Photos showing UNR Geology students in the field working on the seismic line.
Presented as a poster to the American Geophysical Union Fall Meeting, San Francisco, Calif., December 5, 1994.
We attempt to characterize faulting along three strands of the NW trending, down to the west and presumed right-lateral Pahrump Valley Fault Zone (PVFZ), as well as overall basin geometry.
The surveys include detailed conductivity and seismic surveys at the scarps of the PVFZ and valley-wide gravity and magnetic surveys.
We measure apparent conductivity averaged over a depth of 3 to 6 m, and total-field ground magnetism. The seismic profiles employ a high-resolution hammer technique. We use a Worden gravimeter for the gravity survey.
Photos showing UNR Geology students in the field working on the seismic line.
Location map showing Pahrump Valley, with approximate location of survey line marked in red. Dark lines show mapped or inferred faults, while the shaded line shows a hypothesized shear zone. Taken from Hoffard, 1991.
Combined plot of magnetic and gravity data across Pahrump Valley with topography. Vertical shaded lines show the location of the PVFZ scarps.
Combined plot of magnetic and conductivity data across the PVFZ scarps with
topography. The magnetic and conductivity anomalies correlate well with the scarp 1, and are present though less pronounced at scarps 2 and 3.
Scarp 1 appears steepest, and morphologically youngest.
Plot of four offset conductivity profiles across scarp 1, showing continuity of the anomaly along strike. The other two scarps show a similar trend.
Stacked seismic sections across the two western-most scarps. We interpret the strong reflector as an ash bed which we see outcropping near scarp 2.
Interpretations of the stacked seismic sections. While vertical offset is not constrained, the ashbed reflector is clearly truncated in both sections.
Smoothed gravity data and corresponding basin-depth model estimated with
Talwani's method. The thick, dashed line is a general
interpretation of the basin model.
Two attempts at modeling the magnetic data at scarp 1, using a susceptibility
contrast of 0.001 emu. The vertical prism provides the best fit of the many different geometries tried. Also shown is an offset horizontal layer for comparison, which does not match the anomaly shape.
Hoffard (1991) documented numerous occurrences of hot-spring mineralization on the PVFZ, though none active at present.
Minerals such as hematite and pyrite can occur with hot-spring deposits and quantities around 1% above the background level in the lake sediments could account for the observed magnetic anomalies.
The conductivity anomalies could be due simply to changes in water content between the fault and the surrounding sediments, however because the anomaly is so shallow, mineralization may also be important. The same minerals that cause the magnetic anomalies, if present near the surface, could also cause the conductivity anomalies.
The gravity data and basin model imply the PVFZ cuts the basin bottom and is a significant structural feature. We conclude from the presence of sizable magnetic and conductivity anomalies and the truncated ashbed in the seismic sections that the PVFZ is also a recent feature. The seismic sections also indicate most if not all of the recent motion is strike slip.
Relative anomaly magnitude can provide relative fault ages.
This type of survey may reveal fault locations in areas of recent but buried or eroded fault scarps.
Telford, W.M., L.P. Geldart, and R.E. Sheriff, 1990, Applied Geophysics. Cambridge: Cambridge University Press.
Wang, C.-Y., 1984, On the consitution of the San Andreas fault zone in central California, J. Geophys. Res., 89, 5858-5866.
