The Great Basin Altiplano during the middle Cenozoic ignimbrite flareup: insights from volcanic rocks

Abstract

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

The Confusion Range, west-central Utah: Fold-thrust deformation and a western Utah thrust belt in the Sevier hinterland

“…Late Cretaceous to middle Cenozoic paleogeography in the region likely consisted of a high-elevation, low-relief plateau referred to as the "Nevadaplano," with a steep topographic front and foreland basin to the east (Coney and Harms, 1984;DeCelles, 2004;Best et al, 2009;Henry et al, 2012). Paleogene conglomerates, lacustrine limestones, and interbedded volcanics were deposited in local basins and paleochannels, possibly related to early extensional collapse within the plateau (Vandervoort and Schmitt, 1990;Constenius, 1996;Greene and Herring, 1998;Hintze and Davis, 2003;Druschke et al, 2011;Lechler et al, 2013).…”

“…Paleogene conglomerates, lacustrine limestones, and interbedded volcanics were deposited in local basins and paleochannels, possibly related to early extensional collapse within the plateau (Vandervoort and Schmitt, 1990;Constenius, 1996;Greene and Herring, 1998;Hintze and Davis, 2003;Druschke et al, 2011;Lechler et al, 2013). Widespread pyroclastic volcanism, the "ignimbrite fl areup" (Coney, 1978;Best et al, 2009Best et al, , 2013, began in the late Eocene and continued through the early Miocene. Beginning in the early Miocene, predominantly high-angle extensional faulting formed the typical Basin and Range topography observed today (e.g., Dickinson, 2006).…”

“…Large thrust displacements and surfacebreaching faults in the frontal zone created high topographic and structural relief, as indicated by structural culminations, high exhumation, and foreland basins with coarse synorogenic clastic fi ll (DeCelles et al, 1995;Currie, 2002;Long, 2012). In the hinterland west of the frontal zone, folding and predominantly subsurface thrust faulting broadly distributed in regional thrust belts thickened the crust and maintained relatively high elevation (3-4 km) but low topographic relief (DeCelles, 2004;Best et al, 2009;Henry et al, 2012).…”

Petrogenesis of mafic collision zone magmatism: The Armenian sector of the Turkish–Iranian Plateau

“…Note that samples from western Lori (Tashir/Dzoraged) are labelled 'S' in this work as they were collected during a sampling mission largely conducted in Shirak Province in 2011. Yerevan and Odzun/Sanahin/Alaverdi samples were collected in 2012. extensive crustal melting due to contact between lower crust, hot upwelling asthenosphere, and asthenosphere-derived magmas (Babeyko et al, 2002;Best et al, 2009). However, Turkish-Iranian Plateau magmatism is temporally and spatially random in nature (see Fig.…”

Eocene-Early Miocene paleotopography of the Sierra Nevada-Great Basin-Nevadaplano based on widespread ash-flow tuffs and paleovalleys

“…It is now well established that the Great Basin (western United States) was an erosional highland in the middle Cenozoic, following crustal thickening during Mesozoic-early Cenozoic shortening, batholith emplacement, and shallow slab subduction (e.g., Dilek and Moores, 1999;Humphreys et al, 2003;DeCelles, 2004;Geosphere, February 2012Dickinson, 2006Best et al, 2009;Cassel et al, 2009aCassel et al, , 2009bColgan and Henry, 2009;Ernst, 2010;Henry and Faulds, 2010). Major river systems drained this highland both west to the Pacifi c Ocean and east to the Uinta Basin ( Fig.…”