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1. Introduction
The debate over the existence and type of crustal root underlying the Sierra
Nevada Mountains began during 1936 when Lawson (1936) proposed that there should
be a 68 km, Airy-type root beneath the highest topography, Mt. Whitney, based on
density and isostasy calculations. These calculations are plausible considering more
recent data and about other mountain ranges throughout the world, but for the Sierra
Nevada Mountains, the story is much more complicated.
Results from a seismic refraction experiment extending from San Francisco, CA
to Fallon, NV, and then from Fallon, NV down the spine of the Sierras and into Owens
Valley appeared to confirm Lawson’s theory (Eaton, 1963). Then in 1973, from
teleseismic arrivals, Carder proposed a total lack of any crustal root beneath the Sierras.
He propsed that buoyancy from the mantle is supporting the high mountains. The debate
was again brought to the forefront in 1980 when a paper by Pakiser and Brune (1980)
found a 50 km-thick root beneath the entire Sierra Nevada also based on teleseismic
arrivals, except this time the waves were traveling along the spine of the Sierra.
Since this initial work, the debate has continued. More research has been
completed in the souther Sierra, but the data that exists north of Fresno, California is
sparse, and little has been done to connect the evolution of the Sierra Nevada root as
whole. Using new seismic refraction data presented in this paper and from the Northern
Walker Lane refraction experiment (Louie et al. 2004), this thesis builds on the current
models for the southern Sierras to provide an explanation for high topography found
throughout the Sierra Nevada.
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2. Previous work
It is has been hypothesized through several types of geophysical surveys including
seismic and gravity studies, and geological analysis that the root beneath the southern
Sierra Nevada Mountains has delaminated, creating a west centered drip into the mantle
(Zandt et al., 2004; Wernicke et al., 1996; Manley et al., 2000; Farmer et al., 2002; Jones
et al., 2004). It is debated how much of the root foundered, whether the delamination
occurred throughout the entire Sierra Nevada, or whether it progresses northward from
the already foundered southern root. There is also dispute over the actual processes
involved in the foundering of the root. Some argue that negative buoyancy from the
eclogitized root caused the separation (Zandt et al., 2004), while others argue that an
outside force such as extension between Death Valley and the Sierra Nevada caused
stress forming factures within the crust and allowing the heavy root to delaminate
(Manley et al., 2000; Jones et al., 2004).
Extensive geochemical and petrologic work have given insight into the evolution of
the Sierras. The volcanics of this region provide information regarding the timing and the
extent of the delamination of the crustal root. Before 13 Ma, magmatism was located
south of the Garlock fault within the Mojave Desert. Cenozoic volcanism within the
Sierra Nevada is marked by two major pulses. At approximately 12 to 8 Ma magmatism
began within the Sierra Nevada and White Mountains. The first pulse produced typical
subalkalic to alkalic trending basalts, typical of continental eruptions. After a period of
little to no volcanism, a second quick surge of ultra-potassic magmatism began around 4
Ma and quickly ceased at 3 Ma. The alkaltic volcanics erupted within a localized region
of the Sierra Nevada, Owens Valley and the Coso Range, specifically in the Kings and
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San Joaquin volcanic fields (Figure 1). Following another quiet period, lava was erupted
during the Quaternary within the Big Pine, Golden Trout, and Long Valley volcanic
fields. These youner volcanics were of more typical subalkalic volcanics (Manley et al.,
2000; Farmer et al., 2002).
Based on work from Ducea and Saleeby (1996) on the composition of xenoliths
entrained in Miocene and Pliocene volcanics, they suggest that delamination occurred
between 10-3 Ma. From petrologic analysis alone, many cite the start of the delamination
at ca. 3.5 Ma, due to the unique composition of the Pliocene lavas. The concentrated,
short-lived pulse had no progression north or south (Manley et al., 2000). Xenoliths from
the Pliocene volcanics indicate that they were erupted from the mantle, and petrologically
could not have come from crustal rocks (Farmer et al., 2002). These mafic to
intermediate compositional rocks are not only high potassium, but also have the lowest
SiO2 and highest MgO ratios known for any Sierran rocks (Farmer et al., 2002).
Cenozosic volcanism within the Sierra Nevada and its vicinity is coincident with the
beginning of the Cenozoic uplift of the Sierras (Jones et al., 2004). The timing of this
approximately one kilometer gain in elevation also corresponds to extension in Death
Valley, as well as movement along the San Andreas fault (ca. 7 or 6 Ma started
(McKenna and Hodges, 1990 cited in Jones et al., 2004; Wernicke and Snow, 1998).
Other concurrent events include sediment fillin of valleys east of Sierra Nevada (Bacon et
al., 1982, Burchfiel et al., 1987, Lueddecke et al., 1998), folding of the California coast
ranges and initation of the spreading of the Gulf of California (Axen and Flether, 1998).
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3. Idaho-Nevada-California (INC) refraction transect
3.1 INC experiment
In August 2004, the Nevada Seismological Laboratory and the Department of
Geological Sciences and Engineering completed a 600 km-long refraction transect
extending from the Idaho-Nevada border, south of Battle Mountain, Nevada through
western Nevada, across the Long Valley caldera and the central Sierra Nevada, and into
Fresno, California (see Figure 1). Spaced approximately 1.5 km apart along the 600 kmlong transect, 411, 24-bit single channel portable seismographs, known as RefTek
“Texans,” connected to 4.5 Hz, single, vertical geophones that recorded mining blasts and
small earthquakes. The instruments were programmed to record for a total of 96 hours on
the local dates of August 16, 17, 19, and 20, 2004 (Julian days 229, 230, 232, and 233).
Although mine blasts are triggered only during daylight hours, to insure the instruments
would gather earthquakes occurring at night, the instruments were programmed for four
24-hour periods. In the previous 2002 Northern Walker Lane (NWL) refraction
experiment (Louie et al., 2004), instruments were only programmed to record during
working hours, and valuable earthquakes that would have served as refraction reversals
were missed (Louie et al., 2004).
The recorder array consisted of Texan instruments with 64 or 32 megabytes of
memory. The 32 Mb instruments were retrieved halfway through the experiment for a 24hour period to download data and change batteries. The seismographs were programmed
to sample at 50 Hz, slower than commonly used in refraction recording. The recovery
rate for instrument hardware was 99.5 percent. The data recovery rate was over percent.
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The deployment consisted of three download/home stations located in Battle
Mountain, NV, Austin, NV, and Bishop, CA. Several teams of two were responsible for
the deployment and pick-up of approximately 50 Texan instruments each. The teams
buried each instrument at least one foot (0.3 m) beneath the ground surface to help
diminish noise and insulate instrument clocks from changes in temperature. Noise such as
traffic and industrial operations played a minimal role, as the transect only crossed three
highways (US 395, US 50, I-80) in this undeveloped region. In addition, two teams hiked
fifteen of the 411 instruments into the John Muir Wilderness and over the Sierra Nevada
crest, recording as high as 11,500 feet (3500 meter) elevation. This is now the first
continuous refraction experiment to cross the Sierra Nevada. The success rate for
deploying instruments at assigned sites was 100 percent. However, the pick-up rate was
99.3% due to the loss of two instruments during a desert flash flood, and the theft of one
instrument from a wilderness area in the Sierra Nevada.
Modeling our experiment after Harder and Keller (2000) and Louie et al. (2004),
we utilized blasts from several Nevada open-pit gold mines (Barrick GoldStrike, Round
Mountain, and Cortez) as seismic sources for this experiment. However, our experiment
expanded on the original experimental design. Harder and Keller (2000) used one mine
blast to record a high-density profile out to a distance of 150 km, while the INC
experiment took the idea a step further by looking at first arrivals out to a distance of at
least 400 km, and by recording earthquakes.
In open-pit gold mines operating in Nevada, high revenues and levels of
production depend on crushing large quantities of rock. Our experiment reaped the
benefits of recording 200,000 lb (90,000 kg) ANFO blasts at Barrick GoldStrike, located
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near Battle Mountain, Nevada. The open-pit mine is approximately 2 by 3 km across and
has a maximum depth of over 0.5 km. At Barrick GoldStrike explosives were detonated
within arrays of 40-ft (12 m) holes, providing a nicely controlled source. The shot holes
were ripple-fired, but video recording shows all firing is within less than a 0.5-second
interval. We also employed earthquakes located by the California Integrated Seismic
Network and the Western Great Basin Seismic Network. These earthquakes provided
reversals from the Barrick GoldStrike blasts. Mine blasts from Barrick GoldStrike,
located near Battle Mountain, Nevada, served as the primary shots for the 2004 seismic
refraction experiment. In accordance with the mine’s regular blasting schedule, the
instruments captured at least one blast on each day of the experiment.
3.2 Processing
The record has minimal, but standard processing. The processing of the records
included: an amplitude centering function, which consisted of subtracting each vector's
arithmetic mean from the amplitude of elements in the vector; a bandpass filter of 2-16
Hz with a 20-percent taper; a trace equalization gain that gains each vector by the L1
norm of the whole trace- it is equivalent to an AGC (automatic gain control) with the
window length set to the trace length; and, the majority of the picks were made with a
display density clip of 2.5 times the RMS (root-mean-squared) velocity. A dip filter was
also used to verify picks and to view data, but not used for actually picking the data. The
dip filter (Hale, 1980) is usually used in reflection processing to adjust for the
overcorrection of primary reflection and under-correction of secondary arrivals. The filter
rejects high dips with either a negative or a positive slope.
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3.3 Inversion method
Tomography models from first arrival picks were produced using the scheme of
Pullammanappallil and Louie (1994). The program is a Monte-Carlo-type optimization
modeled after simulated annealing. The simulated annealing method uses a fast finitedifference solution of the eikonal equation to produce a velocity model from first arrival
picks in time. In this case, our first arrival picks from mine blasts and earthquakes
included both refracted crustal, Pg, and refracted Moho, Pn, arrivals. The final velocity
model produced by the program is independent of the initial model. The program
continues to produce models until a “global minimum” or an acceptable L1 norm is
reached. Unlike linearized refraction inversion schemes that only reach a low L1 norm
from the initial model, this nonlinear method gives a more representative as a accurate
geologic model by providing several final models to choose.
A common problem addressed by Fliedner et al. (2000) is the smoothing over of a
laminated Moho; however, the optimized inversion is one-step in limiting this type of
error. The synthetics produced by Louie et al. (2004) for this type of refraction
experiment show that this inversion method is reliable and accurate from picks from real
data. The grid spacing used for inversion Models 1 and 2 are 2.5 km vertical by 5 km
horizontal.
3.4 Results
3.4.1 Barrick GoldStrike blast records
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Mine blasts from Barrick GoldStrike served as the primary shots for the 2004
seismic refraction experiment (INC). Although blasts were recorded on each day of the
experiment, this paper only discusses and utilizes the Thursday and Friday blasts for the
model inversion. On August 19, 2004, Julian day 232, Barrick GoldStrike ripple-fired
two blasts: 14007 kg (30880 lb or 10036 kg TNT equivalent); and the second blast,
considerably larger, 62541 kg (137880 lb or 45787 kg TNT equivalent). First arrivals
from the blasts can be seen across the entire 600 km record (Figure 2).
Figure 2b displays the Barrick GoldStrike 232 blast in reduced time (timeoffset/velocity). The time is reduced by 7.8 km/s, the regional average velocity of the
Moho (Bassin et al., 2000). The display of arrivals in reduced time allows the slopes of
the refracted arrivals to be more easily distinguished. In actual time, slopes of the
refracted arrivals often look very similar. In the reduced time records shown, if the
apparent velocity is 7.8 km/s the slope of the refraction is flat. If the refracted arrivals are
slower than 7.8 km /s, they will have a negative slope, and if faster, a positive slope. The
Pg-Pn cross-over occurs at approximately 127 km southwest from the blast location,
suggesting a 30 km-thick crust just southwest of Barrick GoldStrike. This concurs with
the 1986 PASSCAL refraction experiment that found 30 km thick crust within the
vicinity of the INC transect (Figure 7).
Although battery life was tested before deployment, by the last day of the
experiment many of the Texan instrument batteries died, resulting in missed traces from
the Friday (233) recording (Figure 3). Because of the missing traces, the start of the
Barrick GoldStrike blast 233 record was not picked. However, the first arrivals southwest
of Barrick GoldStrike were picked to check the picks of the Barrick GoldStrike 232 blast.
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When picking at distance greater than 400 km from the source, with this type of
experiment, it can be difficult to distinguish between Pn refracted arrivals and noise. By
having more than one record from similar events, the first arrivals are more easily
distinguished. While there are some discrepancies between the blast picks, this also
assures that any errors (less than 0.5 seconds) from picks made from real data are not the
single control for the inversion.
On Friday, August 20, 2004, Julian day 233, Barrick GoldStrike ripple fired three
blasts (Figure 3). The first blast was only 1361 kg (3000 lb, with a 975 kg TNT
equivalent). The second and third blasts were 24405 kg (53805 lb, 17487 kg TNT
equivalent) and 48906 kg (107820 lb, 36567 kg TNT equivalent). Because the first blast
was small, first arrival picks were taken from the start of the second blast. The cross-over
occurred at 125 km from the blast, and again arrivals could be seen out to a distance of at
least 450 km.
Relative to typical northern Basin and Range crust of 30 km with a Moho velocity
of 7.8 km/s (Klemperer et al., 1989), there was no delay in any of the picks from the
Barrick GoldStrike 232 blast. The minimum delay of –0.29 seconds occurs about 20 km
after the cross-over distance, and is within error. A +/- 0.5 second error in a pick only
results in +/- 2 km in depth uncertainty for a Moho with a 7.8 km/s velocity. For a slower
or faster Moho or crustal velocity, the difference is negligible at the scale of this
discussion. The maximum delay of -1.49 seconds occurred past 450 km from the blast
location. These early arrivals are not reflected in the inversion model, but the inversion
program best-fit the earlier arrivals (see Figure 8b). Only Pg picks were made north of
Barrick. It is possible that Pn arrivals exist; however, they were not clearly
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distinguishable. The Barrick GoldStrike 233 picks have a maximum delay of 1.298
seconds near Barrick GoldStrike. This is significantly greater than the earlier Barrick
GoldStrike 232 blast. Possibly resulting crust more like 32 km just southwest of Barrick
whereas the previous inversion model had more like 30 km thick crust. The minimum
delay of –1.298 seconds out toward the end of the record, agreeing more with Barrick
232 record.
3.4.2 Paso Robles, CA earthquake record
During the 2002 Northern Walker Lane transect, the Texan instruments recorded
a quarry blast in Watsonville, California (Louie et al., 2004). The quarry blast was
reported on the United States Geological Survey (USGS) seismic network. This led us to
program for 24-hour increments, as previously mentioned, so the instruments could
capture earthquakes occurring at any time of day. There was also a greater possibility for
capturing smaller earthquakes and more numerous quakes as the transect ran through the
Long Valley area and into Fresno, California. The experiment was fortunate and recorded
two Lake Nacimiento, CA earthquakes, near Paso Robles, CA, directly on strike with the
transect, which provided a reversal of the Barrick GoldStrike blasts.
The two earthquakes, magnitude (M) 2.8 at Paso Robles, CA and M 3.8 at Lake
Nacimiento, have first arrivals that are visible on our records out 750 km from their
epicenters. Although the larger earthquake, the Lake Nacimiento event proved more
difficult to pick Pn first-arrivals. The complicated nature of the larger earthquake
provided more emergent Pn arrivals. The Paso Robles, CA 233 earthquake record is
pickable across the entire record and remarkably continuous. Although the earthquake
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was far enough away to allow visible PmP arrivals, because of the size of the earthquake,
the arrivals are not discernable among the later, high frequency energy within the record.
Because this earthquake occurred during the last day of the experiment many of the
instruments did not record signals due battery failure, making the picking of first arrivals
through the Sierra Nevada slightly more difficult. All of the picks on the Paso Robles 233
record are Pn first arrivals because the earthquake had a hypocenter of 7.8 km depth and
an offset of 151 km from the nearest Texan instrument. Based on first picks, the Pn crossover distance should be approximately 150-250 km from the earthquake’s epicenter. This
calculated cross-over distance is in agreement with Ruppert et al. (1998) who recorded a
cross-over distance of 150-175 km from a surface shot in a refraction study just south of
INC transect.
3.5 Models
3.5.1 Tomography models
The initial tomography inversion, Model 1, used the Barrick GoldStrike 232 blast
and the M 2.8 Paso Robles, CA earthquake. The optimized model yielded a crustal root
beneath the Sierra Nevada Mountains with an approximate depth of 50 km, assuming a
Moho velocity of 7.6 beneath the Sierra Nevada (Fliedner et al., 1996). To the northeast
of the root, the inversion confirms a typical 30-32 km-thick crust in northern Nevada,
thickening to the north toward the Nevada-Idaho state border. The model agrees with the
1986 PASSCAL refraction and the 40N COCORP reflection experiment that both
revealed 30 km thick crust in northern Nevada. Further to the north, a recent refraction
experiment by Lerch et al. (2007) extends west of INC and across northern Washoe and
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Pershing counties, Nevada, showing a deeper, 35-40 km thick crust. This is the reason for
the deepening of the Moho on this paper’s initial model interpretation. The mid-crustal
velocities also seem to support this deepening trend to the north.
In order to confirm and expand on the experiment’s basic model, a second
tomography inversion was completed. The second model, Model 2, utilized re-picked
first arrivals from the Paso Robles, CA earthquake and the Barrick GoldStrike 232 blast,
as well as an additional Barrick GoldStrike blast on Julian day 233. The second inversion
confirms our original picks (without the 2.5 second delay originally added to the Paso
Robles, CA event), and adds clarification to features in the initial version. From Model 2,
we observe a crustal root of only 44 km +/- 2 km with steep sides, centered 40 km west of
the topographic crest at the latitude of approximately 37 N. The calculated depth of the
root partly depends on whether one considers the Moho to have a velocity of 7.6 km/s
below the Sierra Nevada or a more traditional 7.8 km/s Moho velocity for tectonic areas.
This paper uses the convention from more recent papers (Fliedner et al., 1996, Ruppert et
al., 1998, Fliedner et al., 2000), and assigns a velocity of 7.6 km/s to the top of a
laminated or transitional Moho beneath the Sierra Nevada. Within northern Nevada, we
observe 30-32 +/- 2 km-thick crust that is in agreement with Model 1 as well as the 1986
PASSCAL and the COCORP 40 N transect. This gives confidence to the picks and
inversions. There is little transition between the steep-sided root on its eastern side and
the Basin and Range. At the latitude of Long Valley, we observe a steeply dipping 14-20
km step in the Moho. Assuming a root Pn velocity of 7.6 km/s and a Pn velocity in
Nevada of 7.8 km/s, this forces the Moho interpretation of Model 2 to cut across the
velocity boundaries defined by the inversion (Figure 7). Where the change from 7.6 to
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7.8 km/s actually takes place cannot be determined from this study alone. We assume that
the Pn, uppermost-mantle, velocity changes at the eastern boundary of the root where
compositional and/or temperature changes are likely to occur. On the north side of
Barrick GoldStrike, where we have no Pn first arrival picks, we have extrapolated the
Moho to a depth of 35-38 km based on the recent refraction experiment of Lerch et al.
(2007). On the western side of the Sierra Nevada, within the Great Valley, we have again
extrapolated the Moho, this time to a depth of 28-30 km based on a refraction
experiments within the Great Valley and coast ranges (Fliedner et al., 2000, Mooney and
Weaver, 1991).
3.5.2 Comparison and discussion of the models
Overall, the first arrival picks used for the two models were similar, with a few
major changes. These changes led to differences in calculated crustal thickness between
Models 1 and 2. The first obvious difference is that the first model has a complete Sierran
root, while the second model doesn’t provide a clear maximum crustal depth, with the
two roots vary in depth by 5-7 km. Despite lacking a bottom to the Sierra Nevada root,
Model 2 is an improvement for several reasons. Although the depth of the root is not well
defined, it is assumed the depth is approximately 42-45 km, whereas Model 1 had a root
of 50 km. The main reason for this discrepancy is that the Paso Robles 233 earthquake
picks used for the inversion of Model 1 had an added 2.5 second delay. After picking the
first arrivals from the Paso Robles earthquake for Model 1, there seemed to be a
consistent 2.5 second delay across the entire record. This delay was interpreted as an
error in the origin time of the earthquake, and so 2.5 seconds were added to each pick
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time. After re-evaluating the Paso Robles earthquake record, as well as re-picking the
record, this source-consistent error was not present. The picks used for the inversion of
Model 2 did not include any added delays. This difference in pick times clearly
accommodates the shallowing in depth of the crustal root in Model 2 (0.5 seconds=2 km,
2 seconds=8 km).
However, the 2.5 second decrease in pick times does not account for the large
increase in the average velocity of the crustal root seen in Model 2. Theoretically, it
ought to be a velocity-depth trade-off, either a deep root with low velocity, or shallow
root with a faster velocity. Yet, the removal of the 2.5 second delay accounts for the
change in depth alone. This leads us to the second major difference in Model 2. The
Barrick GoldStrike 232 blast picks used for the inversion of Model 2 were consistent,
except that the record was picked across the entire 500 km southwest of Barrick
GoldStrike, whereas in Model 1 the Barrick GoldStrike 232 record was only picked out
to a distance of 350 km. It is possible that these picks with large offsets are too early,
forcing the velocity within the root to be to fast.
The INC inversion Model 2 shows a root with crustal velocities of 7.2-7.7 km/s,
much faster than granulite facies (crustal root rocks), and much faster than the 6.0-6.2
km/s velocity of Fliedner et al., (2000) who show that these slower than typical crustal
root velocities extend down to the Moho. The high velocities are most likely an artifact
from the inversion due to a lack of Pg arrival data within the Sierras. Fliedner et al., 2000
were able to recognize these lower crustal velocities only by modeling the long-offset Pg
arrivals. However, Ruppert et al. (1998) and Fliedner et al. (1996) modeled 6.4-6.6 km/s
velocities to the base of the root from the same experiment, which is more in agreement
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with Model 1. However, Model 2 and Model 1 have comparable velocities and depths
east of the crustal root and extending the distance to Barrick GoldStrike, suggesting that
the entire pick-set is not the source of the problem, but rather a few early picks forcing
the velocity within the Sierra root to be faster. Future work of producing synthetics (as in
Louie et al., 2004), or purposely picking later and inverting the data may prove useful in
determining this error.
Another argument for the faster arrivals within the crustal root are the extensive
ophiolites beneath the Great Valley. Although west of the INC transect, the ophiolites
extend to a depth of at least 25 km. Most of the rays from the Paso Robles earthquake,
with a hypocenter of 7.8 km depth, should bypass the fast ophiolites; however, it is
possible that first arrivals in Fresno, CA are affected as they travel up to the receivers,
affecting the entire inversion with these fast arrivals. In addition, it is under some debate
as to whether or not the ophiolites extend to the base of the crust. Ruppert et al. (1998)
argues that the accreted terrain extends down to the Moho, whereas more extensive work
from Fliedner et al. (2000) argues that there is a lower velocity layer of metasedimentary
rocks, like the Franciscan, between the ophiolites and Moho.
For several reasons, despite the high velocity within the root, a feature of Model
2, it is asserted that Model 2 is still a better model for interpreting the depth of the Moho.
First, the depth of the Sierran root and the crustal thickness within Northern Nevada
generated by Model 2 is consistent with other data (Fliedner et al., 1996, Ruppert et al.,
1998, Zandt et al., 2000, PASSCAL 1986). Second, the high velocity within the crustal
root can be attributed to a smoothing function of the inversion and not to any first arrival
picks from the data, as the hit count plot confirms (Figure 9). There are an infinite
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number of crustal velocity models that could fit the same Moho depth because there are
no crustal, first arrival picks. Third, the lack of definition of the bottom of the Sierra
Nevada crustal root could be caused by the tunneling of rays across the root, or more
likely, the diffraction of seismic rays off the steep sided walls (during the actual
experiment, not just an artifact of the inversion). Analysis of receiver function stations
and PmP arrivals show a “Moho hole” at the location of the thick crust (Zandt et al.,
2004, Fliedner et al., 1996). Zandt et al. (2004) attributes this missing Moho to possible
scattering of reflections and P-S conversions along a boundary that may have some
topography due to deformation caused by delamination. However, small-scale
topography along the Moho should not affect Pn arrivals other than a smoothing over the
area. In addition, refraction experiments in the region were able to clearly define a base to
the root (Fliedner et al., 1996 and Ruppert et al., 1998).
In addition to the better agreement with surrounding geophysical data, as will be
discussed, the preference for Model 2 over Model 1 stems from several basic
observations. The picks from the Paso Robles, CA earthquake have no clear delay
beneath the Sierras, unlike the Northern Walker Lane (NWL) refraction experiment that
saw 3-4 second delays beneath the 55 km root (Louie et al., 2004). A deeper root within
the central Sierra Nevada would require at least a 2 second delay, and one that is not
consistent across the entire record, but localized to the Sierras. Also, the Pg-Pn cross-over
distance for the Paso Robles earthquake is only approximately 150-200 km, that is in
agreement with Ruppert et al. (1998), who also saw a cross-over of 150 km as well, but
from a surface shot. They also found a lack of delay in their PmP arrivals.
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4. Crustal thickness maps
We have compiled contoured crustal thickness maps (Figure 10 and 11) based on
the NWL (Louie et al., 2004) and INC seismic refraction results presented in this paper,
as well as previous geophysical studies within the Great Basin and Sierra Nevada
(literature cited in the comprehensive Braile et al. (1989)). We extracted information
concerning crustal thickness from the crust-mantle (Moho) refraction velocities and
arrival times, crossover distances, and from models presented in the literature. A kriging
algorithm was used to interpolate a crustal thickness map. The colors on the map are
contoured every 1 km, and the black outlines are for every 5 km, showing a more general
trend.
Presented in Figure 10a is the compilation of all the data, including any
discrepancies. In the compiled map of Figure 10, Mooney and Weaver (1989) and
Fliedner et al. (2000) served key references. Mooney and Weaver (1989) provide an
interpretation of crustal thickness within the Sierra Nevada root and surrounding region
based largely on geophysical work before 1989. Their map relies heavily on results from
Eaton et al. (1963), and Pakiser and Brune (1980) for crustal control within the Sierra
Nevada. Fliedner et al. (2000) expanded on the original work of Mooney and Weaver
(1989) map by inverting the 1992 Southern Sierra Continental Dynamic project refraction
results along with other data sets including Savage et al. (1990) for the south-central and
southern Sierras. Fliedner et al. (2000) point out that many older references, specifically
Mooney and Weaver (1989), overestimate the depth of the Sierra Nevada root by using a
different convention for defining the Moho. The current convention is to use the top of a
laminated or transitional Moho, which for the Sierras is a Pn velocity about 7.6 km/s, as
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the depth to represent the base of the crust (Fliedner et al., 1996; Ruppert et al., 1998;
Fliedner et al., 2000). Previously, a Pn velocity of 7.8 km/s had been used to identify the
Moho beneath the Sierra (Pakiser and Brune, 1982), which is most likely the bottom of
any lamination.
From this compiled map, Figure 10, a preferred mode was generated (Figure 11).
The preferred model eliminates most of the discrepancies between adjacent points, but
leaves results in places where no other data exist. Combining these data sets it allows
better visualization of key features, and allows us to better address discrepancies, and
assess how well the proposed models fit the actual data. There are several key features to
note are: one, the crustal root is centered west of the Sierra’s topographic crest; two, the
root deepens north of Fresno, California; three, the crustal root has steep-sided walls; and
four, normal, 30-km thick Basin and Range crust occurs adjacent to the root on its east
side.
5. Discussion
From Model 2, a crustal root of only 44 km +/- 2 km is observed that is centered
40 km west of the topographic crest at the latitude of approximately 37 N. The root is
shallower than that predicted by isostatic compensation (60- 75 km, Lawson, 1936;
Wernicke et al., 1996; Fliedner et al., 2000). This observed crustal root, described as a
“crustal welt” (Zandt et al., 2004) or “small crustal root” (Fliedner et al., 1996) has steep
sides and is bounded below by a Pn velocity of 7.6 km/s. Previous results in the Sierras,
south of the INC transect (36.5 N) are in agreement with the location and depth of this
small crustal root (Ruppert et al., 1998, Zandt et al., 2004, Fliedner et al., 1996, Fliedner
19
et al., 2000). Ruppert et al., (1998) show a 40-42 km root centered 80 km west of
topographic crest at 36.5 N using the same data, but employing different modeling
techniques, Fliedner et al., show a 43 km root east of Fresno, centered 40 km west of
topographic crest. The INC transect only extends to Fresno, CA, but based on previous
work, the crust thins to 28-33 km west of the root beneath the Great Valley (Fliender et
al., 2000; Ruppert et al., 1998; Holbrook and Mooney, 1992).
Because the model for the southern Sierras is well developed, the INC and NWL
refraction experiments (Louie et al., 2004), were used to help tie in major geologic events
and supporting evidence for the evolution of the entire Sierra Nevada root. Between 1210 Ma, the ecologitized section of the root began delaminating across the entire Sierra
Nevada. This coincides and is probably related to extension in the Death Valley region
(Jones et al., 2002, and Jones et al., 1987). The pulse of Miocene volcanism also occurs at
this time across the entire Sierras. We suggest that the root was removed from the entire
Sierras for several reasons. First, the Cenozoic uplift of the Sierras began around 12 Ma,
and has been linked to upwelling asthenosphere (Jones et al., 2002; Wernicke et al., 1996;
Lui and Shen, 1998). Second, for both the southern and northern parts of the Sierras, the
gravity anomalies do not support a thick, Airy-type crustal root at any reasonable depth
(65-75 km). To accommodate the gravity anomalies, some compensation from the mantle
is required (Mavko and Thompson, 1988; Fliedner et al., 1996). Third, seismic refraction
data discussed herein indicate only a small crustal root with a depth of 40 km in the
south, thickening to 55 km in the north. The entire root is centered 40-80 km west of the
topographic crest, as visible on the preferred crustal thickness map (Figure 11). For these
reasons, it is asserted that the delamination of the root formed in conjunction with
20
deformation in the Basin and Range Province (Jones et al., 2003) and not from “mantle
wind” (Zandt et al., 2004). With the incorporation of the more northern results from the
INC and NWL (Louie et al., 2004) transects, this interpretation is made possible.
After a period of quiescence, the second major change in the evolution of the
crustal root abruptly began at 4 Ma. A pulse of potassic volcanism erupted in a localized
area within the Sierra Nevada and east of the range, but quickly ceased around 3 Ma
(Figure 1) (Manley et al., 2000; Farmer et al., 2003). The composition of xenoliths
entrained in the volcanics implies the sinking of the eclogite facies, together with the
metamorphosed mafic lower crust, and the underlying mantle lithosphere beneath the
Sierras within this localized volcanic area (Manley et al., 2000). The petrology of these
xenoliths require not only that they have a mantle source, but that they also formed at
depths of approximately 40 km (Manley et al., 2000). Between the latitude of 35-38 N,
the root is slightly thinner, approximately 42-45 km thick, based on the INC Model 2 and
previous work (Zandt et al., 2004; Fliedner et al., 2000; Ruppert et al., 1998). Because the
root is more shallow here than the in the northern region (55 km, Louie et al., 2004), but
is beneath higher topography, more of the root must have foundered in the southern and
central Sierras. From receiver function analysis; there are no PmP arrivals at the location
of the thickened crust (Zandt et al., 2004). Zandt et al. (2004) attributed this to the
hypothesis that the crustal root is dripping into the mantle, meaning the Moho is actually
“missing.” Because of the evidence from the thickness of the root, the lack of PmP
arrivals, and the localized potassic volcanics, it can be interpreted that the entire mantle
lithosphere was removed from this localized region (Jones et al., 2003). The mantle drip
model is also supported by the drainage patterns in the Great Valley as shown by Zandt et
21
al. (2004). The INC results fall just north of the area of Zandt et al.’s (2004) proposed
mantle drip, but the drip may be pulling the northern root south. Thus, the drainage
patterns to the east of the Sierras at the latitude of the INC transect would not reflect this
sinking and topographic subsidence. Because of these reasons, we concur that the mantle
lithosphere was removed between the latitude of 36-38 N starting at 4 Ma.
As previously mentioned it is unclear how much of the root to the north was
removed. In the north there are no large, Pliocene volcanic fields like those in the southcentral Sierras, but as Manley et al. (2003) points out, there has been little analysis in the
northern region. However, because the northern Sierras lie north of the location of the
proposed Sr line, the compositions of the xenoliths may be different, and the same type of
eruptions cannot be expected (Jones et al., 2004). However, from the COCORP transect
across the northern Sierras, there are no PmP reflections (Nelson et al., 1987). Klemperer
et al. (1989) attribute this to the fact that typically in old continental crust, PmP
reflections are not clear, as opposed to recently extended crust that typically has clear,
continuous reflections as in the Basin and Range. At this time, there is no evidence that
the mantle lithosphere has been removed in the northern Sierras, and the depth of the root
as determined by the NWL transect, 55 km-thick (Louie et al., 2004), suggests that it has
not, but this cannot be confirmed without further work.
6. Future work
Future work on the INC experiment data could add valuable information to the
region. Deep earthquakes near Fresno, CA are not recorded on the seismic networks.
Because no such earthquakes were reported by CISN, all of the 96 hours of recording
22
need to be viewed. At this time, we do not have an efficient method for doing this. A
Fresno, CA earthquake would provide better depth and velocity control in the Sierra
Nevada.
The instruments recorded several small, M 1-2 earthquakes near Tom’s Place and
Mammoth, CA that would provide crustal velocity and depth control in the Long Valley
area. The earthquakes were processed and inverted, but the origin times of the
earthquakes proved to be in error, resulting in unrealistic inversion results. If these
earthquakes were relocated, a more complete model would be achieved.
In addition to the earthquakes and mine blasts in-line with the INC transect, the
experiment recorded off-line earthquakes near Tokop, NV and mine blasts at Round
Mountain. The sources will provide important fan-type shots, as well as PmP reflection
data for further analysis.
7. Conclusions
Results from the Idaho-Nevada-California seismic refraction transect have added
crustal thickness data for the Sierra Nevada Mountains where no previous data existed.
Results indicate the presence of a 43 +/- 2 km thick crustal root centered 40 km west of
the topographic crest. To the east of the Sierra Nevada Mountains, the 30-32 km thick
crust is continuous to the northeast through Battle Mountain, NV. The lack of crustal of a
root in the Sierras at the latitude of the INC transect is attributed to the delamination of
the ecologitized crustal root and all or some of the mantle lithosphere.
8. Acknowledgements
23
This material is based upon work supported by the U.S. Department of Energy under
instruments numbered DEFG07-02ID14311 and DE-FG36-02ID14311, managed through
the DOE Golden Field Office. The instruments used in the field program were provided
by the PASSCAL facility of the Incorporated Research Institutions for Seismology (IRIS)
through the PASSCAL Instrument Center at New Mexico Tech. Data collected during
this experiment will be available through the IRIS Data Management Center. The
facilities of the IRIS Consortium are supported by the National Science Foundation under
Cooperative Agreement EAR-0004370 and the Department of Energy National Nuclear
Security Administration. The California Integrated Seismic Network (CISN) and the
Western Great Basin Seismic Network (USGS Cooperative Agreement 04HQAG0004)
provided earthquake locations used in this experiment. We would like to thank Barrick
GoldStrike Round Mountain and Cortez mines for their cooperation and kind help.
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