Ash-flow tuffs in the Nine Hill, Nevada, paleovalley and implications for tectonism and volcanism of the western Great Basin, USA

Cenozoic ash-flow tuffs are key units for analyzing the tectonic and magmatic evolution of the Great Basin. The tuffs are commonly assumed to have spread nearly radially from source calderas and to have formed nearly continuous, flat-lying deposits. Based on detailed mapping and paleomagnetic investigations of the Nine Hill paleovalley in western Nevada and analysis of the regional distribution of the 27–23 Ma tuffs that crop out in the paleovalley, we find numerous features that differ markedly from these presumed characteristics. Many individual ash-flow tuffs can be correlated from their source calderas in the central Nevada caldera belt westward into the Sierra Nevada of eastern California. Present-day distances from source to distal, primary tuff deposits are as much as ∼295 km. Corrected for extension, original flow distances were as much as ∼200 km. The tuffs flowed, were deposited, and are preserved primarily in deep (as much as 1.2 km) but wide (8– 10 km) paleovalleys. The tuffs were able to flow these great distances because they were channelized, did not disperse, and did not mix with air as much as would tuffs that spread more radially. Tuffs probably spread more radially only within a few tens of kilometers of source. The paleovalleys held major rivers that drained from a “high” plateau in the present Basin and Range Province and Walker Lane and flowed across the Sierra Nevada to the Pacific Ocean in what is now the Great Valley. The Basin and Range–Sierra Nevada structural and topographic boundary did not exist before 23 Ma, the age of the youngest tuff in the Nine Hill paleovalley. Any faulting in western Nevada before 23 Ma was insufficient to disrupt the paleodrainages other than temporarily. Deposition of tuffs in paleovalleys produced several features relevant to interpreting the tectonic evolution of the region. Most tuffs show primary dips, commonly up to ∼20° and locally up to near vertical, because they compacted against gentle to steep paleovalley walls. Angular unconformities, unrelated to tectonism, are common where tuffs were deposited on eroded tuffs that had primary dips. The tuffs are commonly interbedded with coarse clastic deposits that originated either as channel deposits in high discharge, possibly high gradient rivers, or from floods induced by failure of dams, whereby a tuff temporarily blocked drainage. Tuffs show highly asymmetric, elliptical distributions, preferentially west of their source calderas, because they flowed in the westward-draining paleovalleys. Although deposited continuously along the paleovalleys, tuffs were subsequently eroded by the rivers, so that tuff distributions are highly discontinuous both between and along individual paleovalleys. Thicknesses of tuffs vary irregularly with distance from source, and some tuffs are as much as 300 m thick even 150 km (primary distance) from source. Thickness variations probably are in part due to variations in width and depth along paleovalleys. Estimates of tuff volumes should not assume radial distribution or continuous deposition between paleovalleys, which give unreasonably large volumes. In ideal cases where the source caldera is well understood, the best way to estimate erupted volume can be from the volume of caldera collapse (i.e., area within ring fracture × amount of subsidence).