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					 Detailed gravity survey over a low-angle normal fault accommodation zone, Dixie
                                  Valley, Nevada
                                   John N. Louie
                University of Nevada, Reno Seismological Laboratory

                                      Motivation
         The existence of low-angle normal faults in the Basin and Range is contested.
Geologists have argued for them on the basis of field relations and exhumed faults.
Seismologists have dissented, citing frictional constraints and the lack of unequivocal
low-angle mechanisms in the seismic record. Following surface mapping by Caskey et
al. [1996], Abbott et al. [2000] provided the geophysical evidence that the 1954 Dixie
Valley earthquake produced slip on a low-angle normal fault. As such, the Dixie Valley
fault is the only known active normal fault that is characterized by low-angle dip at the
surface. Other possible low-angle fault mechanisms are associated with basement
structures having superjacent high-angle faults (e.g.: 1981 Gulf of Corinth; 1947 Ancash,
Peru), or are under water (Woodlark D’Entrecasteaux rift system, Papua New Guinea).
Therefore, Dixie Valley represents the best area to directly observe the kinematics of a
shallow, low-angle normal fault system.
         Geological mapping of the surface ruptures suggests that low-angle fault
geometry characterizes almost 50% of the 46 km 1954 rupture length. The research
proposed here would explore how the Dixie Valley fault transitions along strike from
high-angle geometry to low-angle geometry through detailed gravity mapping.

Regional and Tectonic Setting
        Dixie Valley, Nevada lies in the north-central portion of the Basin and Range
province (Figure 1). The Basin and Range is a region in which a large amount of
intraplate extension has taken place in the Cenozoic. Much of the extension is
accomplished by high-angle (50°-70°) normal faulting, with many large seismic events
observed historically. The faulting has created a series of predominately northerly
trending mountain ranges and sedimentary basins. Dixie Valley is one such basin; the
Stillwater Range bounds it on the west and the Clan Alpine Range bounds it on the east.
The Dixie Valley Fault, site of the 1954 event, is the east-dipping range-bounding normal
fault along the eastern front of the Stillwater Range.
        The Dixie Valley fault along the southern portion of the Stillwater Range
separates Mesozoic and Tertiary footwall rocks from Tertiary and Quaternary basin fill.
Miocene and Oligocene volcanic rocks and granitic plutons related to the Stillwater
caldera complex [John, 1995] and Mesozoic metasedimentary rocks represent the
“geophysical basement”. The basin fill at the surface consists of Quaternary fan, playa,
and lacustrine deposits [Wilden and Speed, 1974].

The 1954 Dixie Valley Earthquake
        The 16 December 1954 Dixie Valley earthquake was the last of a series of large,
central Nevada earthquakes that took place within a period of 6 months. The preceding
events were the Rainbow Mountain sequence (M=6.6 and 6.4 on 6 July 1954, M=6.8 on
24 Aug. 1954) and the Fairview Peak earthquake (M=7.2 on 16 Dec. 1954). The
Fairview Peak event preceded the M=6.8 Dixie Valley earthquake by four minutes and
twenty seconds.
        Fault plane solutions for the Dixie Valley event are poorly constrained because
the arrivals are obscured by waveforms from the Fairview Peak event. Fault plane
solutions by Okaya and Thompson [1985], and Doser [1986] are not sensitive to change
in fault dip, rake, or strike. Similarly, the epicenter of the 1954 event is only known
within plus or minus 30 km. The absence of a dense seismic network in central Nevada
in 1954 made highly accurate locations of aftershocks impossible. Therefore, mapping
aftershock locations can not define the fault plane. The Dixie Valley fault plane solution
(N8°E, 49°E) of Hodgkinson et al. [1996] using leveling and triangulation benchmarks
suffers from a paucity of data (very few pre-rupture survey benchmarks) in the rupture
region, such that the triangulation network is unable to document slip along most of the
rupture. To summarize, no earthquake or geodetic studies are able to constrain fault
geometry.
Geologic Evidence for Low-Angle Dip on the Dixie Valley Fault
        Caskey et al. [1996] conducted the most recent and detailed study of the surface
faulting characteristics of the Fairview Peak and Dixie Valley earthquakes. The surface
ruptures associated with the Dixie Valley event had an average strike of N17°E along a
46 km rupture length [Caskey et al., 1996]. They noted substantial geologic evidence for
low-angle dip for the Dixie Valley fault along an approximately 20-km-long portion of
the rupture zone. Geologic evidence for a low dip angle lies between Wood Canyon in
the north, to just north of Coyote Canyon in the south (Figure 1).
Results of the 1998 geophysical experiment
        To extend geological observations into the subsurface, an NSF project funded
Louie, Caskey, and Wesnousky to conduct seismic reflection, refraction, and gravity
experiments along the putative low-angle section. Evidence for low-angle faulting along
the geophysical transects (Figures 1, 2) is seen at several scales, from observations at the
surface to over 2.5 km depth.
 Geologic evidence from Caskey et al. [1996], in the form of rupture mapping and
    balanced geologic cross-sections of the rupture graben are valid from 0 to 15 meters
    depth.
 The high-resolution seismic reflection profile (Figure 3) confirms the geologic
    observations and extends the smooth, low-angle fault plane to 75 meters depth.
 The no a priori assumption velocity optimization (Figure 4) shows a surface of
    increasing velocity dipping shallowly to 480 meters depth. The velocity contrast is
    indicative of low density basin fill, in contact with high velocity granite along the
    interface.
 Raw refraction shot-gathers (Figure 5, for example) constrain the fault to be relatively
    planar to 1.5 km and, given reasonable bedrock and alluvium velocity estimations,
    suggest low-angle dip.
 The medium-resolution time-section reflection profile (Figure 6) shows direct fault-
    plane reflections from 50 m to 750 m. In addition, truncations in hanging wall
    stratigraphy seen in the profile allow the interpretation of a slightly listric low-angle
    fault plane to approximately 1.5 km depth.
 The same character is seen in the true-depth cross section of the medium resolution
    profile (Figure 7), extending observations to 1.75 km depth. This true-depth section
    ensures that reflectors are put at their proper depths and the dips are accurate to
    within a few degrees.
 Modeling of the gravity profile along this transect (Figure 8) is valid from the surface
    to the maximum depth of the basin, approximately 2.7 km and is also consistent with
    a low-angle fault geometry.
        All geophysical evidence points to a low-angle fault at the latitude of the
geophysical transects. The data are not compatible with either a smooth, high-angle fault
or “staircase-like” fault geometry in which high-angle sections are interspersed with flat
ramps.
        Very little relevant geophysical evidence exists at latitudes other than that of the
1998 transects, however. Most geophysical fieldwork has been centered near the Dixie
Valley geothermal field, 45 km to the north. The aeromagnetic study of Smith [1967],
includes a portion of the proposed study area, but does not extend through the low-angle
section. The seismic refraction study of Meister [1967] includes six refraction lines in
the study area. The refraction lines do not have the required resolution to constrain fine-
scale structure, but are useful in estimating depth to bedrock in a gross sense. The
reconnaissance gravity survey by Schaefer [1982] will serve as a starting point in the
more complete acquisition of gravity data.

                                     Proposed Study
        Geologic evidence for low-angle rupture (Figure 9) essentially terminates to the
north of Wood Canyon and to the south of Coyote Canyon [Caskey et al., 1996]. Gravity
lows in Dixie Valley mapped by Schaefer [1982] are laterally coincident with the low-
angle rupture terminations mapped by Caskey (Figure 9). This coincidence argues for the
existence of an accommodation zone as the range-front Dixie Valley fault changes from
high angle, to low angle, back to high angle, along this reach. This is not, however, the
only possible conclusion. Several mechanisms can create the observed anomaly, but the
present inadequate sampling of the anomalies makes differentiating among the
hypothetical mechanisms impossible. With increased data coverage we will be able to
constrain models in which the gravity lows are caused by:
        1) Intrabasinal, “scissors-type” faults (Figure 10a). If we assume that the
           range front fault is marked by a change in dip at the mapped geologic
           terminations, east-west striking normal faulting may account for the change in
           basin depth evidenced by the gravity lows. Assuming that slip is constant
           along both high- and low-angle sections of the range front fault, fault offset on
           the intrabasinal, east-west faults will increase to the east. Maximum offset
           will be at the deepest portion of Dixie Valley; minimum offset will be along
           the Stillwater range front. This is analogous to the increasing separation of
           blades away from the hinge in open scissors. Maximum gravity gradients will
           be observed in north-south transects.
        2) Intrabasinal, north-south normal faults. If we assume that the entire
           rupture length of the Dixie Valley fault is low-angle, the gravity lows could be
           explained by north-south striking normal faults (Figure 10b). The gravity
           signature in this scenario should show maximum gravity gradients in east-
           west transects over the anomaly.
        3) Low density sediments in the hanging wall. The gravity lows may be
            caused by density differences in hanging wall sediments unrelated to the
            normal fault system.        The gravity signature in this case will seem
            unpredictable (i.e. have any shape), but the depth of the mass causing the
            anomaly will be shallow. Using the GRAV3D software package we will be
            able to determine the approximate depth to the anomalous mass, as discussed
            later. Therefore we will determine if the anomaly is caused by basement
            deflections (as with faulting) or low-density deposits.
        4) Sub-basins that formed during an earlier depositional phase. This is the
            most difficult anomaly to characterize. The shape of the anomaly can not be
            predicted beforehand and may exhibit the characteristics of any of the above
            three classes.
        The spatial coincidence of the gravity lows and low-angle surface rupture
terminations suggest that case 1) is the most likely scenario. The use of 2.5-D and 3-D
modeling and inversion programs will aid in interpretation. Simple horizontal directional
derivatives of the gravity field will help delineate the strikes and locations of intrabasinal
faults.
Experimental Method
        The density contrast between bedrock and unconsolidated or poorly consolidated
sedimentary rocks allows the study of bedrock structures underlying sedimentary basins.
With good gravity data coverage, only changes in rock density affect the shape of any
gravity anomaly. Basin shape and depth can be inferred from the spatial distribution of
the anomaly. Examples of this general technique can be found in West [1992, p. 200-
209]. Schaefer [1982] modeled the Dixie Valley, Nevada, basin using a similar
technique; many researchers have used this method for hydrologic, geothermal, mineral,
and oil exploration.
        We propose to acquire approximately 1000 gravity measurements with a LaCoste
and Romberg model G gravity meter. The measurements will be made on generally
north-south and east-west transects, centered on the gravity lows in the transition zone
(Figure 9). Transects will jog around small blocks of land owned by the U.S. Navy, if
necassary. Abbott, while working for the mineral exploration industry was able to
average 60 stations per day over varied terrain using an L and R Model G gravity meter
and geodetic GPS. The vast majority of the points will be taken over the essentially flat
basin, allowing ease of access and less time spent on inner ring terrain corrections.
          Vertical control will be provided by a geodetic-quality GPS. The surveys will be
tied to international gravity (IGSN 1971) at a gravity base station in Fallon, Nevada.
Local base stations will be surveyed in Dixie Valley, tied to the Fallon absolute gravity
station. Base station re-occupations will be made on a regular basis to monitor
instrument-related drift. Tidal effects will be removed by algorithm. Terrain corrections
will be estimated by eye in the field from 1 meter to 54 meters horizontally (Hammer
zones B-C) and computed by algorithm from 54 m to 167 km, using 90-meter digital
elevation models. The data will be reduced to complete Bouguer anomaly. The
curvature correction to the Bouguer slab equation will be applied when calculating terrain
corrections beyond 18 km.
        Existing gravity coverage from Schaefer [1982] will be merged into the dataset to
complete our coverage. The additional coverage will allow us to calculate the regional
gradient and avoid edge effects while modeling. The terrain corrections from 54 m to 167
km will be re-computed and re-applied to the existing data, along with the curvature
correction.
         To differentiate gravity effects due to small-scale basins from broader, regional
anomalies, a "bedrock gravity" will be removed from the data set. In the method used by
Abbott and Louie [2000] following Jachens and Moring [1990], all gravity stations will
be classified as "bedrock" or "basin" stations using geologic maps. The Bouguer
anomaly from stations on bedrock will be contoured to define the bedrock gravity field.
This bedrock gravity will be subtracted from the gravity anomaly generated by
contouring data from all stations. Removing the perturbations to gravity caused by
bedrock density contrasts will attenuate the gravity effect of deep density variations
below the surrounding mountain ranges. The gravity effect of a basin extends beyond the
basin boundaries, however, and these are subtracted as part of a “bedrock gravity”
estimate. Thus, basin depths subsequently estimated will be minima.
Error Approximation
         The repeat error of LaCoste and Romberg gravity measurements is estimated to
be 0.02 mGal. GPS horizontal locations, accurate to plus or minus 5 cm, will allow us to
neglect latitude correction errors. Vertical position will be accurate to within 0.1 meters,
and will be confirmed by GPS re-occupations of sites. Inner-ring terrain corrections,
estimated by eye, are negligible on the flat terrain of Dixie Valley. Still, in areas of high
relief along The Stillwater and Clan Alpine Ranges, a 20% error in estimating inner ring
terrain effects is possible. In these rare instances, an error of 0.02 mGal might be
introduced. All considered, an error in observed gravity of plus or minus 0.05 mGal is
possible. The limiting factor in the dataset will probably be the mismatch in existing and
new stations. Most of the existing coverage comes from Schaefer [1982]. All of his
measurements were made using a Worden gravity meter, using an altimeter for vertical
control. Schaefer estimates a maximum error of 0.62 mGal in worst-case scenarios. For
estimation of bedrock gravity and the addition of distal data to stay away from edge
effects, this amount of error is acceptable and the benefits of their inclusion outweigh the
problems caused by decrease in accuracy. Over critical areas (near intrabasinal faults, for
example) inclusion of data will be decided on a case-by-case basis.
Modeling and Interpretation
         The reduced data will be modeled with 3-D inverse and 2.5-D forward modeling
programs. The 3-D modeling will be done with GRAV3D under free academic license
from the University of British Columbia Geophysical Inverse Facility (UBC-GIF)
Outreach Program. Details of the inversion can be found in Li and Oldenburg [1998].
The inversion program has been successfully used on both synthetic and field data with
good results. A crucial element in gravity inversion is the ability to determine the depth
to the anomalous mass. GRAV3D allows the use of a depth weighting function to
separate anomalies caused by density contrasts at varying depths. This will be
particularly useful in discriminating between case 3) and cases 1) and 2) above. A case
history relating to a gravity inversion of the Voisey’s Bay deposit using this program may
be found at
http://www.geop.ubc.ca/ubcgif/casehist/voisey/intro.html.
         Selected transects will be 2.5-D forward-modeled using Northwest Geophysical
Associates’ GM-SYS software package, currently under license to UNR. Abbott and
Louie [2000] used this package to model the subsurface geology of an unexpected gravity
trough in the urban Reno, Nevada area. Initial depth estimates will draw from Abbott et
al. [2000], and Meister’s [1967] seismic refraction study. Meister’s [1967] study does
not provide enough resolution to shed light on the relatively narrow transition zones, but
will be useful in constraining density contrasts based on his depth to bedrock estimates.
Work Plan
1) Scout survey routes, assure complete access (Louie, Abbott, Jan.-March 2001)
2) Test gravity meter for proper repeat errors, send for servicing if needed
    (Abbott, Jan.-March 2001)
3) Test GRAV3D software (Louie and Abbott, Jan.-March 2001)
4) Hire student help (Louie, May 2001)
5) Acquire gravity data (Abbott 40 days, student helper 10 days, May-July 2001)
6) Reduce gravity data to complete Bouguer anomaly (Abbott, daily, while in
    field)
7) Merge new, and existing data; create basin anomaly map; compute and verify
    horizontal derivative map (Abbott, August-September, 2001)
8) Model data using GRAV3D and GM-SYS (Abbott, August-September, 2001)
9) Present preliminary findings at Fall 2001 AGU meeting, San Francisco
    (Abbott and Louie)
10) Prepare and submit manuscript of findings for peer reviewed journal (Abbott
    and Louie, December 2001)

				
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