Paleoseismic Trench Investigation of the Petersen Mountain Fault, North Valleys-Reno, Nevada

Date: 4/24/2019

Time: 06:00 PM

Room: Fifth Avenue

The ~25-km-long Petersen Mountain fault (PMF) bounds the eastern side of Petersen Mountain north of Reno, NV within the northern Walker Lane. The fault is one of a series of north-striking faults that bound basins (known as the North Valleys) oriented oblique to major Walker Lane strike slip faults. The fault has been previously characterized as a late Quaternary normal fault (<130 ka), however information on its style and rate of deformation are poorly understood. Characterizing the PMF is important for assessing regional seismic hazards for the Reno metropolitan area.

Here we present preliminary results from a detailed paleoseismic trench excavated across a 3-m-high fault scarp along the eastern strand of the PMF. The site was selected by evaluation of lidar elevation and aerial photography data, and field mapping. The trench exposed thin alluvial-colluvial sediments overlying massive disintegrated granite sand. The fault trace is characterized by a wide shear zone that extends 10 m west from the main fault seen in the trench. Within the shear zone, faults and fractures dip eastward between 70^o and 80^o, and propagate through the granitic sand into the overlying soil. The footwall consists of disintegrated granitic sands overlain by debris flow alluvium and a thick argillic soil horizon. The hanging wall consists of a massive bouldery colluvium that fines upward into a weak soil, buried soils were not observed. The stratigraphic juxtapositions provide evidence for at least one earthquake that occurred after development of the footwall soil and suggest a component of lateral movement consistent with the presence of linear valleys, and subtle right offsets of alluvial channels south of the trench. Pending Optically Stimulated Luminescence (OSL) and radiocarbon dates will help constrain the ages of sedimentary units and provide constraints on the recency of faulting. We infer that pre-existing normal faults within the northern Walker Lane may be accommodating a component of shear in the contemporary stress field.

Presenting Author: Conni De Masi

Authors