VOLCANOES OF THE MCCULLOUGH RANGE, SOUTHERN NEVADA: A WINDOW INTO THE PRE-EXTENSIONAL HISTORY OF THE COLORADO RIVER EXTENSIONAL CORRIDOR

The McCullough Range is a geologically unique part of the Colorado River extensional corridor. Although adjacent ranges were intensely faulted during Basin and Range extension, the interior of the McCullough Range is relatively undeformed. Thus, volcanic centers in the McCullough Range are well preserved providing an exceptional opportunity to study the pre-extensional history of the Colorado River extensional corridor. The range contains at least four volcanic centers; the McCullough Pass caldera, Hidden Valley volcanic center, central McCullough Range stratovolcano, and Henderson caldera. The McCullough Pass caldera is a club shaped depression formed by the eruption of the McCullough Pass tuff at 14 Ma. Filling the caldera are rhyolite domes, block and ash flows, and volcaniclastic deposits. Caldera formation and infilling occurred over the short period between 14 and 13.9 Ma. The Hidden Valley volcanic center (15.23 to ~12 Ma) is a 13.5-km diameter depression filled by mid-Miocene andesite and dacite domes, flows and pyroclastic flows. Forming the north central McCullough Range is a stratovolcano (~15 to ~12 Ma) composed of a 2500- to 3000-m thick section of andesite interbedded with thin conglomerates, debris flows and volcaniclastic sandstones. The stratovolcano is truncated to the north by the 10-km diameter Henderson caldera. Filling the caldera are biotite dacite and hornblende andesite domes and flows, pumice rich ash-flow tuff, and mesobreccia deposits. The 20- m thick tuff of the Henderson caldera grades from a pumice poor base to a pumice rich top and contains sanidine, quartz, and biotite phenocrysts and andesite xenoliths. A sharp contact between the andesites of the central McCullough Range stratovolcano and a mesobreccia containing < 25 cm andesite clasts in a matrix of the tuff of the Henderson caldera marks the southern wall of the Henderson caldera. Current residential development in the City of Henderson is occurring on and between dacite flow lobes on the north side of the caldera. The ongoing study of the Henderson Caldera and other volcanic centers of the McCullough Range provides geologic data for the development of the Sloan Canyon National Conservation Area as well as providing insight into the evolution of the Colorado River extensional corridor.