STRUCTURAL FRAMEWORK AND LATE OLIGOCENE PALEOTOPOGRAPHY OF THE PAH RAH RANGE, WESTERN NEVADA: IMPLICATIONS FOR ESTIMATING OFFSET ACROSS THE WARM SPRINGS VALLEY FAULT ZONE IN THE NORTHERN WALKER LANE

The Northern Walker Lane consists of a series of left-stepping, NW-striking dextral faults that accommodate ~15-20% of the North American-Pacific plate motion. The Pah Rah Range (PR) is a NW-trending fault block that occupies a left-step between the Pyramid Lake fault (PLF) to the east and the Warm Springs Valley fault (WSF) to the west. As part of an AASG/NSF Mentored Field Research Experience and senior research project, a transect of detailed geologic mapping was carried out across the western PR. The PR contains an ~700 m thick ~31-23 Ma sequence of at least 10 rhyolitic ash-flow tuffs and interbedded fluvial gravels, that filled an ~E-trending paleovalley carved into Cretaceous diorite and older metavolcanics. The relatively steep south margin of the paleovalley is well exposed and characterized by steeper compaction foliations in the tuffs. The ash-flows were erupted from calderas in central Nevada during the ignimbrite flare-up. Mid Miocene mafic lavas and conglomerate associated with the ancestral Cascade arc disconformably overlie the late Oligocene ash-flow tuffs.

Typical of transtensional settings, the western PR is cut by a complex array of multiple, apparently kinematically linked fault sets, including NW- to NNW-striking oblique dextral-normal faults, NE-striking oblique sinistral faults, and NNE-striking normal faults. On the SW flank of the PR, the NW-striking dextral-normal faults generally dip steeply NE and accommodated 15-30^o of SW tilting. Many of the NW-striking faults along the lower reaches of the PR may be splays of the WSF. In the north part of Warm Springs Valley (WSV), a broad WNW-trending syncline plunges moderately (~20^o)west. The fold probably formed through a combination of drag, clockwise rotation, and local shortening induced by dextral motion on the WSF. The main strand of the WSF may be buried in WSV directly south of the fold.

The paleovalley margin in the western PR serves as a potential piercing-line with which to estimate slip across the WSF. A strikingly similar paleovalley margin is exposed at Dogskin Mt on the southwest side of the WSF. If correlative, this would imply ~20 km of dextral offset on the WSF. However, other correlations implying less offset are possible across the WSF and may be more likely considering the apparent termination of the WSF only 10 km SE of the study area.