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Group A, Poster #093, Earthquake Geology

The Bonham Ranch fault, Nevada: paleoseismic observations along the eastern margin of the northern Walker Lane

Rich D. Koehler
Poster Image: 

Poster Presentation

2022 SCEC Annual Meeting, Poster #093, SCEC Contribution #12255 VIEW PDF
The Bonham Ranch fault splays off the northwest trending Pyramid Lake fault (a major strike-slip fault of the northern Walker Lane) and extends north for >50 km along the western margin of Smoke Creek Desert, Nevada. In this area, approximately 5-7 mm/yr of dextral shear is accommodated across the northern Walker Lane and a small amount of shear and extension is partitioned to faults in the western Basin and Range. The Bonham Ranch fault accommodates part of this deformation by normal faulting. Prior paleoseismic trenching has not been conducted along the fault and paleoseismic rupture parameter data has only been inferred from reconnaissance mapping.

Here, I present m...
apping and paleoseismic trenching observations along the Bonham Ranch fault based on interpretation of lidar data and two trench exposures. Preliminary mapping results indicate that the Bonham Ranch fault is expressed as continuous east-facing 1-5 m scarps that displace latest Pleistocene and Holocene lacustrine and alluvial fan deposits along the western side of Smoke Creek desert. The range front was submerged and buried in places by deposits associated with pluvial lake Lahontan during its highstand (~15 ka), however, the range front does not exhibit active faulting morphology typical of Basin and Range normal faults (i.e. triangular facets, wineglass canyons). Trench exposures show deep water lacustrine clays interbedded with fine sand and two distinct tephra layers that are faulted across two primary fault strands. The tephra layers may correlate to the Wono and Trego Hot Springs tephras (~25 ka) which are widespread in the area. Two packages of post faulting deposits bury the lacustrine deposits on the hanging wall, the oldest of which is faulted providing evidence for the occurrence of at least two latest Pleistocene/Holocene events. Radiocarbon, optically stimulated luminescence, and tephrachronologic analyses are in progress and will place constraints on the timing of the earthquakes. This ongoing work will contribute towards a better understanding of regional seismic hazards and the role of the Bonham Ranch fault in partitioning slip along the margin of the northern Walker Lane.