Abstract
Geophysical experiments conducted in sedimentary basins along the northern Walker Lane, a
complex and poorly understood zone of northwest trending strike slip faults, are used to better
understand basin geometries and depths. Gravity, TEM, seismic, and GPS surveys constrain the
depth to basement in the local sedimentary basins and can be used to further understand the
faulting mechanisms behind their development. The integration of TEM, gravity and seismic
provides relatively good accuracy in depths. In Warm Springs Valley, basement depths are
observed between ~ 150 and 750 m in a half graben geometry. An anomalous high gives us the
very shallow depth, which is attributed to the location of the prominent Warm Springs Valley
Fault, a dextral fault that has experience dip slip motion during extension of the area. Hungry
Valley shows depths up to 568 m with typical graben geometry composed of dominantly Tertiary
diatomite. Other valleys of study, Red Rock and Long Valley, are not well constrained due to
lack of gravity points and supporting geophysical methods.
Introduction
The Walker Lane is a region that accommodates both dextral slip and extension between the
Sierra Nevada Batholith and the Basin and Range province to the west (figure 1). The Basin and
Range province is characterized by generally north-south trending ranges and valleys (Stewart,
1988). The dextral strike-slip faulting accommodates approximately 20% of the Pacific/North
American relative plate motion (Cashman, personal comm.). Interactions between these two
tectonic domains in terms of timing and predominant style of deformation, kinematics of
deformation, and driving mechanisms behind deformation are not well constrained.
Figure 1 – The Walker Lane as outline by Stewart (1988). Regional structural blocks and major
faults and fault zones in the belt. Arrows indicate relative movement on strike-slip faults. Heavy
lines are major faults and fault zones, dotted where inferred.
Faulting within the northern Walker Lane was active incrementally between 30-20 Ma, and the
present dextral slip began between 12 and 10 Ma (Dilles and Gans, 1995; Dilles and Proffett,
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1991). Estimates of displacement within these three northern domains are not well constrained,
but displacement probably was on the order of 30-40 km, based on offsets of a sequence of
Cenozoic tuffs (Bonham, 1969).
In order to characterize the structural style of the northern Walker Lane we conducted a series of
geophysical experiments within Warm Springs Valley, Hungry Valley, Red Rock, and Long
Valley. Specific methods used include Transient Electro-Magnetics (TEM), seismic reflection
and refraction, gravity, and GPS (Figure 2). In addition, we conducted a microgravity and TEM
survey in Virginia City where little to no gravity or other geophysics has been done. The results
from these experiments provide estimates of depth to bedrock within local basins, which may be
used later to constrain displacement along local faults.
Figure 2 – Location map of study area north of Reno. Red dots represent GPS survey points and
specific areas of interest.
Geologic Setting of Tertiary Basins in the Northern Walker Lane
The area along this northern section of the Walker Lane is comprised of Paleozoic and Mesozoic
metamorphic rocks, Mesozoic plutons, Tertiary volcanics and non marine sedimentary rocks, and
Quaternary basin fill (Bonham, 1969). The basins in this study, Warm Springs, Hungry Valley,
Red Rocks, and Long Valley are predominantly Tertiary non marine sediments with interlayered
volcanic sequences such as ash-flow, air-fall, and water-lain tuffs of the Hartford Assemblage
(Bonham, 1969). Occasional basalt flows are interlayered with lacustrine diatomite, siltstone,
shale, and sandstone. The Tertiary sediments in the valleys north of Reno are Pliocene in age,
based on mammalian fossil discoveries, and fall into one or both of the following formation
names, Coal Valley Fm., and Truckee Fm. There is much discrepancy about formation names
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even though there seems to be clear differences between the two. In general, the Tertiary
sediments in the northern Walker Lane are probably that of the Truckee Fm. We analyzed
sedimentary basins in an effort to look at the structural and lithological controls of local basins.
Multiple questions exist regarding the timing of Basin and Range extension and tectonic factors
controlling crustal-scale extension in the Walker Lane. The relationship between extension
faulting and the south-southwest migration of Cenozoic silicic volcanism are commonly cited to
help explain the evolution of the Basin and Range (Axen et al., 1993; Dilles and Gans, 1995).
Additionally, the direction of extension shifted from west-southwest to northwest between 11-9
Ma (Hardyman and Oldow, 1991; Dilles and Gans, 1995). These evolutionary changes in Basin
and Range deformation are assumed to have resulted from the influence of plate motions along
the Pacific-North American boundary (Wernicke et al., 1988)
Geologic Setting of Virginia City
Virginia City Mining District is located in west-central Nevada, about 27km southeast of Reno
(figure 2) is the site of the famous Comstock Lode. Approximately 225,000 Kg of Gold and 7
million Kg of silver were produced from the district (1862 – 1953) (Vikre, 1989). The area is
mountainous with elevations ranging from 1280 m in the valley to 2,360 m on Mt. Davidson.
The oldest exposed lithologies are Mesozoic metasedimentary and metavolcanic rocks, which are
intruded by Cretaceous granodiorite These units are unconformably overlain by Oligocene and
early Miocene silicic ash-flow tuffs, thick andesite flows and associated breccias of the Miocene
Alta Formation. Overlying the Alta Formation are andesite flows, breccias, and accompanying
dikes and stocks of the Kate Peak Formation. The Alta Formation is the main host of orebodies
in the district (Thompson, 1956) and is the unit most affected by hydrothermal alteration.
Mineralization and hydrothermal alteration of the Comstock Lode are associated with the NorthSouth trending Comstock, Silver City and Occidental faults. The main ore zones are located
along the Comstock fault and associated cross faults. Thick veins of crushed quartz with silver
sulfosalts, native silver and native gold are found in proximity to the faults. The main ore
mineralization episode at Comstock is middle Miocene (Vikre, 1989). Vikre (1989) also
suggests that the high-sulfidation mineralization is older than the main Lode mineralization.
Previous Work
Little geophysical work has been done in the northern Walker Lane or Virginia City areas.
However, a gravity study of Warm Springs Valley conducted in 1967 (Gimlett, 1967) and a deep
seismic reflection study in 1984 by COCORP, the Consortium for Continental Reflection
Profiling, provides some background and comparisons to this study of the valleys north of Reno.
Virtually no geophysical work is done in the Virginia City area.
The COCORP line is a 65 km traverse from Granite Springs to the Sierra Nevada Batholith,
transecting Warm Springs Valley in a west southwesterly trend. Three crustal-scale features
associated with the Walker Lane and the transition from the Basin and Range extension are
observed in the data set. The first is a moderately west dipping reflector that underlies the
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Walker Lane beneath the Warm Springs fault and the Pyramid Lake fault, which accommodates
strike slip and normal displacement down to a mid-crustal level. Second, the Moho appears to
gradually deepen westward through the Walker Lane. And third, the reflective characteristics of
the crust change across the Walker Lane, from a more reflective lower crust in the Basin and
Range to a less reflective lower crust in the Sierra Nevada (Knuepfer, 1987).
COCORP data shows reflections that are generally discontinuous and appear to define
asymmetric wedge-shaped basins marked principally by east-dipping and subhorizontal
reflections (Knuepfer, 1987). The COCORP line crosses the mapped traces of the Warm Springs
Fault in Warm Springs Valley according to Bonham (1969) and has been modeled by Gimlett’s
(1967) gravity survey to be a steep east dip, which is consistent with the COCORP reflection
profile. Reflections in Warm Springs show a steep east dip of the Warm Springs fault truncating
observed subhorizontal reflections, possibly Tertiary or Quaternary basin sediments that
terminate beyond 0.5 s two way time (Knuepfer, 1987). The west dipping reflector projects to
the surface where the Pyramid Lake fault is mapped. However, the Warm Springs Valley fault is
antithetic to and appears to terminate into the west dipping reflection.
Gimlett’s (1967) report is a complete gravity survey throughout Warm Springs valley that also
includes a regional gravity survey, and geologic information. The regional gravity serves
primarily to determine the regional gradient across Warm Springs Valley, however it also
functions as an unknown mass distribution, possibly to yield estimates of the depths to the
disturbing masses, the thickness of the earth’s’ crust, and the degree of isostatic compensation in
the area.
In addition to Gimlett’s (1967) gravity survey, there is another regional gravity report conducted
by the USGS (Plouff, 1994), which is used in this study.
Methods and Results
GPS (Global Positioning System) Survey
Methods
The position of each gravity station was measured using GPS, with particular attention paid to
obtaining accurate elevation data. The aim was to achieve heights with 10cm precision.
Two Trimble 4000SSi, dual-frequency, carrier-phase, geodetic receivers were used as a base
station and rover in order that post-processed, double-differenced positions be acquired. A
choke ring antenna, useful for eliminating multipath, along with tripod and tribrach for stable
centering over benchmark points, was used at the base station, whilst a compact-L1/L2 antenna
was used at the rover, mounted on a pogo stick. Although the difference in antennas may have
caused a small error of several mm due to antenna phase offset (a difference in electrical centers
of the two antennas), there was only one Compact L1/L2 antenna available and the gain in speed
from using this for the roving station was deemed a worthwhile tradeoff.
4
Base stations were selected at known benchmarks, within 8 km of the gravity stations for each
survey, in order that the survey be accurately translated to UTM coordinates. Base station
receivers were left continually running throughout the survey. Gravity stations were visited by
the roving receiver with 10-minute occupation times. The occupation time for the roving station
at the Virginia City micro-gravity survey points was increased to 20 minutes, and a choke-ring
antenna and tripod used, so that elevations of increased accuracy be obtained.
Data was downloaded from the receivers to the Trimble GPSurvey software. Baseline distances
between the base station and gravity points were then processed in the Trimble WAVE software.
The zenith delay correction for baseline processing was not used because station occupation
times were so short (with files of less than 2 hours the tropospheric correction can degrade,
rather than improve, the measurements). The fact that the receivers are dual-frequency meant
that ionospheric effects, which are frequency dependent, could be modeled for the longer
baselines. The broadcast ephemeris was used to define satellite positions and elevation mask set
to 15o. The maximum fixable cycle slip was set to 600 seconds.
A least squares adjustment was then carried out in TRIMNET. No weighting was applied, but all
stations obtained FIX status and all surveys passed the chi-squared test at 95%. Base station
coordinates were fixed to known UTM values (that correspond with the base map) before
adjustment and the coordinate system and datum changed to UTM Zone 11 and NAD27
respectively.
Results
VDOP values were recorded in the field at all sites. When compared with the results, there was
little correlation with achieved precision, indicating the importance of other errors to the survey.
Multipath errors should also be minimal, in that most of the survey was situated in open spaces
and care was taken to distance the vehicle from the survey points. This was proven by
examination of the error ellipses, most of which have similar orientations (rather than mixed
orientations as a result of multipath), with the axis of maximum error directed perpendicular to
the base station. In a well-planned survey with three receivers this axis will have random
direction, and ellipses will be near circular, so it is indicative of the precision lost by only having
one base and one rover that all axes point the same way and many ellipses are elongate.
Regardless of these error sources, all least squares standard deviations for elevation indicated a
precision of better than 5cm, with the majority less than 2cm. Horizontal precisions were all less
than 1.5cm. Further error sources, however, such as the fact that the rover was rarely placed
exactly on the gravity point and pogo stick was not held exactly vertical will have degraded these
precisions somewhat, but they should still be well with the 10cm limit. Refer to figure 2 for
location of GPS survey points in the study.
Gravity
Methods
A LaCoste-Romberg (L&R) model G gravimeter was used for all gravity measurements. A total
of 24 gravity measurements were taken in an east-west profile across Warm Springs Valley, 21
5
were used for modeling. A total of six measurements were taken in a northwest – southeast
profile across Hungry Valley, six were taken in an east-west profile in Red Rock Valley and four
in Long Valley. A total of 25 measurements were taken throughout Virginia City. Warm Springs
Valley data is compared to the 1967 data collected by the Nevada Bureau of Mines. Eight of the
Virginia City data points are used to determine an estimate of the sediment density using the
Nettleton method. The remaining seventeen points will be used to create a Bouguer anomaly
map.
Vertical control for all measurements was provided by a geodetic quality GPS. Tidal variations
and instrument related drift was corrected for by regular re-occupation of local base stations.
Hammer zone terrain corrections B and C (1 meter to 54 meters) were estimated by eye in the
field. Bouguer earth density of 2.67 g/cm3 and a density contrast of -0.5 g/cm3 was used in the
reduction of all data. Regional anomaly effects were corrected for by a simple linear
approximation.
Initial basin depths were estimated using a Bouguer slab approximation. This method gives a
minimum depth to bedrock. However, the infinite slab approximation works best when the
lateral extent is much greater than the depth of the basin (Abbott and Louie, 2000). All basin
depths were calculated by three methods, Talwani inversion, infinite slab approximation and by
GM-SYS, a forward modeling program developed by Northwest Geophysical Associates. Crosssectional models followed the transect of the gravity stations as closely as possible. All models
were created using GM-SYS.=20.
The LaCoste and Romberg gravity measurement errors are estimated to be 1.17 g/cm3 for Warm
Springs Valley, 0.18 g/cm3 for Long Valley, Red Rock Valley and Hungry Valley and 0.21
g/cm3 for Virginia City. The transect in Virginia City to be used for near surface density
calculations had an error of 0.08 mGal. Battery problems account for the larger error in the
Warm Springs Valley data. Much of the error from all the valleys is due to more than one
gravimeter operator in any data set. Elevation measurements are accurate to within 0.15- 0.20
meters introducing a ~ 0.04 mGal error. Terrain corrections estimated in the field occasionally
approached 0.2 mGal. A 25-30% error is possible for the estimated terrain corrections,
producing a possible error of ~ 0.06 mGal. The total estimated anomaly error is 1.35 mGal for
Warm Springs Valley, 0.29 mGal for Hungry Valley, Red Rock Valley and Long Valley, and
0.32 mGal for Virginia City. The transect for near surface density contains less elevation error.
The elevation is estimated to be within 0.05-0.10 meters. The over all error for this data is
estimated to be ~ 0.14 mGal.
The magnitude of gravity depends on: latitude, elevation, topography of the surrounding terrain,
earth tides, and density variations in the subsurface. The largest source of error in our results
comes from the estimation of rock densities (the slab is of uniform density) and lateral variations
(it is of infinite horizontal extent). These two assumptions were made calculating the Bouguer
correction: neither is really valid. The lack of any density data of the basin fill introduces
significantly large error because local variations in the densities of rocks near the surface
produce small changes in the gravity field. The basement rock has a significant lateral variation
from 2.70 on the west to 2.38 on the east as well (Gimlett, 1967). It is possible that the depth
6
error may be as high as 50% in valleys were data points are limited. There is also, significant
errors in modeling the data, for example, when modeling with GM-SYS, it is possible to have
two completely different models that will fit the same line graph.
Results
Initial basin depths were estimated using a Bouguer slab approximation yielding a minimum
depth to bedrock, if density is known. The results for the Warm Springs Valley, Hungry Valley,
Red Rock Valley, Long Valley were 509m, 475m, 146m, and 1220m, respectively. Basin depths
computed using Talwani inversion method for the above valleys were found to be 557m, 523m,
210m, and 1267m, respectively. The forward modeling program GM-SYS, thought to produce
overestimates of maximum depth, gave results of 750m, 568m, 170m, and 1270m, respectively.
Gravity modeling results from the Warm Springs Valley, Hungry Valley, Red Rock Valley, and
Long Valley are shown in figure 3, as basin depth models. The steep slope seen on the western
side of the Warm Springs Valley is most likely false and is probably due to the method of
correction for the regional trend and the lack of uniqueness inherent in GM-SYS.
A)
B)
C)
D)
Figure 3 – GM-SYS gravity models of A) Warm Springs Valley, B) Hungry Valley, C) Red Rock,
and D) Long Valley.
Table 1 outlines the results obtained for each method and includes a summary of the average
depth of each valley. The average values reflect none of the potentially large uncertainty in
density structure and lateral variation that may be further constrained by seismic and EM in
Hungry Valley and Warm Springs Valley. However, the uncertainty in Red Rock Valley may
be only 20%, but could be at least 50% in Long Valley because of very few data points.
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There are clear differences between each method where the Talwani inverse usually would
produce the worst overestimate of maximum depth, but the best estimate of average depth. . The
Bouguer Slab method, if density is known, would be an underestimate and GM-SYS appears to
produce overestimates of maximum depth as well. Maximum depth is very hard to determine
from gravity, but it seems that the large GM-SYS value at Warm Springs is a "maximum
overestimate" fitting the data but beyond a 2-sigma error. The uncertainty of Long Valley is
constrained by very few measurements (Louie, 2000).
Method
HV
LV
RRV
WS
Talwani
Bouguer
slab
GM-SYS
523 m
475 m
1267 m
1220 m
210 m
146 m
557 m
509 m
568 m
1270 m
170 m
750 m
170+-30 m
560+-50 m
Summary
520+-50 m 1270+-200 m
Table 1 – Maximum depths of valleys computed three various ways. HV – Hungry Valley, LV –
Long Valley, RRV – Red Rock Valley, WS – Warm Springs Valley.
Previous work done in Warm Springs Valley give a maximum depth of 676 to 780 meters across
the same transects (Gimlett 1967). These results are in good agreement with our maximum depth
of 750 meters.
The Virginia City data collected for near surface density calculations had an error of 0.8 mGal,
and less elevation error, within 0.05 to 0.10 meters. The overall error for this data was estimated
to be  0.14 mGal. A near surface sediment density of 2.00 g/cc was obtained using the
Nettleton method. This low density is possibly associated with an 11% void below the surface
due to tunneling or mine shafts. The area has been previously mapped as Alto Formation with an
approximate density of 2.53 to 2.69 g/cc (Gimlett, 1967), which is much higher than that
obtained using the Nettleton method. Data in Virginia City is hand contoured within the regional
gravity report of Plouff (1994). The data falls along a semi steep gradient, which parallels the
Comstock fault and similar trending faults, and has values between –179 and –180 mgals (plate
1).
Transient (Time-domain) Electro-Magnetics (TEM)
Methods
The ability to asses such quantities as the depth of the water table, basin geometry, location of
faults and fracture zones, depth of clay units, and location of other conductors helps to
characterize the geology and dynamics of a site. In this study, TEM measurements were taken in
two of the northernmost valleys of the Walker Lane and in Virginia City to achieve these goals.
Taken concurrently with gravity and seismic profiles when possible, this data may contribute
some constraint to near surface structures observed through various means. In addition to useful
site information, this TEM study implements two different modeling techniques for comparison.
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This provides one the ability to compare a simplistic transformation scheme proposed by Meju
(1990) and an inversion technique called EINVRT written by Sandberg (1990).
In Warm Springs Valley and Hungry Valley, TEM was set up in an offset sounding pattern. The
transmitter (Tx) loop was set as a single loop in square pattern (50m sides). The receiver (Rx)
loop was placed about 10 m away from the Tx Loop (figure 4a).
A)
B)
Figure 4 – A) diagram showing geometric layout of TEM instrument in the field. B) Diagram
showing the theory behind taking a TEM reading.
Transient (Time-Domain) Electromagnetic methods work on the principle of electromagnetic
induction and subsequent magnetization of the layer horizons. When the TEM instrument is
activated, the transmitter produces magnetic field where magnetic eddy currents extend into the
sounded layers. The transmitter shuts of and the receiver begins to record the induced magnetic
field in the sounded layers (figure 4b). The resistivity of the layers is directly related to their
composition and water saturation.
The Simplistic Method as proposed by Meju, 1990. Mirroring skin-depth theory, Meju notes the
definition of a “diffusion depth” of TEM that can be approximated by the following formula:
(
 eff 
2t a
0
1
)2
(1)
2.3
Here t is the sampling time, a is the apparent resistivity, and 0 is the permeability of free space.
TEM data has been correlated with magnetotellurics data to produce an effective resistivity
transformation defined as follows:
(2)
eff  2.3ae(1 ) ,0.15    0.2
Another technique is the “Inversion Technique” by Sandberg (1990). This simplified inversion
reduces galvanic and EM data. The initial depth versus resistivity model is modified iteratively
9
by reducing the l2 norm between field data and forward-calculated response of a homogenous
layered earth.
Errors associated with TEM data collection are attributed mostly to user errors and lack of
knowledge concerning the operations of the “machine”. When operating the TEM instrument,
many different parameters must be input to the system depending on the local conditions. If
these parameters are incorrect data results may be affected. The accuracy of the geometric set up
for the instrument to record from may also play an important role in acquiring reliable data.
Results
In Warm Springs Valley, TEM data collection followed an approximately east to west line that
coincides with the gravity stations 102, 111, 115, 119, and 121. The approximate location of a
fault scarp observed in the field near is located near station 115. Also, it should be mentioned
that sources familiar with the area suggest that the water table lie at a depth of 15-20m out in the
center of the Valley, or at an elevation of 1248-1253m.
Data from station 102 (elevation 1286m) suggests a decrease in resistivity with depth. This is
generally the expected result in a sedimentary basin where the basement is too deep to be sensed
by the TEM instrument. Meju’s method identifies one layer from 38-104m with resistivity
decaying from 47m to 40m. A conductive interface is reached at that depth and the
resistivity decays to 14-15m by 154-157m. EINVRT, modeling the data with two layers over a
half space, results in the following model: 68-70m layer to 25-27m, 33-35m layer from 2527m to 165-180m, and 4m layer below this depth. The resistivity drop between the first two
layers corresponds to the interface of the water table. The absolute elevation is 1259-1261m,
which is within 6m of the estimate given above. Thus, it seems as though the water table
resistivity may be around 33-35m according to the EINVRT model. The second conductivity
drop may be the product of a clay layer, but more information is required to assess the validity of
this hypothesis.
The results from station 111 (elevation 1269 m) show approximately a 16-20m layer that
extends down to 66-67m in the Meju model and to 66-84m in the EINVRT result. The slightly
more conductive interface drops the resistivity to 5-7m starting from 111-127m in the Meju
model and from 86-103m in model obtained form EINVRT.
On the other hand, station 115 may resolves the edge of the basin or a shallower block of
basement material within the conductive sediments. The Meju transformation shows an
intermediate layer at station 115 (elevation 1273m) that is lower in resistivity than the two layers
surrounding it. The resistivity drops from 26m at 31m to 13m at 105-106m. From that point,
the resistivity starts to increase slightly until it finally reaches 15-17m at 183-189m. This data
is modeled by EINVRT as follows: 21-31m to 34-44m, 12m from this depth to 150-180m,
and 13-18m below 150-180m. The Meju model predicts a depth of interface of 105-106m,
while the model obtained from EINVRT suggests a depth of 150-180m. Recall that the station is
also coincident with the fault trace observed in the field. Assuming that the intermediate layer is
a result of the water table at an elevation somewhere between 1226-1239 m, which is 9m below
the estimate given above.
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Station 119 (elevation 1314m) is located near the West End of Warm Springs Valley exhibits
one layer of resistivity according to the Meju model. From 38-39m at 52m, the resistivity falls
slightly to 30-32m by 142-147m. From that point, a conductive interface is reached and the
resistivity drops to 16-20m at 165-185m depth. This layer is modeled by EINVRT from 2426m and ranges in depth from 34-39m to 203-245m. The drop observed may be due to a clay
layer. The water table is not resolved at this site.
Since station 121 (elevation 1357m) appears to be located over bedrock, and so the expected
resistivity should be higher than all other stations in the line. From 213-228m at 81-84m, there
is a resistivity increase to 251-264m at 120-123m according to the Meju model. A conductive
interface is reached, resulting in a resistivity of 31m at 260m. The EINVRT model suggests
the following layering: 241-256m down to 81-84m, 85-142m from this depth to 206-211m,
and 3-20m below 206-211m.
a) Station 102
b) Station 111
c) Station 115
Caption on next page
d) Station 119
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Figure 5 – TEM models of Warm Springs
Valley. A) through E) follows the West
to East profile followed during data
acquisition. The left two boxes are
models of the time (10-4 to 10 –2 sec) vs.
apparent resisitivity (101 to 102 m) and
the right two boxes are modeled data
according to Meju and EINVRT,
resisitivity (100 – 102 m) vs. depth (103
to 101 m).
e) Station 121
The Hungry Valley data set, as with Warm Springs Valley, has a water table between 15-20m
deep in the valley’s center, at an absolute elevation of 1490-1495m (Widmer, oral comm.).
Data at station 201 reveals a highly conductive interface at 41-49m. The results obtained from
EINVRT suggest a drop from 161-172m above the interface to 2-8m below. This area is
assumed to be covering bedrock, so this conductive layer may be the result of a fracture zone. In
this case, the Meju model does not give reliable information.
The data obtained at station 202 (elevation 1528m) does not transform well via the Meju method.
Resistivities greatly vary in the EINVRT models but the depths obtained are somewhat
consistent. It is possible that either user error or inhomogeneous layering could be to blame for
the inability to model this site effectively. If one believes the EINVRT model of depth for the
second layer, though, it results in a resistivity drop at an elevation of 1490-1492m, which is
consistent to the range of the water table estimates, resulting in a resistivity of the water table
between 8-13m.
Station 204 (elevation 1509m) is located over the deepest portion of the basin. The Meju model
reveals decreasing resistivity with depth where the resistivity of one layer ranges between 510m and 24-78m depth. The EINVRT scheme calculated the following model: 22-30m to
16-18m depth, 5m at 16-18m to 104-111m depth, and 2-3m below this point. The EINVRT
model agrees with the resistivity range of the water table and results in an absolute elevation of
1491-1493m. The Meju depth is within 5m of the proposed depth of the water table.
Located at the southeast end of the line, station 205 (elevation 1531m) shows signs of relatively
shallow bedrock. The Meju model suggests a single layer with resistivity varying from 1016m from 21-151m while the EINVRT model suggests a resistivity increase from 9-10m to
30-40m at a depth of 100m. If this trend is believable, it appears as though the bedrock may be
within 100m of the surface at this site.
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a) Station 201
b) Station 202
c) Station 204
c) Station 205
Figure 6 – TEM models of Hungry Valley. A) through C) follows the northwest/southeast
profile during data acquisition. Left two boxes are plotted time vs. apparent resistivity
and the right two boxes are plotted resisitivity vs. depth. Number ranges vary per plot, if
not visible.
In an attempt to establish a means of mineral exploration via TEM, measurements were taken in
the ore rich Alta formation that underlies Virginia City. Unfortunately, though, the presence of
conductive materials at the surface was problematic. One cannot resolve a resistor below a
conductor since all of the eddy currents induced by the magnetic field collect along the
conducting surface. Signal saturation in the first several measurement gates followed by little to
no signal does not reveal any information of underground structures. It was also the case that
user error may have contributed to this trend.
Seismic
Seismic investigations utilize the fact that elastic (seismic) waves travel with different velocities
in different density materials. Generating seismic waves at a point and observing the arrival
times of the waves at a number of other points along a linear trace, it is possible to determine the
velocities of individual waves and locate subsurface interfaces where the waves are reflected or
refracted. Calculated velocities can then be correlated with time measurements and supply
information regarding basic lithology, thickness, and dip of strata, as well as depth to basement.
Methods
In Hungry and Warm Springs Valleys, reflection and refraction lines were oriented in a general
east-west direction. Strategic location of these arrays at both the valley center and the valley
13
edge provide information to characterized velocity of sediments, velocity of basement rocks, and
information about basin depth and geometry.
A Bison 48 channel seismograph with a 75 MHz data processor was used for seismic acquisition.
Refraction surveys were conducted using two 375 meter cables with 24 takeouts along each
cable. Geophones operating at 8 Hz were laid out at a 15 meter spacing and seated 6 inches
below the surface. Three source points were used along refraction lines with hit locations at the
east and west ends of the line and at the center point of the line. Records were recorded for one
second. Reflection surveys were conducted using six 25 Hz geophone arrays at each takeout
with spacing between takeouts set to four meters. Thirty-four source points were used along the
reflection line with hit locations beginning at the eastern end of the line and spaced at eight meter
intervals along the length of the line. Six off end hit locations were also taken with 50, 60, 70,
and 90 meter spacings from each end of the line. Data was recorded for one second. The shot
source for all experiments was a 12 pound sledge hammer with both radio trigger and hard wire
signal connection. Hits were made in line with the layout of the seismic line.
Post recording processing for refraction and reflection data was done using Viewmat, a
windows-based processing program. Processing included such filter passes as trace-equalization
gain, automatic gain control, and clipping data at 3* the calculated root-mean-square.
Reflection data required additional processing using band pass filtering which was optimized
between 50 and 100 Hz. Reflection data was then compiled through constant velocity stacking,
analyzing normal move-out velocities between 500 and 1500 m/s.
Error within this portion of the study is dominated by computer problems. The bulk of the Warm
Springs Valley seismic data remains locked in an unusable format. Data collected in the DOS
version of the bison unit is free of computer / equipment induced error. Sources of error outside
of these initial concerns include noise within the recorded data, generated from wind, traffic, our
own movements during recordings, and variability of source input. Poor resolution within some
of the records also carries over into poor computer and manual picks with interpreting velocities
within refraction data and translates into inaccurate estimates of depth for reflectors and
estimates of lithologic thickness.
Results
The refraction and reflection surveys conducted in Hungry valley produce meaningful yet
indefinite results of the basin geometry. The refraction survey allowed us to calculated depth
and dip of Tertiary sediments. Using a time depth conversion based on a two layer velocity
model,
T0 = 2h1/V1,
(3)
the following velocities are calculated for the first and second layers (Table 2; figure 7). Using
the depth equation following
Depth = Z1 = t1/2[V2V1/(V22 – V12)1/2],
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(4)
we’ve calculated the approximated depth to Z1, which is within the range of 25-54 m below the
surface and based on the following dip equation,
Dip =  = ½[sin-1(V1/V2min) – sin-1(V1/V2max)]
(5)
the second layer is calculated to be virtually horizontal, with only a slight dip of 6.3 to the east.
Refraction
Forward Shot
V1 = 681 m/s
V2 = 937 m/s
Z1 = 24.8 m
Reverse Shot
V1 = 937 m/s
V2 = 1102 m/s
Z1 = 53.4 m
Dip = 6.3 east
Table 2 – Velocity, depth, and dip values of both forward and reverse shots calculated from the
refraction data in Hungry Valley.
Figure 7 – Seismograph from the refraction survey in Hungry Valley, forward shot to the left
and reverse shot to the right. Note the three visible waves.
Stacked data from the reflection survey suggest a depth to Tertiary sediments on the order of 2040 meters below the surface (figure 8). Oddly enough, this correlates with the refractor
somewhere between 25-54 m below the surface. The depth to which this survey penetrates is
only 280 m due to low source energies. The high velocities layers, interpreted as Tertiary
diatomite, extends at least to the greatest depth of the survey and probably further. In general, it
appears that Quaternary basin fill makes up the first 0-20 m, underlain by 20-40 m of Tertiary
sediments, which is then underlain by 40 to possibly >280 m of diatomite.
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Tertiary sediments
Tertiary diatomite
Figure 8 – Stacked, normal move out velocities from the reflection survey in Hungry Valley. The
shallow upper reflections represent Tertiary sediments between depths of 20-40 m. The rest is
Tertiary diatomite
Soil Field Spectroscopy
The main suites of minerals targeted in spectroscopy and remote sensing are iron oxides and
hydroxides and clay minerals presumed to be associated with hydrothermal alteration or soil
formation. Most of the Iron minerals have subtle spectral features in the 400-700 nm range.
Charge transfer between O-2 and Fe+3 ions causes strong absorption in Iron minerals at
wavelengths smaller than 400nm. The hydroxyl (OH) group has characteristic absorption
features in the 2200-2400 nm range, which is caused by hydroxyl ion stretching. Silicate
minerals have no detectable features in the 400-2400 nm spectral range.
Methods
As a side note, soil spectra were collected in Warm Springs and Hungry Valley using portable,
full range, Visible and Short Wave InfraRed (400-2500 nm) ASD spectrometer. The instrument
was calibrated for in-situ conditions using Spectralon TM panel. Data source points have no
geometric significance to the rest of the experiments.
Results
Warm Springs spectra exhibit narrow absorption feature near 2160 nm and 2200nm
characteristic of O-H band stretching in clay minerals. The spectra, however relatively flat
which also indicates possible mixture with silicates. These absorption minima are characteristic
of the mineral illite. Hungry Valley spectra indicate very little or no O-H band absorption and
appear to be predominantly flat, indicating possibly silicate composition. Warm Springs valley
fill appears to be mixture of clays and diatomite, while Hungry Valley fill appears to be almost
purely diatomite.
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A)
B)
Figure 9 – Graphs of spectra in the A) Warm Springs and B) Hungry Valleys.
Discussion
Multiple observations can be made about the data collected in the northern Walker Lane. Most
importantly, the integration of TEM and gravity data has aided in placing constraints on basin
depths and geometries, given there are sufficient data points, and a depth to Tertiary sediments is
observed from both seismic and TEM data in a local basin. The use of TEM data constrains the
water table in both the Warm Springs and Hungry Valleys. Important observations about basin
depths and geometries reveal half graben basin geometry in Warm Springs Valley, whereas the
other valleys display typical graben basins.
Very good comparisons are present between TEM and gravity data in Warm Springs and Hungry
Valleys. The TEM data has successfully constrained the water table approximately 15 m below
the basin surface, which has been further confirmed by hydrology studies in both of these valleys
(Widmer, oral comm.). Basin depths in both Warm Springs and Hungry Valleys are in direct
agreement for the most part. A few anomalous station locations give conflicting evidence for
basin depths, For example
The basin geometry has not been entirely constrained in Warm Springs Valley as there is still
some seismic data to process that may give better estimates of the shallowest depth to Tertiary
sediments. The overall geometry modeled by gravity suggests an uplifted block of basement
approximately halfway across the profile. The gravity model correlates well with the interpreted
depth to basement from the TEM data set (station 115) approximately 152 m, similarly the
maximum depth of the basin is modeled to be approximately 710 m below the surface for both
TEM and gravity models. Further confirmation of the basin depth and the anomaly on the
western half, Gimlett (1967) describes a gravity low over the entire length of Warm Springs
Valley however an anomalous “hump” is observed in the basin (figure 10). The high is possibly
evidence of a buried, upfaulted northern extension of the Curnow Range. This is further
explained by a regional gravity low in the northern Hungry Valley and a steep gravity gradient
along the eastern flank of the Curnow Range. The steep gradient is interpreted as the fault zone
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where elevation changes has occurred. Overall, Gimlett (1967) reports a minimum of
approximately 593 m and a maximum of approximately 1024 m throughout the whole Warm
Springs Valley in comparison to the maximum depth of 750 m estimated from this survey. The
COCORP data is not useful to the local interpretations about basin depth of this study, however
their data does help constrain the dip of the Warm Springs Valley fault as an steeply east dipping
structure that is intercepted at depth by a strong west dipping structure. The west dipping
structure does not show vertical offset so as to infer the cross cutting relationships of the two
faults (Knuepfer, 1987).
“hump”
“hump”
a)
b)
Figure 10 – Comparison of gravity data in Warm Springs Valley. A) gravity model from this
survey shows TEM location points and depth in blue, and b) gravity model from Gimlett’s (1967)
report.
In Hungry Valley, Gimlett (1967) observed a smaller anomaly (~10 mgal) that he says is due
almost entirely to a thick section of Truckee sediments, which probably fill a rather shallow
basin. However, no detailed work was done on this anomaly in his report. No direct
comparisons can be made from the Gimlett’s observations and the results of this study. We
observe a slightly shallower basement of 568 m below the surface.
Integrating gravity, TEM, and seismic data in Hungry Valley proves reliable when constraining
geometry and depths in the basin. TEM and seismic data correlate well on the shallow reflector
approximately 40 m depth and TEM and gravity data again correlate well with depth to
basement. An exception to this is at TEM station 205, where basement is observed at
approximately 100 m depth, but the gravity modeled the basement at the surface, which was not
evident in the field so the point was discarded. TEM is a very good method for acquiring depth
to basement information in conjunction with gravity, because where one lacks data the other can
pick it up fairly well. The seismic data did not prove particularly useful in constraining depth of
basement in this experiment however it is at least know that diatomite does continue beyond 280
m depth.
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Figure 11 – Gravity model of Hungry
Valley that shows TEM locations and
depths to basement in blue.
The Long Valley deserves further analysis, based on the general model developed from the few
gravity points. The valley appears to have a half graben geometry similar to Warm Springs
Valley as well as an anomalous high in the middle of the valley. The basin has much greater
depths to basement than the others. If comparisons can be made between basin geometries
alone, it would seem evident that the "Long Valley fault" is similar to the Warm Springs valley
fault. Red Rock seems to have simple basin geometry with relatively shallow depths though it is
uncertain whether this survey extended from bedrock to bedrock. The lack of gravity points in
Red Rock and Long Valley provides only minimal results of depths and geometries in these
basins. Furthermore, there is a lack of other geophysical methods, like TEM and seismic, to
reliably constrain the character of the basins.
These experiments provide valuable data for understanding the evolution of basins in the Walker
Lane. The development of these basins is formed as a result of left-stepping right lateral
faulting, particularly in the Warm Springs Valley. The Warm Springs Valley fault is mapped on
the west side of the northern end of the basin and the east side of the southern end of the basin,
therefore extension between en echelon faults produces depositional basins. Yet, more detail is
still needed.
Valuable insights could be acquired from more seismic lines across this Warm Springs and
Hungry Valley transects, as well as the Red Rock and Long Valleys. Stronger source energies
are pertinent for such an experiment in order to attain greater depths. Particular efforts should be
focused on Long Valley, which appears to have interesting geometry, similar to Warm Springs,
which would essential to constraining fault displacements and basin geometry associated with
movement along the Walker Lane.
Acknowledgments
We would like to acknowledge various individual and groups for their use of equipment: Robert
Karlin for the Gravimeter, Geoff Blewitt and the Nevada Bureau of Mines for the GPS
equipment, Ken Taylor for the TEM –osaur, and the Seismology department, particularly Rob
Abott for the use of the seismic equipment.
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