The west margin of the Sierra Nevada structural block is placed at the present east margin of the Sacramento-San Joaquin Valley. (a) Estimates assuming second stage of major uplift starting in late Cenozoic time (e.g. Wakabayashi and Sawyer 2001). Slope of gravel with confidence limits extrapolated from Yeend (1974) using method of Lindgren (1911) for western sector of 4.5 ^ 0.5 m/km (20–25 feet/mile) and using Yeend’s estimate of 11.8 ^ 0.5 m/km (60–64 feet/mile) eastward. Distances and elevations shown for present day and reconstructed distances adjusted to 50% extension in the Great Basin. (b) Estimates assuming that nearly all uplift occurred prior to deposition of Eocene Auriferous Gravels, so that present slopes in the Sierra Nevada are nearly the same as existed in late Eocene time (Mulch et al. 2006). Overall, slopes of Auriferous Gravels to elevation of 1.7–1.8 km in northern Sierra (Mulch et al. 2006) are linear interpolations to Donner Summit. Distances eastward from east margin of Sierra Nevada structural block are adjusted assuming 50% extension in the present Carson Sink and Walker Lane Belt after emplacement of middle Cenozoic ignimbrites. Elevations of tuffs are adjusted to fit a hypothetical slope between Donner Summit and our assumed elevations of ignimbrite sections in Stillwater, Paradise, Clan Alpine, and New Pass mountain ranges approaching 4 km. Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 622

California Nevada Utah

Great Basin “altiplano”

Wet western slope Dry plateau (3-4 km high) Deeply incised, tuff-filled valleys Poorly integrated drainage

Line of section Best M.G. Topographic Ignimbrite flattened surface barrier 4 CNCC IPCC 2 0 al. et Elevation (km) 20 Sierra Nevada Oceanic Phanerozoic batholith 40 lithosphere accreted terranes Proterozoic crust ? ? ?

Depth (km) 60 Subcontinental lithospheric mantle

Figure 17. Conceptual WE cross-section through the middle Cenozoic Great Basin Altiplano at approximately 38.58 N latitude. International Geology Review 623

New Pass Range. These adjustments imply overall slopes less than that of the present Sierra Nevada result and result in a rather even gradient from sea level to nearly 4 km elevation in middle Cenozoic time. Although the construction of Figure 16(b) involves a circular argument it is our intent to show the much greater plausibility of the newer model for timing of uplift of the Sierra Nevada in the context of a middle Cenozoic Great Basin Altiplano. Moreover, the form of Figure 16(b) is not entirely different from the Andean Pilcomayo profile displayed in Figure 15(a), with due allowance for the difference between a hypothetical profile and one based on real topography. Figure 17 portrays our concept of the Great Basin Altiplano or orogenic plateau in a west-east cross-section near 38.58 N Latitude. The topographic barrier to dispersal of ash flows lies at the crest of and between the western slope of the Altiplano and its smoothed interior. The present crest of the Sierra Nevada lies roughly midway in that slope.

Demise of the Great Basin Altiplano If the middle Cenozoic Great Basin crust was 60–70 km thick during the ignimbrite flareup, as the composition of the lavas suggest, how was the transition made to the 30 ^ km thickness today? With an approximately 50% province-wide extension (McQuarrie and Wernicke 2005) since the flareup the crust should now be in the order of 40–47 km thick, assuming a closed system. If we assume an open system, as discussed in the introduction, then how is it possible to eliminate about 10–17 km of crust? McQuarrie and Chase (2000) suggest that the Colorado plateaus crust has been thickened by intracrustal flow of hot ductile rock from the eastern Great Basin, causing it to thin. They do not specify exactly how much crust was swept eastward, driven by the topographic head of the orogenic plateau (Altiplano), but the lower part of the range of 10–17 km would appear more reasonable. Another mechanism to thin the Great Basin crust since the middle Cenozoic is by delamination of the lower part. Although the details of delamination are complex, Kay and Kay (1993) conclude that, where the crust is more than about 50 km thick rocks of basaltic composition in the lower crust consist of a dense eclogitic phase assemblage that can readily separate from the overlying crust if the underlying lithospheric mantle has delaminated, or delaminates with the crust. In the Great Basin area, a considerable volume of mantle-derived basaltic magma must have been intruded into the lower crust

Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 to power the silicic magma generating systems from which tens of thousands of cubic kilometres of dacitic and rhyolitic ash were erupted. However, geophysical studies of the eastern part of the Great Basin (Zandt et al. 1995; Hasterok et al. 2007) reveal that a few tens of kilometres of mantle lithosphere currently underlie the crust, thus precluding its delamination during the middle Cenozoic. Interestingly, the subcrustal lithosphere beneath the Andean Altiplano, where the ignimbrite flareup died within the past few million years, has not been completely removed (Beck and Zandt 2002). In a variation of the delamination model that overcomes the barrier imposed by the mantle lithosphere, the mass of dense garnet-pyroxene-bearing mafic rocks and ultramafic cumulates that remains after silicic magma generation in the lower crust develop Raleigh- Taylor convective instabilities that allows it to ‘drip’ into the underlying less dense mantle (Jull and Keleman 2001; Dufek and Bergantz 2005). Although many factors govern the instability, reasonable boundary conditions, particularly for thicker crust, predict such piecemeal thinning on a time scale of several millions of years. 624 M.G. Best et al.

Conclusions Geologists over the past two decades have hypothesized from different lines of evidence that the Great Basin area was a high orogenic plateau after contractile deformation and thickening of the crust during Mesozoic–early Cenozoic mountain building. Although many are of the opinion that the plateau experienced gravitational collapse and crustal thinning soon after the deformation, that is, before and during the middle Cenozoic ignimbrite flareup, we have reason to doubt this timing. The prodigious ‘supervolcanic’ volume of silicic calc-alkaline magmas erupted during the ignimbrite flareups in the Great Basin area and in the central Andes since the middle Miocene are believed to be a direct consequence of an unusually thick crust (Maughan et al. 2002; de Silva et al. 2006). Because the lower to middle crust was unusually hot, it was a fertile site for silicic magma generation as vast amounts of mantle-derived magma were intruded into it. The unique monotonous intermediates, or crystal-rich high-K dacite tuffs, that constitute a significant proportion of the ignimbrites in both locales (Maughan et al. 2002) are best explained by evolution in pancake-shaped magma chambers resulting from magma accumulation within a thickened crust, where compressive tectonic and gravitational stresses are more or less balanced. To provide an independent assessment of the crustal thickness in the Great Basin we examined chemical and Sr-isotopic parameters sensitive to crustal thickness in 376 samples of 42–17 Ma mafic lava flows in the Great Basin and compared these to .6000 analyses of similar but mostly late Cenozoic lavas extruded in continental arcs, where the thickness of the crust has been geophysically determined. The compositional parameters clearly indicate that the middle Cenozoic crust in the Great Basin was significantly thicker than the present 30 km, likely 60–70 km. Isostatically, this thickened crust would have had a high surface elevation, thus indirectly supporting palaeobotanical and stable isotopic data claimed by many workers to indicate high elevation during the middle Cenozoic. The widespread areal extent of ignimbrite sheets, the small variation in palaeomagnetic directions between sample sites within an individual sheet, and the absence of intervening sedimentary deposits between sheets imply the depositional surface, especially during the closing of the flareup, was a smoothed landscape. Altogether, the character of volcanic rocks is consistent with a high, relatively flat plateau, or Altiplano, in the Great Basin during the middle Cenozoic. After the ignimbrite flareup the orogenic Altiplano experienced significant collapse and crustal thinning to 30 ^ km by extensional faulting. The deep crust could have been thinned by ‘dripping’ of dense residues from the silicic Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 magma generating systems that created the ignimbrite flareup into the underlying less dense upper mantle and possibly by intracrustal ductile flowage into adjacent regions. In south-central Nevada, the systematic eccentric position of source calderas within most ignimbrite outflow sheets is consistent with a NS topographic barrier on the Altiplano that extended southward from just east of Austin and governed the dispersal of pyroclastic flows across the landscape. Ash flows erupted from. the central Nevada caldera complex were dispersed eastward, whereas ash flows erupted from sources west of the barrier flowed westward and at least some entered into stream valleys draining the western margin of the Altiplano. The exact nature and origin of the topographic barrier are uncertain. Possibilities include a nearby and essentially parallel chain of domical uplifts and intrusions along the western edge of the Precambrian basement overrun by Palaeozoic–Mesozoic thrust sheets and a topographic ridge between the western slope of the Altiplano and its smooth interior that resulted from an interplay of heavy orographic precipitation on the slope and consequent focused denudation and isostatic uplift. International Geology Review 625

Acknowledgements This work is an outcome of a larger project on the middle Cenozoic Great Basin ignimbrite province ﬁnancially supported by the National Science Foundation through grants EAR-8604195, -8618323, -8904245, -9104612, and -9706906 awarded to M.G. Best and E.H. Christiansen. The support of Brigham Young University and, in the early stages of the project, the US Geological Survey is also gratefully acknowledged. Olivier Bachmann brought the paper by Dufek and Bergantz (2005) to our attention. We are indebted to David John and Chris Henry for very constructive reviews of an early version of the manuscript that provided a sharper focus on its content.

Note †Present address: US Department of Energy, Ofﬁce of Civilian Radioactive Waste Management, 1551 Hillshire Drive, Las Vegas, 89134 Nevada.

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