IMPLICATIONS OF WIDESPREAD, PALEOVALLEY-FILLING ASH-FLOW TUFFS OF THE WESTERN GREAT BASIN FOR PALEOTOPOGRAPHY, REGIONAL TECTONICS, AND TUFF VOLUMES

Myron Best has been a leader in studying ash-flow tuffs of the ignimbrite flareup, the most voluminous volcanic rocks of the Great Basin. His contributions to understanding the character, distribution, sources, and volumes of these rocks have been essential in resolving both the magmatic-tectonic history of the region and magmatic-tectonic processes in general. Our mapping, geochronology, and paleomagnetism document that individual 31-23 Ma ash-flow tuffs of the western Great Basin commonly flowed as much as 200 km (estimated pre-extension distance) from sources in the central NV caldera belt. Enormous, preferentially westward flow distances resulted from focusing flow in paleovalleys that drained westward toward the Pacific Ocean, in the Great Valley at the time, with a probable north-trending paleodivide through east-central NV. Paleovalley flow inhibited radial dispersion and cooling by mixing with air. The paleovalleys were deep (0.5-1.5 km) but broad (commonly 8-10 km) and separated by relatively low-relief interfluves, consistent with a "high" but mature Nevadaplano as proposed by many. The Sierra Nevada constituted only a western flank to this high plateau in the mid-Cenozoic. Tuffs at least as young as the correlative, 23.1 Ma tuff of Toiyabe/Santiago Canyon Tuff from the Toiyabe caldera flowed these great distances, which precludes significant disruption of the paleovalleys by either normal or strike-slip faulting pre-23 Ma. Preferential flow and deposition of tuffs in paleovalleys resulted in highly asymmetric distributions, offset westward from source calderas, and having thicknesses that decrease very irregularly with distance from source. These complexities hinder calculation of eruption volumes but imply much lesser volumes than if the tuffs were assumed to have spread more radially. The best way to estimate erupted volumes is probably from the area and amount of collapse of source calderas. For example, the 27.1 Ma Mickey Pass Tuff, which flowed ~210 km from its source in the Toquima caldera complex, is as much as 300 m thick 150 km from source, which could imply an implausible, radially determined volume of about 21,000 km³ (V=πr²t=π150²x0.3). The ~400 km² caldera has probably undergone several km of collapse, suggesting a more realistic V=1200-1600 km³.