NEW OBSERVATIONS ON THE PALEOSEISMIC HISTORY AND SENSE OF SLIP ALONG THE PETERSEN MOUNTAIN FAULT, NORTH WESTERN NEVADA

The Petersen Mountain fault (PMF) is a ~25-km-long range-front fault system within the North Valleys area of Reno, Nevada in the northern Walker Lane. At this latitude, geodetic observations indicate ~7 mm/yr of northwest directed shear that is primarily accommodated along major northwest-striking strike-slip faults that bound the North Valleys. Within the North Valleys, low rates of oblique extension have been geodetically observed across Quaternary active north-striking normal faults, however evidence of lateral deformation has not been previously reported. The PMF consists of two parallel strands and had been previously characterized as a Quaternary normal fault. Here we present results of a Quaternary geologic mapping and trenching study along the PMF and provide new information on the paleoseismic history and style of deformation.

We excavated a paleoseismic trench across a 3-m-high fault scarp along the eastern strand of the PMF. The trench exposed thin alluvial debris flow deposits overlying massive disintegrated granite sand in the footwall, a 10-m-wide shear zone, and a massive boulder alluvial deposit in the hanging wall that fines upward into a weak soil. A thick, well-developed argillic soil is developed into the footwall package. Within the shear zone, faults and fractures dip eastward between 70^o and 80^o, and propagate through the granitic sand to the overlying soil. Vertically offset units, colluvial wedges, and buried soils commonly associated with normal displacement were not observed. Thus, based on the juxtaposition of stratigraphic units and soils, we infer that the exposure records evidence for strike-slip displacement and interpret the occurrence of at least one late Quaternary earthquake that postdates the age of the footwall argillic soil (OSL ages pending). New mapping of geomorphic features consistent with strike-slip displacement include right deflected alluvial channels, a NW oriented fault parallel linear valley, a small pull-apart basin and left-stepping en echelon springs. These observations suggest that pre-existing normal faults within the North Valleys may be accommodating a component of Walker Lane shear.