Geophysical Investigation of Recent Faulting in the East
Gardnerville Basin
Weston Thelen, Jim B. Scott, Matthew Clark and John N. Louie
Seismological Laboratory, University of Nevada, Mail Stop 174, Reno, NV 89557
Abstract
Using three simple and inexpensive geophysical methods, we have characterized small active
faults in the east Gardnerville Basin, Nevada. Shallow seismic reflection, total field ground
magnetics and apparent resistivity identify poorly understood features and, in some cases, their
three-dimensional geometry. Faulting in the area is associated with slightly magnetized zones,
and in one case, silicification. Saturated sediments, producing an apparent-resistivity anomaly
across the faults are due to impermeable clay-rich fault zones. In one area, seismic imaging
confirms faulted stratigraphy in the area of both resistivity and magnetic anomalies. Our results
contribute to understanding the tectonic history of this part of the Walker Lane. The presence of
active faulting in the east Gardnerville Basin could significantly alter local earthquake hazard
analyses.
Introduction
The east Gardnerville Basin is situated in the
transition zone between the Sierra Nevada mountain
range and the Basin and Range province (Fig.1). At
the base of the Carson Range to the west, lies the
Genoa Fault, the most active fault zone in western
Nevada (Ramelli et al., 1999). In the eastern part of
the basin lies a 14 km-wide fault zone adjacent to the
Pine Nut Mountains (Maurer, 1984; Bell and Hoffard,
1990). The Gardnerville Basin represents one of the
largest and deepest basins in the western Great Basin
(Trexler et al, 2000). Understanding the tectonic
evolution of the basin is therefore important in
understanding the most recent evolution of the Basin
and Range.
Great Basin extension initiated near the center
of the province and reached the Gardnerville Basin
approximately 12 Ma (Dilles and Gans, 1995; Henry,
2001; Surpless et al., 2002). Additionally, a second
phase of extension may have occurred at 7 Ma (Dilles
and Gans, 1995; Henry, 2001; Surpless et al., 2002).
The east Gardnerville Basin is located in the Walker
Lane, a prominent strike-slip feature in the western
Great Basin (Muntean, 2001).
DePolo et al. (2000) mapped the east
Gardnerville Basin with a multitude of short north
trending, east-dipping normal faults. Bell and Hoffard
(1990) suggested the fault zone is due to antithetic
faulting associated with the west-dipping range front
fault of the Pine Nut Mountains. Muntean (2001)
attributes the faulting instead to an accommodation
zone from the maximum offset of the Genoa Fault
across the valley. In 1988, Bell et al. (1989) reported
1 km-long extensional cracks along the fault zone that
they suggested were due to fault creep in the area.
Bell and Hoffard (1990) also suggested through
exploratory trenching, that the latest fault
displacement occurred in the mid to late Holocene.
Our field area is located approximately 10
miles to the northeast of Gardnerville along Buckeye
Canyon Road (Fig. 1). The area crosses 18 mapped
faults in the north and 3 mapped faults in the south.
Fault scarps in the area are up to 30 meters in height.
The purpose of our study is to establish geophysical
evidence for faulting. After confirmation of faulting,
a secondary goal is to evaluate the displacement of the
fault and other characteristics of the fault zone such as
mineralization and fault timing. This information will
be important in the geologic history of the basin and
in seismic hazard analyses of the area.
Methods
We performed two magnetic transects in
parallel, west-northwest trending lines to identify
major anomalies in the area so we could further focus
our geophysical methods. We then analyzed the
largest of the magnetic anomalies with resistivity
profiling.
After evaluating the magnetic and
resistivity data together, we chose a seismic reflection
site. We took the location of our anomalies using two
handheld global positioning units, each with a
maximum accuracy of 3 m.
Magnetic Survey
Our magnetic survey deployed a Scintrex
ENVI magnetometer and a Scintrex MP-2
magnetometer as rover instruments and a Scintrex
ENVI magnetometer as a base station. We carried out
the survey in two roughly parallel lines at
approximately 070 degrees heading. Each line was
between 3 km and 4 km long. We took magnetic
measurements on each line at 50 m intervals. When
we found large anomalies, the area was re-sampled at
as little as 12.5 m intervals. Rover values were
linearly corrected based on observed values at the
base station before and after each line was completed.
Several sources of error may be present in our
magnetic readings.
Diurnal errors stem from
approximating the diurnal magnetic variation with a
line. Errors, estimated from plots of the base station
readings throughout the day, suggest errors of up to
10.5 nT.
Errors also originate from magnetic
microbursts that originate in the Ionosphere. In our
base station record, our data suggest that this error
contributed up to 8.5 nT to the overall error.
Resistivity Survey
Our resistivity survey analyzed two magnetic
anomalies on the south line and one anomaly on the
north line. For the survey, we used a L and R
Instruments SN-110 Mini-Res resistivity instrument.
At each survey point, we used a Wenner array, a
configuration that locates both the source and receiver
electrodes at a distance a apart. In our study, we used
an a spacing of 20 m, giving our study a depth
penetration of 15-20 m. Each site consisted of two
approximately perpendicular measurements in order
to reduce the effect of buried pipes and wires in our
analysis. At each anomaly, we took measurements
perpendicular to the mapped strike of the anomaly
until a local baseline was found. In this paper, the
apparent resistivity is plotted against the location of
the center of the array. The apparent resistivity is
found by the equation,
aV/I
where  is the apparent resistivity, a is the spacing
between the electrodes, V is the voltage, and I is the
current.
Seismic Reflection Survey
Our seismic reflection profile covered the
largest of the magnetic and resistivity anomalies,
Waypoint 76.
We used a Bison Instruments
recording instrument with a 48-channel array. We
employed 100 Hz reflection geophones at 3 m spacing
with a 5-geophone overlap. The survey geometry was
such that the center of the largest anomaly was located
in the center of the survey with the recording
instrument located on the west side of the line. An
acceleration-based generator on the sledgehammer
handle initiated the recording cycle synchronously for
each of the five hammer blows at every third station.
At each end of the line, we recorded 20 hammer
blows. In processing the data, we used a band-pass
filer and a dip-filter before stacking. In order to pick
the stacking velocities, we used 100 m/s stacking
intervals. We then binned common depth points at 3
m intervals, which reduced to a common depth-point
stack, that was then Stolt migrated using the picked
stacking velocities. The Stolt migrated image was
dip-filtered at 15 samples per trace.
Results
All three techniques employed in the field
produced anomalies near the location of mapped
faults. Since each method gives insight to different
properties of the crust at different depths, using all
three properties together provides for a more robust
and complete analysis of the geologic situation.
The magnetic surveys revealed a multitude of
anomalies on both the north and south lines. On the
north line, at waypoint 229, we found a magnetic
anomaly of approximately -30 nT over a horizontal
distance of 50 m. Coincident with the apex of the
anomaly is a north-trending gully about 20 m deep.
Forward modeling using GM-SYS for waypoint 229,
is shown in Figure 2. The best fitting model (lowest
root mean square (rms) error) suggests a shallow, lowsusceptibility wedge in sediments with a magnetic
susceptibility of 5.51x10-4. An offset layer with a
normal separation of about 20 m also fits the data
reasonably well, if the layer has a magnetic
susceptibility of 0.001. Attempts to fit the anomaly
with a magnetized prism failed to fit the data within a
reasonable amount of error. At waypoint 229,
galvanic profiling produced a 32% apparent resistivity
increase over the same area as the magnetic low
(Figure 2). The resistivity anomaly, paired with the
magnetic anomaly discussed above, appears to favor
the low susceptibility wedge over the offset layer
model.
Magnetic data at waypoint 318 also shows a
magnetic anomaly of approximately -25 nT. The
magnetic anomaly is highly asymmetric and appears
to represent two anomalies instead of just one.
Attempts at forward modeling with GM-SYS alone
were unable to determine the genesis of the anomaly
(Figure 3). Both models have similar rms error even
though we propose two, very different models. The
first of the models suggests two magnetized zones of
susceptibilities of 0.001. The second model proposes
a layer that has been offset twice, with a magnetic
susceptibility of 1.3x10-5. The major discrepancy in
the models is in the western feature. In both models
the western feature is deeper than the eastern feature,
however in the first model, the feature is east-dipping
while in the second model, the feature is west-dipping
(Figure 3). Surface morphology gives no insight to
the equivalence. Resistivity profiling produces a high
and a low apparent resistivity, from west to east, near
the same locations as seen in the magnetic anomalies
(Figure 3). Because of the depth penetration of the
resistivity method employed, the magnetized zones
are favored. The offset layer is too deep to be
detected by our technique while the mineralized zone
is at a depth that could affect our profile. It should be
noted that the anomalies reported at waypoint 318 are
substantiated by the least number of points of any
feature reported in this paper.
Waypoint 76 has the largest magnetic
anomaly of any area we surveyed, approximately 98
nT. The area modeled (Figure 4) was best modeled
using four independent features. A large area was
modeled in order to take into account the effect that
one anomaly may have on another. The preferred
model proposes four east-dipping magnetized zones
for each of the four anomalies, each with a
susceptibility of 1.5x10-5. An offset magnetized layer
model, similar to Figure 3, could not be fit to the
observed data within an acceptable degree of error.
The location of the west anomaly in Figure 4 may be
the extension of the feature analyzed at Waypoint 229,
although forward modeling differs between the north
and south anomalies. No surface expression is present
in the immediate area, however a prominent eastdipping escarpment exists immediately to the north of
Waypoint 76 that appears to strike into the area
surveyed. The apparent resistivity survey assessed the
same area as the magnetic survey. The apparent
resistivity data shows a gradual decline to a minimum
at approximately the same location as the magnetic
low (Figure 4). The apparent resistivity rises abruptly
to the east of the minimum. The anomaly shows a
total drop in apparent resistivity of 100 ohm-m. The
magnitude of the anomaly suggests a shallow feature
is present near the location of Waypoint 76. It should
also be noted that the apparent resistivity shows a
similar anomaly to that of Waypoint 229, however at a
much lesser magnitude.
Due to the magnitudes of the magnetic and
apparent resistivity anomalies at the site near
Waypoint 76, we chose to undertake a seismic
reflection survey. The results of our survey are shown
in Figure 5. The alluvium-bedrock contact is evident
in the section and appears to show some separation.
Using stacking velocities of 400 m/s for the alluvium,
the depth to the upper (west) reflector is 8.24 m while
the depth to the lower (east) reflector is 13.72 m.
Underneath the prominent basin interface, westdipping sediments appear to be present.
The
basement rock is probably the west-dipping
sedimentary Sunrise Pass Formation described by
Muntean (2001). Our section loses resolution at 30m
depth due to the energy partitioning at the basinbedrock interface.
Discussion
Several studies in the East Gardnerville Basin
have recognized faults based on air photo mapping
and exploratory trenching (Bell et al, 1989; Bell and
Hoffard, 1990; DePolo et al, 2000; Muntean, 2001).
Given the combination of magnetic, resistivity and
seismic data, we believe we have found geophysical
evidence for the existence of normal separation
faulting in the east Gardnerville Basin. A study by
Shields et al (1998) used a nearly identical
combination of geophysical characteristics to identify
the existence of the Pahrump Valley Fault Zone in
southern Nevada.
At Waypoint 76, we believe there is strong
evidence for at least one normal-separation fault. The
seismic evidence suggests a bedrock offset of about
5.5 m within 10 m of the surface. Our magnetic data
suggests that some very weak magnetized zones are
present at slightly deeper depths, between 10 and 20
m. One mechanism for the occurrence of a
magnetized zone may be through the deposition of
iron oxides by water, whose transport has been
facilitated by fault gouge or fracturing near the fault
plane. A similar mechanism was proposed for the
Pahrump Valley Fault by Shields et al. (1998). The
apparent resistivity data in the area suggests a
shallower feature than indicated with either the
magnetic or seismic data. The resistivity anomaly
was the largest we encountered in the area. The
simplest explanation for a broad change in apparent
resistivity, is the saturation of the shallow sediments
near the survey point (Telford, 1990). If the anomaly
is due to water backed up behind an impermeable
layer, then we suspect that the shallow sediments of
quaternary age are offset, as well as the bedrock. An
impermeable fault zone can easily be created by
incorporating clay into the brittle deformation process.
The fault imaged in this survey appears to be the most
active fault in the area due to the large electrical and
magnetic anomalies. The presence of a large scarp to
the north also suggests active and recent faulting.
At Waypoint 229, the combination of an
apparent resistivity high combined with a magnetic
low suggests that some very shallow structure beneath
the surface is present (~1 m deep). One material that
possesses low magnetism and high resistivity is quartz
(Telford et al, 1990). We therefore propose that the
structure is a wedge of partially silicified material,
possibly created by deposition of quartz by shallow
groundwater that has been collected behind an
impermeable zone of fault gouge. An extension of the
proposed fault may be extending into the western
portion of the Waypoint 76, however such a
connection is highly speculative without further data.
At Waypoint 318, we propose the presence of
two slightly magnetized zones facing each other at
approximately 15 m depth. The drop of apparent
resistivity to the east of the eastern anomaly could
represent the presence of groundwater back up behind
an impermeable fault zone. It is important to note that
uphill is to the east. We propose a saturated area in
the zone between the magnetized areas. The presence
of faulting in the area is questionable due to the lack
of data in the area.
Conclusion
Combining magnetics, apparent resistivity
and seismic data, we have found evidence for normal
separation faulting in the east Gardnerville Basin. At
all three zones where our methods were concentrated,
evidence is present that suggests offset layers in
quaternary sediments. These faults appear to coincide
with mapped faults in the area. We also present a set
of geophysical methods to find hidden or questionable
faults. The presence of active faulting in the east
Gardnerville Basin has pronounced effects on the
seismic hazard analysis of the area and our study
provides the preliminary analysis for a larger seismic
study in the future.
Acknowledgments
A special thanks goes to those who have helped in the completion
of this report. Dr. John Louie, James Scott, Matt Clark and Shane
Smith for their participation in field work. Dr. Robert Karlin who
supplied the magnetic instruments. Dr. Ron Petersen for loaning
out the resistivity instruments. Pat Cashman and Jim Trexler also
helped in evaluating the seismic data.
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