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. 120˚ 117˚ 114˚ 111˚ 109˚ Nevada aker 50 Wyoming 41˚ 40 Utah Colorado t l Salt e Luning Fencem b Lake t City s u 50 r Reno h Golconda t d 50 n a ountains Austin d 40 l 39˚ Ely o f Downloaded By: [Brigham Young University] At: 20:48 15 September 2009 r e i v e Roberts M S f o t n ro Tonopah Cedar F City Caliente 37˚ 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.