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Geophysical confirmation of low-angle normal slip on the
historically active Dixie Valley fault, Nevada
Robert E. Abbott
Seismological Laboratory and Department of Geological Sciences, University of Nevada, Reno
1664 North Virginia Street, Reno, NV 89557; Voice: 775-784-4251, FAX: (775) 784-1833, rabbott@seismo.unr.edu
John N. Louie
Seismological Laboratory and Department of Geological Sciences, University of Nevada, Reno
1664 North Virginia Street, Reno, NV 89557; Voice: 775-784-4219, FAX (775) 784-1833, louie@seismo.unr.edu
S. John Caskey
Department of Geosciences, San Francisco State University
1600 Holloway Avenue, San Francisco, CA 94132, Voice: (415) 405-0353, FAX: (415) 338-7705, caskey@sfsu.edu
Satish Pullammanappallil
Optim L.L.C., University of Nevada, Reno Seismological Laboratory
1664 North Virginia Street, Reno, NV 89557, Voice: (775) 784-6613 FAX: (775) 784-1833,
satish@optimsoftware.com
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Geophysical confirmation of low-angle normal slip on the historically active
Dixie Valley fault, Nevada
Abstract. The 16 December 1954 Dixie Valley earthquake (MS=6.8) followed the nearby
Fairview Peak earthquake (MS=7.2) by four minutes, twenty seconds. Waveforms from the
Fairview Peak event contaminate those from the Dixie Valley event, making accurate fault-plane
solutions impossible. A recent geologic study of surface rupture characteristics in southern Dixie
Valley suggests that the Dixie Valley fault is low-angle (<30) along a significant portion of the
1954 rupture. To extend these observations into the subsurface we conducted a seismic reflection
and gravity experiment. Our results show that a portion of the Dixie Valley ruptures occurred
along a fault dipping 25° to 30°. As such, the Dixie Valley event may represent the first large,
low-angle normal earthquake on land recorded historically. Our high-resolution seismic
reflection profile images the rupture plane from 5 to 50 m depth. Medium-resolution reflections,
as well as refraction velocities, show a smoothly dipping fault plane from 50 to 500 m depth.
Stratigraphic truncations and rollovers in the hangingwall show a slightly listric fault to 2 km
depth. Gravity profiles conservatively constrain maximum basin depth and define overall
geometry. Extension along the low-angle section may have occurred in two phases during the
Cenozoic. Current fault motion post-dates a 13 to 15 Ma basalt, imaged in the hangingwall, and
inherits from a fault formed during an earlier extensional pulse, concentrated at 24.2 to 24.4 Ma.
The earlier extension suggests extraordinary slip rates as high as 18 mm/yr, resulting in the
formation of the low-angle fault break. Sections of the Dixie Valley fault where there is no
evidence for current low-angle slip correlate well with areas where no pre-15 Ma slip has been
documented.
Introduction
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Despite growing geological and geophysical evidence arguing for the existence of low-angle
normal faults that have accommodated large amounts of extension, the paradox of the nearcomplete absence of low-angle normal-mechanism earthquakes in the seismic record remains.
Slip on low-angle normal faults is not predicted in Andersonian theory [Anderson, 1942] and
studies of earthquake focal mechanisms, both regional [Doser and Smith, 1989] and global
[Jackson, 1987], show an extreme scarcity of large (M>5.5) normal fault mechanisms with dip
less than 30°. However, several researchers [e.g. Abers et al., 1997; Hatzfeld et al., 2000;
Johnson and Loy, 1992] have presented compelling evidence that substantial extension has
occurred along low-angle normal faults in the brittle upper crust.
Theories to resolve the seismicity paradox fall into two categories: those that do not
require brittle slip at low angles (e.g. “rolling hinge” models [Wernicke and Axen, 1988] and
flexural rotation [Buck, 1988]), and those that argue either for long earthquake recurrence
intervals [Wernicke, 1995], or a current rarity of active low-angle faults [Burchfiel et al., 1992].
Compelling evidence that brittle slip is possible on at least one low-angle normal fault
would have important ramifications for both fault mechanics theory and seismic hazard
calculations. Here, we present the results of a seismic reflection and gravity experiment to test
whether part of the 16 December 1954 Dixie Valley Earthquake (MS=6.8) produced slip on a
low-angle normal fault.
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 that has experienced a large amount of intraplate extension
in the Cenozoic. Much of the extension is accomplished by high-angle (50°-70°) normal faulting,
with several large seismic events recorded historically (e.g. 1915 Pleasant Valley, 1954 Fairview
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Peak-Dixie Valley, 1983 Borah Peak). The faulting has created predominately north-southtrending mountain ranges and sedimentary basins. Dixie Valley is one such basin; bounded by
the Stillwater Range on the west and the Clan Alpine Range on the east (Figure 1). The Dixie
Valley fault, site of the 1954 Dixie Valley earthquake, is the east-dipping range-bounding normal
fault along the eastern front of the Stillwater Range.
The 1954 fault trace lies along the southern portion of the Stillwater Range and separates
Mesozoic and Tertiary footwall rocks from late 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 alluvial fan 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 earthquakes that
took place within a period of 6 months in central Nevada. The preceding events were the
Rainbow Mountain sequence (MS=6.6 and 6.4 on 6 July 1954, MS=6.8 on 24 Aug. 1954) and the
Fairview Peak earthquake (MS=7.2 on 16 Dec. 1954). The Fairview Peak event preceded the
MS=6.8 Dixie Valley earthquake by four minutes and twenty seconds. Focal mechanisms for the
Fairview Peak and Rainbow Mountain events indicate NNW-striking normal-oblique faults with
dips ranging from 60 to 78°.
Fault plane solutions for the Dixie Valley event are poorly constrained because the
arrivals are obscured by waveforms from the Fairview Peak event. Doser [1986] used waveform
modeling to determine fault geometry with a strike of N10°W and a dip of 60°E; however, due to
contamination of the Dixie Valley waveforms, “large changes in strike and dip did not
significantly change the waveform shape” [Doser, 1986]. Similarly, Okaya and Thompson’s
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[1985] solution of N11°W, 62°E cannot be relied upon. They noted that: “Of the four focal
parameters (depth, dip, strike, and slip), only changes in depth are significant; changes in fault
plane strike, dip, or slip have negligible effect” [Okaya and Thompson, 1985]. 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.
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. They noted substantial
geologic evidence for low-angle dip for the Dixie Valley fault along an approximately 20-kmlong portion of the rupture zone. Geologic evidence for a low dip angle lies between The Bend in
the north, to just north of Coyote Canyon in the south (Figure 1). Geological evidence for lowangle dip at the surface in Caskey et al. [1996] include: 1) three point fault plane reconstructions,
2) shallow subsurface modeling of the rupture-trace graben, 3) fault-parallel fracture sets in the
footwall, and 4) geometry of the Stillwater rangefront.
Previous Geophysical Work in Dixie Valley
Dixie Valley has been the subject of numerous geophysical investigations. The studies primarily
focused on the northern part of the valley, around 40 km north of our study area. Okaya and
Thompson [1985] combine seismic reflection and gravity data to model northern Dixie Valley as
a half-graben with one major normal fault dipping 50°E to the northwest (the Dixie Valley fault),
and three smaller, west-dipping normal faults to the southeast. This model is inconsistent with
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Smith’s [1967] aeromagnetic study north of The Bend (Figure 1). Smith maps pre-Tertiary
basement under Dixie Valley as a graben within a graben.
Schaefer [1983] collected widespread gravity data throughout Dixie Valley. A portion of
Schaefer’s [1983] Bouguer anomaly map is reproduced in Figure 2. As can be seen in the figure,
two 4-5 mGal local gravity lows are near the latitudes of Coyote Canyon and Wood Canyon,
consistent with a more moderately dipping rangefront fault (or uplifted bedrock) between these
two latitudes. It should be noted that the shape of these anomalies is very poorly constrained and
the gravity lows may have other explanations unrelated to a change in rangefront fault dip. An
east-west linear transect of 24 gravity points near the latitude of Little Box Canyon (Figure 3) is
consistent with a low-angle Dixie Valley fault, assuming reasonable bedrock-alluvium density
contrast.
Meister [1967] and Herring [1967] conducted seismic refraction experiments near the
latitude of IXL Canyon (Figure 3). Meister [1967] interpreted southern Dixie Valley as a
composite graben, based on several short refraction lines parallel to the rangefront, and an eastwest cross-valley profile. The east dipping Dixie Valley fault was interpreted to be a
combination of high-angle normal faults and flat terraces, resulting in a “staircase-like” fault
geometry. These data allow for an alternate interpretation, however. In our evaluation of
Meister’s [1967] data, a single low-angle dip normal fault can replace the previous stair-step
structure. Herring’s [1967] experiment assumed, a priori, high-angle dip and the assumed
geometry was used to test a “sideswipe-refraction” technique.
Methods
Field Data Acquisition
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Figure 3 shows the location of our geophysical transects. The Cattle Road profile consists of two
seismic reflection profiles and a gravity profile, while the Scarp profile consists solely of gravity
coverage. The trend of the Cattle Road profiles is within 10 of the gradient of the gravity field
near the rangefront. If the gravity gradient reflects the direction of true dip of the Dixie Valley
fault, than apparent dips measured from the geophysical profiles will be underestimated by no
more than 0.5.
The medium-resolution Cattle Road profile extended 3.6 km into the basin and utilized 8
Hz geophones. It was composed of 4 stationary setups of 48 receivers with 15.2 meter spacing.
The 132 source points were 2-7 kg of high explosive buried 2 m below the surface. Off-end and
longer offset shots were recorded to increase fold and improve deep velocity information.
Maximum source-receiver offset for the medium-resolution line was 2.8 km, and maximum fold
was 24.
The high-resolution Cattle Road profile overlapped the medium-resolution profile close
to the 1954 rupture. It was conducted within 150 m of the rangefront scarp using 100 Hz
geophone groups with 2 m spacing. 67 sledgehammer source points were rolled through the
array. Six inline geophones per group were used to reduce noise from ground roll. Maximum
source-receiver offset for the high-resolution line was 124 m. Maximum receiver fold was 32.
Gravity data along Cattle Road were acquired with a LaCoste and Romberg Model G
gravity meter. Gravity coverage started at the scarp and extended eastward 12.5 km into the
basin at a 250 m average station spacing. Vertical control was supplied by a geodetic-quality
GPS.
Gravity data were also collected parallel to the rangefront scarp (Scarp Gravity, Figure
3). The Scarp profile was located along a line where depth-to-bedrock was assumed to be
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approximately constant, and therefore any gravity variations would be largely due to density
variations within the geophysical basement. In this way, errors in depth-to-bedrock calculations
from our 2.5-D forward model along the Cattle Road profile can be estimated.
Seismic Data Processing
Conventional processing and post-stack migration- Conventional seismic data processing
techniques were used to remove noisy traces, mute direct waves and attenuate other unwanted
arrivals. The medium-resolution line was bandpass filtered (6-8 Hz, 100-120 Hz trapezoidal
filter), and then filtered with a polygonal, 48 trace, 500 ms f-k domain filter. The f-k filter was
designed to eliminate 400 m/s Rayleigh waves. After AGC, constant velocity stacks at 200 m/s
intervals were used to pick stacking velocities. Common depth points were binned at 15.2 m
intervals, with no amplitude variation with fold. The subsequent CDP stack was then Stoltmigrated at a constant 2000 m/s velocity.
The high-resolution seismic line was bandpass filtered (8-12 Hz, 200-202 Hz) and f-k
domain filtered (48 trace, 125 ms). Stacking velocities were chosen using the same method as
with the medium-resolution line. Common depth points were binned at 2 m intervals and the
resulting CDP stack was Stolt-migrated using the rms stacking velocities.
Velocity Analysis- Due to complex geometry, strong lateral velocity variation, and steep dips
(for seismic imaging) in the subsurface at Dixie Valley, we chose to compute pre-stack depth
migrations. For input into our pre-stack migrations, we obtained a detailed velocity image of the
subsurface by performing a nonlinear optimization on first arrivals picked off raw shot-gathers.
The
optimization
technique
employs
a
generalized
simulated
annealing
algorithm
[Pullammanappallil and Louie, 1994] to invert first arrivals for subsurface velocity structure. We
9
used a commercial package, SeisOpt @2D™ (Copyright Optim LLC, 1998-1999), that
implements this method.
The simulated annealing algorithm is a Monte-Carlo based estimation process that has the
property of being independent of the starting model and has the ability to find the global
minimum (i.e. solution) for a highly nonlinear problem. These characteristics make the algorithm
a very effective tool for velocity estimation. Travel time inversion is a highly nonlinear problem
because any perturbation in the velocities alters the path of the ray propagation, changing the
travel times recorded at the surface geophones. This non-linearity makes linear methods
dependent on the starting model; that is, the accuracy of the final velocity model is dependent on
a good initial guess. The method employed by SeisOpt @2D™ “tests” several thousand models
before arriving at the optimized velocity model. The only inputs required by the algorithm are
the first arrival picks and survey geometry (source and receiver coordinates). In addition to the
final velocity model, SeisOpt @2D™ outputs a ray coverage or “hit count” plot that shows what
parts of the model were sampled by the seismic array. The algorithm outputs only the velocities
in the subsurface that have been sampled by the rays.
A total of 6117 first-arrival picks from 134 shot gathers were used for the optimization.
SeisOpt @2D™ can handle only two-dimensional array geometry. Hence, in order to overcome
a bend in the medium-resolution seismic line, we project the source locations to a straight line
while maintaining the true offsets of the source-receiver pairs. As a result of this projection, the
optimized velocities might show some lateral smearing in the vicinity of the bend in the profile.
The resulting velocity model was used in a pre-stack migration algorithm to image the seismic
reflectors directly.
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Pre-stack migration- In order to use the optimized velocities to perform a pre-stack Kirchhoff
migration, we first extended the velocities down to a depth of 2.0 km. Like Pullammanappallil
and Louie [1994] and Chavez-Perez et al. [1998] we extended the optimized velocity models for
migration by finding the maximum constrained velocity value in each column of the velocity
model, and substituted that value into the column of the model everywhere below the depth of
the maximum velocity. We then performed a severe lateral smoothing below the depth of first
arrival constraint.
The resulting velocity model was used in a pre-stack migration algorithm to image the
seismic reflectors directly. The pre-stack Kirchhoff-summation algorithm was originally used to
image the San Andreas fault zone by Louie et al. [1988], and modified by Louie and Qin [1991]
to account for reflection ray paths that may bend significantly through strong lateral velocity
variations. In Dixie Valley we did not attempt to image near-vertical structure, as previous work
did, but we needed to account for strong ray bending through the velocity contrasts at the edge of
the basin, and through any lateral variability in the Tertiary basalts within the basin section. So
we added to the algorithm a dip-dependent obliquity factor.
Another addition is the migration-operator antialiasing criterion of Lumley et al. [1994],
which leads to high-cut filtering of the seismic traces. Completely preventing the spatial aliasing
of the migration operator leads to discontinuous coverage of depth points for our mediumresolution survey, and detracts from the lateral continuity of reflections. Thus, for the aliasing
calculation, we used a receiver spacing that is half of the actual 15.2 m spacing. This apparent
spacing yields a mild operator antialiasing effect that only removes the worst-aliased frequencies
of the most steeply-dipping structure, while retaining the lateral continuity of the unaliased nearhorizontal structure.
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We migrated the f-k filtered data also used for stacking, with additional 8-100 Hz
bandpass filtering, locations projected to a straight line, and AGC with a 0.5 s window for
amplitude balancing. Along with migration of the data we estimated noise data in the manner of
Harlan et al. [1984] by resampling to destroy pre-stack trace-to-trace coherency. Harlan et al.
[1984] provided a Bayesian pre-stack coherency measure computed through statistical
comparison of the data migration against the noise migration. We screened the data migration
through the coherency image to yield an enhanced structural image. The enhancement
emphasizes those structures that produce the most coherent reflections in the pre-stack shot
gathers.
Gravity Data Processing- We reduced gravity data to simple Bouguer anomaly using standard
techniques. Terrain corrections out to 54 m (Hammer rings A-C) were estimated by eye in the
field. We used a density value of 2.67 g/cm3 for both the Bouguer slab correction and for terrain
correction. Elevations are accurate to within 5 cm, as seen in site re-occupations. Analysis of
loop closures indicates a maximum drift of 0.16 mGal, forming the limiting error of these data.
This amount of error is acceptable, given the magnitude of the basin anomaly (approximately 35
mGal).
Results
High-Resolution Line
Figure 4 is a no vertical exaggeration common-depth-point stack of our high-resolution line near
the 1954 scarp. The Dixie Valley fault is the very prominent reflector dipping to the east from 40
to 160 ms, at 28°. This is interpreted as the bedrock-alluvium contact. The fault surface is
approximately 16 m below the surface at the rangefront scarp. This depth-to-fault is nearly
identical to that predicted by the balanced geological cross-section in Caskey et al. [1996], their
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Figure 12d. The balanced section is reproduced at the location of the graben in Figure 4. There is
no evidence for staircase-like fault geometry at this scale. Surface-parallel reflections are seen
early in the section (above 60 ms) and represent layering in the alluvial fan above the fault. The
coherency of these reflections is disturbed in the vicinity of the 1954 scarp (at 25 m, 30 ms),
possibly in relation to the formation of the rupture graben.
Medium-Resolution Line
Fault Refraction- Fault geometry can be inferred from arrivals seen in several raw shot gathers.
The headwave arriving at the same time at all stations in the gather of Figure 5 limits the range
of possible fault dips. If we use our optimization velocity model (Figure 6) as a guide, maximum
and minimum bedrock-to-alluvium velocity ratios can be placed at 2.1:1 and 1.4:1. After
adjusting for elevation changes along the array, fault dips of 21° to 39° could result in the
vertically propagating headwave seen in Figure 5. Using a more reasonable velocity estimates
than the minimum and maximum results in a fault dip of 29° to 30°. Using the same reasonable
velocity estimation, but assuming fault dips of 21° and 39°, results in the synthetic arrivals
represented by dashed lines in Figure 5.
These results are in general agreement with Meister’s [1967] velocity model of The Bend
area, generated from his Dixie and IXL Canyon refraction lines. His bedrock-alluvium velocity
contrast of 1.9:1 results in a fault dip of 25°. The coherency of the first arrival on Figure 5
indicates that the fault is smoothly varying and relatively planar down to 500 m depth. This is in
contrast to the segmented first arrivals that would be generated by refraction and diffraction
along a staircase-like fault geometry. Meister’s [1967] model in which he modeled a staircaselike fault geometry had insufficient resolution (cross-basin spacing of approximately one
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geophone per 400 m as compared to one geophone per 15 m in this study) to resolve down-dip
fault geometry at a fine scale.
Migrations- A post-migrated stack of our medium-resolution Cattle Road profile is presented in
Figure 7. The data were Stolt time migrated at a constant velocity of 2000 m/s. The interpreted
fault plane reflector, dipping eastward at 25° to 30°, can be traced to the Dixie Valley fault
reflector seen in the high-resolution profile (Figure 4). Reflections sub-parallel to the fault can be
seen in the footwall, suggesting the Caskey et al. [1996] foliation in the footwall bedrock. The
fault reflections are obscured below a highly reflective hangingwall layer at about 400 ms. This
first, strong basin reflector has been interpreted to be a “capping” basalt layer by Hastings [1979]
in a reflection profile 25 km northwest, in the Carson Sink (Figure 1). This interpretation is
supported by drilling logs that intersected the profile. Okaya and Thompson’s [1985] reflection
profile near the Dixie Valley Geothermal Field (Figure 1) also shows a similar reflection, which
they interpret to be from the same reflector seen in Hastings [1979]. This basalt is the
culmination of a Tertiary volcaniclastic sequence that is seen in all the ranges surrounding the
Carson Sink and Dixie Valley. The sequence appears to be locally thickened in the southern
Stillwater Range, with thicknesses approaching 500 m [Page, 1965]. We interpret the strong
series of reflections starting at 500 ms and ending at 900 ms to be originating from this sequence.
Below the capping basalt, hangingwall stratigraphy can be traced to its termination
against the fault at depth. Mapping the terminations allows us to extend our fault plane
interpretation to 1.25 seconds. Many of the reflections show increased westward dip close to the
fault, forming rollover anticlines. The rollover anticlines form in response to listric fault
geometry.
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Figure 8 shows the enhanced pre-stack migration that results from imaging the mediumresolution data through the extended 8 m velocity model shown in Figure 6. Because of the lowpass filtering inherent in the antialiasing criterion, this section does not have as high a vertical
resolution as the post-stack migrated section, and may not image more steeply dipping structures
such as anticline flanks. As demonstrated in Death Valley, California, by Chavez-Perez et al.
[1998], however, the pre-stack migration through an optimized velocity model does assure that
this section puts structures at their true depths. Thus any dips interpreted from Figure 8 will be
accurate to within a few degrees.
The top of the Tertiary basalt at 0.5 km depth, and its bend into a west-dipping rollover
anticline against the Dixie Valley fault about 1 km east of the 1954 scarp, can be interpreted in
Figure 8. The pre-stack image shows a reflection sequence at 0.8 km depth, which the stacked
data could not image without velocity pull-down effects. These reflections also show some
evidence of rollover, and may originate at the bottom of the Tertiary volcanic sequence, at the
top of an earlier-Tertiary basin-fill sequence. The image shows only parts of the Dixie Valley
fault plane itself, at 0.5, 1.4, and possibly 1.7 km depths, dipping east from the 1954 scarp at 28°.
The low-frequency response of the antialiasing obscures the fault plane above 0.5 km depth. The
prestack migration, like the post-migrated stack, failed to clearly image the fault plane below the
strong reflectivity of the capping basalt. Truncations of the deeper basin stratigraphy do support
the linear, shallowly-dipping fault geometry.
The imaging of a flat basin fill sequence at 1.1 km, in onlap relation to the Dixie Valley
fault, suggests that no appreciable rotation of the fault plane has occurred since its formation. It
also suggests that significant extension had occurred before the initiation of volcanism. The
flanks of any rollover anticlines at this depth could have been hidden by the migration
15
antialiasing criterion. Alternatively, a constant average fault dip could have avoided the
formation of rollover anticlines in the deep basin. The two upwarps in the fault profile suggested
by Figure 7 between 0.5 and 1 s two-way time may have resulted in rollovers forming only updip
of each. Although the fault reflector itself cannot be seen below the capping-basalt reflector,
Figure 8's imaging of a continuation of onlap truncations along the projected fault plane to 1.5
km depth supports our interpretation of an early, now deeply buried, but unrotated episode of
basin filling.
Gravity Results
Bouguer gravity along Cattle Road is agrees with Schaefer’s [1983] study. 2.5-dimensional
gravity modeling (Figure 9) is consistent with a shallowly dipping fault. In the cross-section the
Dixie Valley fault is modeled as dipping 26°. Density values generally follow those in Speed
[1976] and Thompson [1959] as summarized in Okaya and Thompson [1985]. A value of 2.5
g/cm3 was chosen for the volcaniclastic units. Higher in the section, an average value of 2.3
g/cm3 was chosen for volcanic units intercalated within sedimentary units, as seen in Hastings
[1979] well log in the Carson Sink. The gravity effect of topography (terrain correction) was
modeled in the plane of the cross-section.
Maximum basin depth is approximately 2700 m, using our density scheme. The Dixie
Valley fault merges with the steeper Clan Alpine range-bounding fault. It is likely that the Clan
Alpine range-bounding fault shown in Figure 9 is actually a series of faults as seen in Okaya and
Thompson [1985], rather than the one large fault modeled. Due to our lack of accurate density
information or seismic profiles in this area, an effort was made to produce the simplest model
that fits the anomaly. Although a staircase-like fault geometry to the east cannot be ruled out
solely on the basis of gravity data, the absence of short wavelength anomalies suggests no rapid
16
changes in fault dip. Results from the Scarp profile (Figure 2), not shown, indicate that
intrabasement density contrast is negligible along this reach of the fault.
Discussion
Evidence for low-angle faulting is seen at several scales, from observations at the surface to over
2.5 km depth. Moving from shallow to deep, the evidence consists of:
1) Geologic evidence from Caskey et al. [1996], in the form of rupture mapping and geologic
cross-sections are valid from 0 to 10 m depth.
2) Our high-resolution profile (Figure 4) confirms the geologic observations and extends the
smooth, low-angle fault plane to 75 meter depth.
3) The no a priori assumption velocity optimization (Figure 6) shows a surface of increasing
velocity dipping shallowly to 480 m depth.
4) Raw shot-gathers (Figure 5, for example) constrain the fault to be relatively planar to 500 m
and, given reasonable velocity estimations, suggest low-angle dip.
5) The medium-resolution time-migration reflection profile (Figure 7) shows direct fault-plane
reflections from 50 m to 750 m. In addition, truncations in hangingwall stratigraphy seen in
the medium-resolution profile allow the interpretation of a slightly listric low-angle fault
plane to approximately 1.0 km depth.
6) The same character is seen in the Kirchhoff depth migration of the medium resolution profile
(Figure 8), extending observations to 1.75 km depth.
7) Gravity mapping by Schaefer [1983] (Figure 2) and this study (Figure 9) are valid from the
surface to the maximum depth of the basin, approximately 2.7 km. The gravity data, too, are
consistent with a low-angle fault geometry.
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Timing of extension
The recognition of an early basin-fill sequence below the mid-Tertiary capping basalt (Figure 8)
suggests that extension in the southern Stillwater Range started earlier than extension to the
north. Early sediments have been mapped in the Stillwater Range at the latitude of our transects
by John [1995]. Seismic reflection profiles near the Dixie Valley geothermal field [Okaya and
Thompson, 1985] and the northern Carson Sink [Hastings, 1979] record no such sequence. The
pervasive capping basalt layer, dated as 8 Ma [Hastings, 1979] and 13 to 17 Ma [Nosker,
1981], was interpreted to be pre-extension. In southern Dixie Valley, a rapid, but spatially
limited, pulse of extension is recorded by tilted fault blocks in the Stillwater Caldera Complex
[John, 1995; Hudson et al., 2000]. The tuffs, flows, and plutons associated with the complex
have tilts of 60° to 70°. Similar deposits in the southern Clan Alpine Mountains and northern
Stillwater Range dip less than 30°, suggesting very localized uplift and tilting. Hudson et al.
[2000] estimate over 200% extension in a brief period from a balanced cross-section near IXL
Canyon (Figure 2). The extension is well constrained to have started at 24.2 to 24.4 Ma [Hudson
et al, 2000]. Parry et al. [1991], based on fluid inclusion and alteration mineral studies, also
estimate extension starting at 20 to 25 Ma. The extension direction was, in general, perpendicular
to current structural strike, although some vertical axis rotation has occurred since early Miocene
time [Hudson and Geissman, 1987; Hudson et al., 2000].
Two contrasting models of early Miocene extension have been suggested. Parry et al.
[1991] suggest the current Dixie Valley fault is the major fault accommodating extension and
localized uplift and has been active since the early Miocene. Similarly, King and Ellis [1990] cite
the Dixie Valley fault as a potential field example of a fault accommodating dramatic, localized
uplift, starting in the early Miocene. The rotated crustal blocks mapped in the Stillwater Range
18
represent rotation of footwall blocks of the Dixie Valley fault in this interpretation. In contrast,
Hudson et al. [2000] consider it unlikely that the current Dixie Valley has been active longer
than 13 to 15 Ma (post-dating the capping basalt). As evidence, they point to a sharp
accommodation zone between two dip domains in the Stillwater Range [Hudson et al, 2000].
The sharply defined accommodation zone separates domains of east and west dip in the rotated
caldera deposits, requiring two large normal faults of opposing dips to form. The current Dixie
Valley fault cuts across both the east and west dip domains. Instead, they favor a model in which
extension along a west dipping (now inactive) detachment rotated the caldera deposits, which are
synthetically and antithetically dipping in the hanging wall of the low-angle detachment [Hudson
et al., 2000]. Current Basin and Range extension subsequently cut the resulting structure in the
middle Miocene [Hudson et al., 2000].
We favor early Miocene initiation of the southern Dixie Valley fault from our reflection
profiles and evidence for low-angle dips. Evidence for early extension comes from the basin-fill
and volcanic sequences seen in Figures 7 and 8 and mapped in the Stillwater Range John [1995].
This flat-lying sequence is directly below a thickened Tertiary (early to middle Miocene)
volcanic section. The volcanic section is locally thickened in the southern Stillwater Range
[Page, 1965]. The volcanic sequence may be thickened in our profiles because it filled a preexisting basin at this latitude. The reflection profiles are adjacent to the west-dipping blocks in
the Stillwater Range, which require a large east-dipping normal fault in precisely the same
location as the current Dixie Valley fault.
Formation at low-angle may have been in response to rapid extension. Abers et al. [1997]
found a correlation between low-angle faulting and increasing strain rate in the WoodlarkD’Entrecasteaux rift system, where strain rates of up to 10-8 s-1 are seen. Hudson et al. [2000]
19
find minimum strain rates in the early Miocene of 10-13 s-1 in the southern Stillwater Range. That
strain rate may be underestimated because the exact duration of extension is not known.
Nevertheless, strain rates at least an order of magnitude greater than usually encountered may
have played a role in forming the low-angle structure. Current Basin and Range extension along
the low-angle reach may have inherited the same fault that accommodated early Miocene
extension. North and south of the low-angle section that formed in the early Miocene, extension
had no favorably oriented structures to inherit and formed a new steeply-dipping normal
rangefront fault at approximately 13 to 15 Ma.
Magnitude of slip
Estimates for fault slip from our seismic lines can be broken into two phases (Figure 10). One
phase is constrained by the thickness of the early Miocene basin, and the other by the current
elevation of the capping 13 to 15 Ma basalt. Our gravity results indicate a maximum basin depth
of 2.7 km. The bottom of the Tertiary volcanic sequence and top of the early Miocene basin is
interpreted to be at 1.1 km depth from our Kirchhoff migration (Figure 8). Hudson et al. [2000]
estimate, using dates and tilts of caldera deposits, that this phase of extension lasted at most 4
million years. The majority of the rotation was most likely accomplished in approximately 0.2
million years. The 1.6 km of vertical offset along a 30° fault in approximately 0.2 Ma
corresponds to slip rates of 18 mm/yr (extension rate of 9 mm/yr). This is an extraordinarily fast
slip rate, but it has a current analog in the Woodlark-D’Entrecasteux extensional province, which
has extensional rates of over 15 mm/yr and as high as 40 mm/yr in an area of low-angle normal
faulting [Abers et al., 1997]. However, using the most conservative estimate of extension
duration from Hudson et al. [2000] yields a more typical slip rate of 0.9 mm/yr in the early
20
Miocene. Errors in the gravity model and the duration of extension can make these fault slip
rates uncertain, however.
The second phase of extension post-dates the middle Miocene capping basalt (13 to 15
Ma). The basalt is seen at 400 m below the surface (650 m elevation) in Figure 8 and at an
elevation of 2500 m in the Stillwater Range. Accommodating the 1850 m of vertical offset along
a 30° fault in 13-15 million years requires a slip rate of 0.28 to 0.32 mm/yr. These slip rates
compare favorably with Meister’s [1967] estimation of 0.3 mm/yr over 15 million years for
southern Dixie Valley. Okaya and Thompson [1985] found a fault slip rate in northern Dixie
Valley of 0.47 mm/yr along a 50° fault (using Hasting’s [1979] basalt age of 8 Ma). This would
be equivalent to a 13 million year slip rate of 0.29 mm/yr. Bell and Katzer [1990] report a
Holocene vertical-slip rate of 0.5 mm/yr in the Bend area.
Conclusions
Our results indicate that slip along a section of the 16 December 1954 Dixie Valley earthquake
rupture took place along a fault plane of unusually low dip (<30°). In this regard, it is the first
large historical earthquake on land for which slip on a low-angle normal fault has been
documented. Evidence for the low-angle fault plane is seen at several different scales, from 0 to
2.7 km depth. A computed velocity model, with no a priori assumptions, supports a low-angle
hypothesis, as does gravity modeling. Our results suggest extension in the southern Stillwater
Range had two distinct phases. The first period is marked by rapid extension that initiated a lowangle fault. The second period of extension began at 13-15 Ma and inherited a portion of the
previous Dixie Valley fault along the low-angle section. Fault sections with steep dip formed 13
to 15 million years ago.
21
ACKNOWLEDGEMENTS. This project was funded by the National Science Foundation
under grant EAR-9706255 to J. Louie, S. J. Caskey, and S. Wesnousky. The W. M. Keck
Foundation donated seismic equipment, computers, and modeling software. K. Miller and G.
Abers provided careful reviews and helpful comments. The 1998 Geophysical Applications class
at the University of Nevada, Reno performed all geophysical fieldwork. Class participants were:
A. Cadena, T. Rabe, M. Herrick, M. Johnson, A. Rael, T. Blechen, and E. Hobson. C. Mann, J.
Ollerton, and J. Oswald rendered additional field assistance.
22
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27
Figure Captions
Figure 1. Map of Dixie Valley, Nevada and surrounding areas. Gray areas are mountainous,
white areas are basins. Dashed box is area of Figure 3. WC, Wood Canyon; CC, Coyote Canyon;
DVGF, Dixie Valley geothermal field.
Figure 2. Complete Bouguer gravity map of Dixie Valley after Schaefer [1983]. Gravity
contours are at 2 mGal intervals, hachures are on closed minima. Evidence from fault mapping
for low-angle Dixie Valley fault dip is between Wood Canyon and Coyote Canyon. Local
gravity high between these two latitudes may represent less vertically-offset basement rocks.
Figure 3. Map showing our geophysical transects, the area corresponds to dashed box in Figure
1. The two gravity transects are oriented approximately perpendicular to and parallel to the 1954
ruptures. Our medium-resolution seismic line along Cattle Road starts at the white box and
terminates at the white star. The white box bounds the high-resolution seismic line along Cattle
Road.
Figure 4. CDP stack of our high-resolution Cattle Road profile. Section is one-to-one at 1700
m/s. The superimposed line-drawing predicting depth-to-fault is from Caskey et al. [1996].
Figure 5. Raw shot gather from our medium-resolution seismic reflection line. The highlighted,
simultaneous refraction arrival can be used to limit the range of fault dip, given velocity
information. The dashed lines correspond to predicted headwave arrival times from faults
dipping 21 and 39. The smooth nature of the arrival suggests that the fault surface is planar
down to 500 m depth. The 48 receivers are within 730 m of rangefront scarp on Cattle Road. The
source is at SP101 on Figure 3.
28
Figure 6. Velocity model obtained by first arrival optimization of the medium-resolution data.
The one-to-one velocity model reveals deepening of low velocities towards the center of the
profile (east, towards the valley) and shallowing of high velocities towards the rangefront. White
line is the deepest penetration of refracted first arrivals. After testing several resolution settings,
an 8 m grid spacing resulted in the most robust model. The model is extended down to 2 km for
use in migration (note broken depth axis).
Figure 7. Stolt-migrated stack of our medium-resolution seismic reflection profile. Depths are
calculated using the 2000 m/s migration velocity. Darker lines trace the most prominent
reflectors. Dashed line represents the interpretation from Meister [1967]. See text for description.
Figure 8. Kirchhoff pre-stack migration of our medium-resolution seismic reflection profile,
with interpreted section below. See text for description.
Figure 9. 2.5-D gravity model of Dixie Valley at Cattle Road (Figure 3). Circles are observed
gravity, dashed line is model misfit. Fault dips to east at approximately 26°.
Figure 10. Schematic diagram showing fault geometry and activity since the late Oligoceneearly Miocene. The fault offset from active Basin and Range deformation is estimated from the
elevation of basalt deposits in the Stillwater Range (Af) and depth of the same unit (Ah)
interpreted from our seismic profiles. The fault offset from the late Oligocene-early Miocene
pulse of extension is calculated from the total basin depth interpreted from our gravity results
(C), and the depth at which reflections interpreted as coming from volcaniclastic deposits change
to reflections interpreted as coming from basin-fill deposits (B). Constraints on duration of
faulting are provided by isotopic age dating and paleomagnetic rotation of rock units [Hudson et
al., 2000; John, 1995].