TECTONIC IMPLICATIONS OF ASH-FLOW TUFFS AND PALEOVALLEYS IN THE WESTERN US

Widespread ash-flow tuffs are some of the best deposits to evaluate tilt, extension direction and magnitude, and vertical axis rotation (when combined with paleomagnetic measurements). Geologists generally assume that compaction foliation in tuffs indicates paleohorizontal at the time of deposition.

Our investigations, including detailed mapping in western and northern Nevada and reconnaissance in the eastern Sierra Nevada, show that mid-Cenozoic (~40-20 Ma) tuffs erupted mostly from calderas in central Nevada. Near source, the tuffs spread relatively radially, but even proximal deposits were affected by significant paleotopography. Farther from source, tuff flow and deposition were focused into broadly W-trending paleovalleys that drained to the Pacific Ocean. Almost all preserved deposits in western Nevada and the Sierra Nevada are within these paleovalleys. The paleovalleys were major drainages before, and continued to be after, tuff deposition, so tuffs were rapidly eroded. Damming of drainages, as seen in Quaternary tuffs worldwide, probably induced catastrophic floods, rapid and deep erosion, and coarse clastic deposition.

These circumstances may produce features that can confuse interpretation of Cenozoic geology. (1) Primary dips are common. E.g., the widespread Nine Hill Tuff (NHT) has primary dips up to 70° in the essentially untilted Sierra Nevada. At its type locality, NHT dips ~50° inward on opposite sides (separated by ~1 km) of an at most gently tilted (*10° W), WSW-trending paleovalley. (2) Ash-flow tuffs are commonly interbedded with extremely coarse clastic deposits. Clasts up to 2 m in diameter are common and reach 12 m in paleovalleys north of Reno. (3) Both from irregular deposition and subsequent erosion, tuffs and interbedded clastic rocks show abrupt, along and across valley changes in thickness, including abrupt pinch out of units.

These features have led, in some cases, to misinterpretation of the presence of (1) angular unconformities, (2) faults where units thin and pinch out abruptly, (3) extensional or other faulting during ash-flow deposition (in W NV, all significant extension postdates ~16 Ma), and (4) vertical axis rotations because of incorrect tilt corrections. Although compaction foliation in tuffs may generally record paleohorizontal, this should not be assumed.