THE GREAT BASIN ALTIPLANO DURING THE MIDDLE CENOZOIC IGNIMBRITE FLAREUP: INSIGHTS FROM VOLCANIC ROCKS

The configuration of middle Cenozoic ignimbrite sheets as well as the compositions of calc-alkaline lava flows show that the Great Basin was a relatively smooth plateau underlain by unusually thick crust, probably persisting after Mesozoic and earliest Cenozoic contractile deformation. A comparison of the compositions of intermediate-composition lava flows in the Great Basin with those from continental volcanic arcs with known crustal thickness indicates that the middle Cenozoic Great Basin crust was as much as 60-70 km thick. If isostatic equilibrium prevailed, this unusually thick continental crust must have supported high elevation. This high terrain was progressively smoothed as successive ignimbrite outflow sheets were emplaced, creating a relatively flat, high plateau similar in origin and features to the present-day Altiplano-Puna in the central Andes. Outflow ignimbrite sheets deposited from 35 to 23 Ma in southeastern Nevada were derived from source calderas to the west. Of the twelve major sheets, nine are distributed unevenly east of their eruptive sources while the remaining three sheets spread about as far east as west of their sources. This eccentricity of sources and sheets suggests that a north-south trending topographic barrier in central Nevada restricted westward dispersal of ash flows. Eastward dispersal of ash flows from sources farther west also seemed to be impeded by this topographic barrier. In addition, westward dispersal of the ignimbrites 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 traveled as far west as the western foothills of the Sierra Nevada. The nature and origin of the implied topographic barrier is uncertain. It is possible that heavy orographic precipitation on the western slope of the altiplano and consequent focused denudation and isostatic uplift created a north-south-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 Proterozoic basement and to a zone of thermal-diapiric domes that were spawned in thickened crust as the basement edge was over run by Late Paleozoic-Mesozoic thrust sheets