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 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.