Paleoseismic Investigation of the Freds Mountain Fault and the Western Lemmon Valley Fault Zone, North Valleys-Reno, Nevada

Date: 4/24/2019

Time: 11:15 AM

Room: Vashon

A series of fault-bounded basins north of Reno, NV, collectively known as the North Valleys, form an important structural linkage between some of the most active faults in the Basin and Range: the Carson Range fault system to the south; and dextral faults of the northern Walker Lane to the north. To improve characterization of the seismic hazard posed by the faults this study used new lidar data to remap faults in Lemmon and Antelope Valleys and paleoseismic trenching to determine the earthquake history and slip-rate of the Freds Mountain fault (FMF).

On the west side of Lemmon Valley a trench was excavated across a prominent scarp on a mapped, range-front fault. The trench exposed lacustrine sediments and no evidence for faulting. It was determined that the scarp is a wave cut shoreline formed by a Pleistocene lake in Lemmon Valley. Identification of the shoreline suggests that the highstand of Lake Lemmon reached the basin outlet elevation and spilled into the topographically lower Lake Lahontan in the latest Pleistocene. Recognition of the larger lake extent and the existence of fault parallel shorelines facilitated new mapping of the Western Lemmon Valley fault zone (WLVFZ), including removal of some previously mapped fault traces. New vertical separation measurements on WLVFZ fault scarps in nearby late Pleistocene fan deposits range from 5.6-6.7 m, yielding slip-rate estimates of 0.05-0.6 mm/yr. These fault scarps cross the lake highstand elevation with sharp morphology suggesting post-highstand deformation, weighting the higher slip-rate estimates.

To the north in Antelope Valley a second trench was excavated across a scarp on the FMF. The trench exposed faulted alluvial-fan deposits, evidence for 4 surface rupturing earthquakes, and yielded an average total vertical displacement estimate of 9.6 m. Age estimates of the paleo-earthquakes and displaced stratigraphy are pending OSL and radiocarbon analyses. New lidar-based mapping of the FMF suggests significant along-strike variability in slip-rate indicating the fault may have a segmented rupture history.

Presenting Author: Seth Dee

Authors