7 Contributions from Geophysics to Better Understanding the Structure and Tectonics of the Western United States

Monday, 11 May 2009
1:00 PM - 5:00 PM

INVESTIGATING SHALLOW FAULTING IN THE AMARGOSA DESERT (NYE COUNTY, NEVADA) USING SH-WAVE REFLECTION PROFILES

Paper number: 7-6
Program: Theme Sessions
Time: 2:40 PM – 3:00 PM

Authors

Abstract

Fault patterns have previously been suspected to govern the discharge of ground waters in the Amargosa Desert in southwestern Nevada. For example, alignments of springs have been interpreted previously as indicating the influence of a buried fault trending parallel or sub-parallel to mountain range fronts. Having developed in an extensional tectonic regime, such faults would be assumed to have experienced normal displacement and to offset shallow sedimentary strata. In order to test such a hypothesis, we conducted horizontally polarized shear-wave (SH) seismic reflection profiles across one of the major faults mapped in the area, the Gravity Fault, where it intersects a road near the northern margin of the Ash Meadows National Wildlife Refuge. The SH-wave profiles were conducted using a 12-channel “land-streamer” array of horizontal geophones with two phones combined for each channel so as to cancel compressional wave energy. The advantage of this technique is that it is easy to cover a significant area with a small field crew, is non-invasive, and provides results that are independent of the water table or any other fluid or gas effects. Our results thus provide a framework for interpreting shallow structural anomalies associated with geologically recent faulting. Three separate profiles were acquired over a three-day period. Two of the profiles crossed two separate strands of the Gravity Fault as previously mapped by the U. S. Geological Survey. These profiles also crossed linear geomorphic anomalies mapped independently. A third profile was surveyed where no fault had been previously mapped, as a “control” experiment. The Gravity Fault appears to correlate with subtle structural and/or stratigraphic changes where the traces of the previously interpreted fault strands are projected downward through the seismic profile. The “control” profile shows little or no evidence for fault displacements or other structural anomalies. Our findings indicate the ability of the SH-wave seismic reflection method to detect subtle shallow structural anomalies related to geologically recent faulting. The Gravity Fault offsets and disrupts shallow geological strata and thus may be expected to influence shallow discharge of springs. Partial support for this project was provided by Nye County, Nevada.