LATE OLIGOCENE PALEOTOPOGRAPHY AND STRUCTURAL EVOLUTION OF THE PAH RAH RANGE, WESTERN NEVADA: IMPLICATIONS FOR CONSTRAINING SLIP ON THE RIGHT-LATERAL WARM SPRINGS VALLEY FAULT IN THE NORTHERN WALKER LANE

Left-stepping, NW-striking dextral faults in the northern Walker Lane accommodate ~15-20% of the North American-Pacific plate motion. Tuff-filled Oligo-Miocene paleovalleys provide piercing lines from which to gauge offset across the strike-slip faults. To constrain offset across the NW-striking dextral Warm Springs Valley fault (WSF), detailed mapping and structural analysis were carried out in the Pah Rah Range (PRR) on a highly faulted ~750 m thick, 31-23 Ma sequence of ash-flow tuffs that fill an ~E-trending paleovalley carved in Mesozoic basement. The tuffs are locally interbedded with fluvial gravels and pinch out southward against a well-exposed paleovalley margin. Mid Miocene mafic lavas and conglomerate disconformably overlie the Oligo-Miocene tuffs.

A complex array of kinematically linked NW-, NE-, N-, and E-striking faults and NW- to WNW-trending folds deform strata in the PRR. A large, open, upright, gently NW-plunging anticline near the range crest is probably extensional in origin and marks the intersection of oppositely dipping systems of NW-striking normal faults and associated tilt domains. Proximal to the WSF, however, strata are deformed into tight, WNW-trending, gently plunging folds in both the PRR and northern Curnow Range (NCR). These folds probably resulted from a combination of drag, CW rotation, and N-S shortening along the WSF. The SW limb of the extensional anticline in the PRR is overprinted by a WNW-trending fold near the WSF, suggesting that regional extension preceded the onset of strike-slip faulting. Paleomagnetic data indicate negligible vertical-axis rotation of the PRR but significant CW rotation (>40 deg) within ~2 km of the WSF.

The paleovalley in the PRR may correlate with other paleovalleys across the WSF, either on the NW flank of Dogskin Mt (DM) or in the NCR. A correlation with the DM paleovalley implies ~20 km of dextral offset on the WSF. However, some stratigraphic differences between these paleovalleys and the apparent termination of the WSF only 10 km SE of the study area argue against such a correlation. Instead, similar stratigraphy and structures favor a correlation between paleovalleys in the PRR and NCR. Such a correlation indicates ~4-8 km of dextral offset along the southeast part of the WSF.