FAR-FLUNG ASH-FLOW TUFFS OF YERINGTON, WESTERN NEVADA ERUPTED FROM CALDERAS IN THE TOQUIMA RANGE, CENTRAL NEVADA

Ash-flow tuffs near Yerington are well known because their steep tilts led to some of the earliest recognition of high-magnitude extension in the western US. Despite considerable individual thickness and a composite thickness of ~2 km, all tuffs are outflow deposits from previously unknown sources. Our new geologic, 40Ar/39Ar, and petrographic data allow correlation of many Yerington tuffs with intracaldera tuffs and source calderas in central Nevada. The oldest cooling unit of the Mickey Pass Tuff (MPT; Guild Mine 1; 27.00±0.06 Ma) is petrographically unlike the rest of the unit and may correlate with the tuff of Ryecroft Canyon (27.08±0.09 Ma), an intracaldera unit in the eastern Toquima Range. The main part of the Guild Mine member (units 2, 3, and 4; 26.94±0.02 Ma, n=4) correlates with the tuff of Moores Creek and the lower tuff of Mount Jefferson (26.90±0.05 Ma, n=5), which erupted from the Toquima caldera complex. A correlative intracaldera tuff is not recognized for the upper, Weed Heights member of the MPT (26.76±0.06 Ma, 26.77±0.06 Ma). However, the intracaldera, upper tuff of Mount Jefferson (26.67±0.02 Ma; n=3) correlates with the Lenihan Canyon Tuff (26.60±0.06 Ma) near Carson City. The Singatse Tuff, the youngest major unit near Yerington, consists of two cooling units (26.52±0.05 Ma; 26.50±0.06 Ma). They probably correlate with the tuff of Round Mountain (26.52±0.02 Ma; n=5), which erupted from a caldera along the west flank of the Toquima Range. For all correlated units, phenocryst assemblages and proportions match well except for mafic phenocrysts in Guild Mine 1 – Ryecroft Canyon.

The Yerington tuffs are ~200 km west of their source calderas, and several tuffs occur as much as 60 km farther west. Even allowing for 100% extension, an unlikely total across the whole area, the tuffs flowed at least 130 km from source. This indicates that Oligocene topography was unlike the present basin and range topography and supports major extension beginning after 26.5 Ma in the western Basin and Range. In part, the tuffs flowed in paleovalleys that drained westward to the Pacific Ocean in what is now the Great Valley. Deposition in paleovalleys promoted long transport distances and great local thicknesses of outflow tuffs. Such paleovalleys are potential piercing points to measure lateral displacement across the Walker Lane.