STRUCTURAL AND TOPOGRAPHIC EVOLUTION OF THE SIERRA NEVADA BASED ON FAR-TRAVELED OLIGOCENE-MIOCENE IGNIMBRITES

The paleoelevation and uplift history of the Sierra Nevada, including differences between the northern and southern Sierra Nevada (NSN, SSN), are controversial. My published work on ignimbrite distribution in the NSN and Great Basin show that ignimbrites erupted from the western half of a present-day NW-SE belt of calderas from western Nevada to western Utah astride the interpreted Nevadaplano (NP). The western half of the caldera belt lies west of a paleodivide in the NP. Tuffs erupted from the western half flowed westward down paleovalleys across what is now the Basin and Range – NSN structural–topographic margin. The paleovalleys in the NSN are the famed auriferous channels. The ignimbrites, the world’s most far traveled, flowed as much as 280 km (pre-extension distance) from source. Although great flow distances are consistent with calderas at 3-4 km elevation in the NP, source elevations greater than ~1.5 km cannot be proven by ignimbrite or paleovalley data alone. Confined, paleovalley flow probably is the critical factor in great flow distance.

Cenozoic magmatism and caldera-forming eruptions swept southwestward across the Great Basin. Because erupted tuffs were channelized, paleovalley sequences young southward: e.g., tuffs in paleovalleys north of Reno are 31.5-25.3 Ma; south of Reno, 27.6-23.3 Ma. Tuffs are exposed in paleovalleys as far south as 37.8°. Tuff distribution across the NSN demonstrates that the eastern, fault-bounded flank could not have existed before 23 Ma. Farther south in the SSN, mid-Cenozoic deposits have been eroded.

Published data show that 36-20 Ma tuffs occur in Death Valley (DV). Based on these ages, the tuffs must have erupted from the coeval part of the caldera belt in central and eastern Nevada, northeast of the younger southwestern Nevada volcanic field (~15.7-7.6 Ma). Although some deposits are pyroclastic-fall or reworked tuffs, some are pyroclastic-flow deposits that could only get to DV by paleovalley flow. Whether these tuffs ponded in interpreted “old” extensional basins in the DV area or flowed into or across what is now SSN is unknown. Equivalent tuffs are not preserved in the deeply eroded SSN. Correlation of tuffs from source calderas to DV and examination of coeval deposits in the San Joaquin Valley can help resolve the uplift history of the SSN and tectonic evolution of DV.