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Introduction
The debate over the existence and/or the type of crustal root underlying the Sierra
Nevada Mountains began during the 1950s. Some guy proposed that there ought to be a
deep root based on elementary gravity and compensation calculations. These calculations
are plausible considering given know data today about other mountain ranges throughout
the world, but for the Sierra Nevadas, the story is much more complicated.
In 1973, Carder proposed that in fact there is no root based on teleseimic arrivals
over the seimic network. His paper is still cited by some authors today. However, the
truth is with the unrealistic velocity model used in his calculations and that the data was
unreversed, his model cannot be verified. In 1980, Pakiser and Brune also thought this,
and wrote a analytical paper on the travel times through the Sierras and caluclating
several arrival times and only then choosing the best model. Their results, a 50 km root in
beneath the Sierra Nevadas in the northern and southern Sierras.
Since the initial bodies of work, the debate continues. More extensive work has
been completed in the southern Sierras, but little has been done to connect the evolution
of the Sierra Nevada root as whole for one primary reason, the data that exists north or
Fresno, CA is sparse. This paper aims at expanding on the current and well thought-out
model for the southern Sierras with seismic refraction data as a control, and what the
Sierra’s geophysical structure may appear toward the north of most previous work.
The root beneath the Sierra Nevada Mountains should be around 75 km thick given
the estimated Mesozoic elevation, the size of the batholith, and the petrology of the
xenoliths entrained during volcanic eruptions (Fliedner et al., 2000, Jones et al., 2004).
Crustal roots formed at an active continental margin are typically composed of calc-
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alkaline granite facies. Because of the crustal root’s depth (temperature-pressure gradient
and gravity), it often differentiates into more of less intermediate and mafic parts.
Either way, it is agreed upon the magmatism is a result of decompression melting
from the delaminated root and that there is a small amount of melting at the base of root
by asthenosphere (Manley et al., 2000).
It is has been shown through several types of geophysical work that the root beneath
the southern Sierra Nevada Mountains has delaminated, creating a west centered drip into
the mantle (Fliedner et al., 2000, Zandt et al., 2004, Wernicke et al., 1996). It is debated
as to how much of the root foundered and whether the delamination occurred throughout
the entire Sierra Nevada, or whether it’s progressing northward from the already
foundered southern root. There is also debate over the actual process and cause of the
foundering of the root. Some argue that negative buoyancy from the eclogized root
caused the separation (Zandt et al., 2004), while others argue that an outside force such as
extension between Death Valley and Sierras caused stress forming factures within the
crust and allowing the heavy root to founder (Manley et al., 2000).
Extensive geochemical and petrologic work as been completed within the southern
Sierra Nevada and its eastern vicinity. The volcanics of this region provide key insight to
the timing and possibly the extent of the delamination of the crustal root. Based on work
from Ducea and Saleeby (1996) the composition of xenoliths entrained in Miocene and
Pliocene volcanics, they suggest that delamination occurred between 10-3 Ma.
Petrologically, the Sierras, at one point in time must have had a complete crustal root in
order to have produced the volcanics from the Mesozoic batholith, in addition to the
required support for the paleo-elevation of the Sierras (Manley et al., 2000). Although the
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Sierra have gained over a kilometer of elevation during the Cenozoic, the Sierras had an
elevation of at least X (based on paleo-botany of seeds within the Sierras, as well as the
amount of erosion and sediments that were deposited into the Great Valley).
This bracket of volcanism is coincident with the beginning of the Cenozoic uplift of
the Sierras (Jones et al., 2004). This over one kilometer gain in elevation also
corresponds to extension in Death Valley (ca. 7 or 6 Ma started (McKenna and Hodges,
1990 cited in Jones et al., 2004, and Wernicke and Snow, 1998). Other concurrent events
include the valleys east of Sierras filling with sediment during the late Miocene and early
Pliocene (Bacon et al., 1982, Burchfiel et al., 1987, Lueddecke et al., 1998), folding of
the California coast ranges and the spreading of the Gulf of California during the
Pliocene (Axen and Flether, 1998).
Cenozoic volcanism within the Sierra Nevada are marked by two major pulses. The
first, occurring in the Miocene, 12 to 8 Ma, produced typical subalkalic to alkalic
trending basalts- typical of contential eruptions. After a period of no volcanism, a quick
spurt of magmatism between 4 to 3 Ma produced ultra potassic rocks, occurring in a
limited area of the southern/central Sierras, specifically in Kings and San Joaquin
volcanic fields. Followed by quiet, lava was again erupted during the Quaternary within
the Big Pine and Long Valley volcanic fields, however, this time once again producing
more typical subalkalic volcanics (Manely et al., 2000).
From petrologic analysis alone, many cite the start of the delamination at ca. 3.5 Ma,
due to the unqiue Pliocene lavas. They were just a pulse, short-lived, no progression
north or south, as moving of plates, only concentrated in southern Sierras and Owens
Valley (Manley, et al., 2000). Low Nubd. K-metasomatized, mantle not involved in
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Sierra magmatism during Cenozoic (Farmer et al., 2002). Xenoliths also indicated that it
was shallow (40 km). Newley studied rocks with mafic to intermediate compositions
indicated that they have K and P, but also the lowest SiO2 and highest MgO for any
known Sierran rocks (Farmer et al., 2002). Highly potassic mafic to intermediate rocks
are constrained within the San Joaquin and Kings volcanic fields during the Pliocene
(Farmer et al., 2002). Not parent from lower crustal rocks, must have come from the
mantle. Miocene volcanics in the central Sierras derived from LILE, Pb-metasomatized,
High Nb peridotite, upwelling asthenosphere, or post-Mesozoic lithospheric mantle.
Melting from extension in Death Valley and Slab window (Farmer et al., 2002).
(From Manley et al. (2000) based on their compilation of over 500 samples, they
created a simple timeline for volcanism within the Sierra and surrounding region. Before
13 Ma, also magmatism south of Garlock fault (Mojave desert). Ca. 13 Ma, Sierra
Nevada, White mtns. And some in Mojave. Ceased in ca. 9 Ma. Abrubtly began and then
ceased between 4-3 Ma. Only within Sierra Neavda, Owens Valley, Coso Range (all high
potassic). Activity in ca. 1.5 Ma Coso, Big Pine Long Valley, Golden Trout, which are
volcanic fields we see today, all east of Sierra frontal fault.)
Crust began delamination process at ca. 3.5 Ma determined by the plus mafic potassic
volcanism within and east of the Sierras preceded and followed by times of no volcanism
(Manely et al., 2000). Farmer et al.(2002) have mapped these potassic rich volcanics and
they do extend north to about 38 N. And in the INC study area alone there are volcanics
in the Long Valley area. The volcanic eruption of these xenoliths, high potassium
occurred south of Long Valley between 3-4 Myr ago, aiding a time bracket of when the
initial foundering, or detachment of the ultra mafic rocks began (Zandt et al., 2004). In
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the study area of Zandt et al. (2004), they show the presence dense garnet pyroxnenite
xenoliths in mid-Miocene volcanics within the Sierra Nevada. These ultramafic rocks are
much more dense than typical mantle peridotites, and Zandt et al. (2004) states that these
are prone to separation from their granitoid counterparts.
Jones et al. (2004) requires more than just the weight of the elclogized crust, but also
a shear. Zandt et al. (2004) does suggest shear interms of mantle anisorty in negative
polarities where the root has already separated.
Cenozoic volcanism within the Sierra Nevadas are marked by two major pulses.
Before 13 Ma, magmatism was located south of the Garlock fault within the Mojave
desert. Around ca. 13 Ma magmatism began within the Sierra Nevada and White
Mountains. The first, occurring in the Miocene, 12 to 8 Ma, produced typical subalkalic
to alkalic trending basalts- typical of contential eruptions. Adburbty ceased. After a quiet
period of volcanism, a quick spurt of magmatism between 4 to 3 Ma within Sierra
Nevada, Owens Valley, and the Coso Range. produced ultra potassic rocks, occurring in
a limited area of the southern/central Sierras, specifically in Kings and San Joaquin
volcanic fields. Followed by quiet, lava was again erupted during the Quaternary within
the Big Pine, Golden Trout and Long Valley volcanic fields, all easr of the Sierra frontal
fault.However, this time once again producing more typical subalkalic volcanics.
(Manely et al., 2000).
However, potassic volcanism itself, doesn’t indicated delamination, however they are
similar to Andes and Tibet though, says more about crustal root composition (Manley et
al., 2000). The question is raised as what is happening in the southern and northern
Sierras. It is clear from geophysical data that there is no southern root (Zandt et al., 2004,
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Fliedner et al., 2000). However, the Kern Volcanic field erupted Pliocene rocks that are
similar to Miocene rocks (Farmer et al., 2002), not from delamination in opinion and
results. Also, there is no strong evidence for these Pliocene, K-rocks north of 38 N, but
the root was most likely root removed along the entire the Sierra Nevada (Jones et al.,
2004) because all Sierras have been uplifted during the Cenozoic. Maybe no volcanics
north b/c old craton, Sr line or area that the entire mantle lithosphere was removed.
(Jones et al., 2004). OR just not enough evidence in north for the rocks in Lake Tahoe,
west 2-4 Ma (D.S. Harwood reported in Saucedo and Wagner, 1992 in Jones et al., 2004).
Subsidense area determined by flow of drainage into the Great Valley. North of
Fresno flow northward, while drainage flows into the Tulare basin. This subsidense
possibly began 3-4 Myr and moved southwestward (Zandt et al., 2004). However the INC
experiment does not directly follow this lodgic since the drainage pattern into the Tulare
basin is south of the INC crustal welt. A possible explanation is that the delamination and
drip is more than just the separation of the root, but as Jones et al. (2004) suggests also
due to a horizontal shear, aiding the delamination by fracturing the crust, or some other
process. If the deepest part of the delamination is still within the Tulare basin region, also
associate with the highest SIerran topography, then welt moving into the mantle would
have a greater pull on the crust to the north, having a zipper effect.
However drainages did incise, because of uplift everywhere
From the most recent inversion model, we observe a crustal root of only 44 km
+/- 2 km centered 40 km west of the topographic crest at the latitude of approximately 37
N. The root is shallower than what the depth the root ought to be of at least 75 km by
Fliedner et al. (2000). This observed crustal root as described as a “crustal welt” (Zandt et
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al., 2004) or “small crustal root” (Fliedner et al., 1996) has steep sides and a velocity of
7.6 km/s. Previous results in the Sierras, just south of the INC transect (36.5 N), are in
agreement with the location and depth of the crustal root (Ruppert et al., 1998, Zandt et
al., 2004, Fliedner et al., 1996, Fliedner et al., 2000). Ruppert et al., (1998) show a 40-42
km root centered 80 km west of topographic crest at 36.5 N and based on the same data,
but employing different modeling techniques, Fliedener et al., show a 43 km root east of
Fresno that 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 (well data from Wentworth, 1990 in Fliender et al., 2000,
Ruppert et al., 1998, and Holbrook and Mooney, 1992).
INC Refraction Experiment
In August 2004, the Nevada Seismological Laboratory with the University of
Nevada, Reno completed a 600 km-long refraction transect extending from the IdahoNevada border, through Battle Mountain, Nevada and 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 km-long transect, 411, 24-bit single
channel portable seismographs, known as RefTek “Texans,” connected to 4.5 Hz
geophones recorded mining blasts and small earthquakes. The instruments were
programmed to record for a total of 96 hours (four 24-hour periods) on the 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, in order to insure the instruments would
gather earthquakes occurring at night. In the previous 2002 Northern Walker Lane
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(NWL) refraction experiment, instruments were only programmed to record during
working hours. 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 24-hour 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%. The data
recovery rate was over 99%.
The deployment consisted of three download/home stations located in Battle
Mountain, NV, Austin, NV, and Bishop, CA. Eight teams of two students were
responsible for the deployment and pick-up of approximately 50 Texans instruments. 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 at 11,500 ft (3500 m) 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%. However, the pick-up rate was 99.56% due to
the loss of two instruments during a desert flash flood.
Modeling our experiment after Harder and Keller (2000), we utilized blasts from
Nevada gold mines: Barrick GoldStrike, Round Mountain, and Cortez, as seismic sources
for this experiment. However, our experiment tested the limitations of the original
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experiment; Harder and Keller (2000) used one mine blast to record a high-density
profile out to a distance of 150 km. This experiment takes the original idea a step further
by looking at first arrivals out to a distance of at least 400 km, and by recording
earthquakes. In disseminated gold deposits, high revenues and levels of production
depend on crushing large quantities of rock. Our experiment reaped the benefits with the
recording of 200,000 lb (90,000 kg) ANFO blasts. 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 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, as well as crustal refraction values along the interior of the
transect. Mine blasts from Barrick GoldStrike, located near Battle Mountain, Nevada,
served as the primary shots for the 2004 seismic refraction experiment. Following the
mine’s regular blasting schedule, the instruments captured at least one blast on each day
of the experiment.
Processing
The record has minimal, but standard processing. The processing of the records
included: a center function, 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 clip of 2.5 times the RMS (root-
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mean-squrared) 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 is usually used in reflection
processing to correct for the overcorrection of primary reflection and undercorrection of
secondary arrivals. The filer rejects dips that curve down with offset, positive dip
Inversion
Tomography models from first arrival picks were produced using the optimized,
finite differencing scheme of Pullammanappallil and Louie (1994). The program
continues to produce models until a global minimum in terms of error is reached. This
optimized method does not limit the model product as a typical refraction inversion
might. 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. Grid spacing, 2.5x5
km
Monte Carlo-based optimization “general simulated annealing, invert first arrivals
to get velocity. Fast finite-difference solution of the eikonal equation. Convergence of of
model is independent of the initial model. Not a linear methods. Produces many final
models with least-square error. nonlinear
Results
Barrick GoldStrike blast records
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Mine blasts from Barrick GoldStrike, located near Battle Mountain, Nevada,
served as the primary shots for the 2004 seismic refraction experiment. Following the
mine’s regular blasting schedule, the instruments captured at least one blast on each day
of the experiment. The Barrick GoldStrike mine is an open pit with approximately 2 by 3
km dimensions and a maximum depth of 5 km from the local elevation.
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. First arrivals from
the blasts, the first blast, 14007 kg (30880 lb or 10036 kg TNT equivalent), and the
second blast, considerably larger, 62541 kg (137880 lb or 45787 kg TNT equivalent), can
be seen across the entire 600 km record (Figure 2a). Figure 2b, displays the Barrick
GoldStrike 232 blast in reduced time (time-offset/velocity). The time is reduced by 7.8
km/s, the continental average velocity of the Moho. Displaying 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 too similar. In the reduced time records shown,
if the velocity is 7.8 km/s the slope 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 crossover occurs at approximately 127 km from the blast location, suggesting a 30 km-thick
crust just southwest of Barrick GoldStrike. Typical cross-over distances range from 150200 km for average 35 km thick, continental crust. This also concurs with the 1986
PASSCAL refraction experiment that found 30 km thick crust within the vicinity of the
INC transect (see Figure 9b).
Although battery life was tested before the deployment, by the last day of the
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experiment many of the Texan instrument batteries died, resulting in missing traces
within records from the Friday (233) recording. Because of the missing traces, the start of
the Barrick GoldStrike blast 233 record was not picked. However, the first arrivals to the
southwest of Barrick GoldStrike were picked to self-check the picks of the Barrick
GoldStrike 232 blast on the day before. Often when picking past 400 km from the source,
it can be tricky 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 assures that the inversion
model isn’t over-fitting the data.
On Friday, August 20, 2004, Julian day 233, Barrick GoldStrike ripple fired 3
blasts. 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 (in
comparison) first arrival picks were taken from the start of the second blast. The crossover occurred at 125 km from the blast, and again arrivals could be seen out to a distance
of at least 450 km, only because so many Texan records were missing.
For a typical, northern Basin and Range crust of 30 km with a Moho velocity of 7.8
km/s, there was no delay in any of the picks. The minimum delay of –0.29 seconds
occurs about 20 km after the cross-over distance, and is within in error. A +/- 0.5 second
error in a pick only results in +/- 2 km in depth for a Moho with a 7.8 km/s velocity. For a
slower or faster Moho or crustal velocity, the difference is negligible for 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
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program best fit the earlier arrivals (see plot). Only Pg picks were made north of Barrick.
It is possible that there are one Pn arrivals, however they weren’t clearly distinguishable.
The picks have a maximum delay of 1.298 seconds near Barrick GoldStrike. This is
significantly more than the other Barrick 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.
Paso Robles, CA earthquake record
During the 2002, Northern Walker Lane transect, the Texan instruments recorded
a quarry blast in Watsonville, California. The quarry blast was reported on the USGS
seismic network. This led us to program for 24 hour increments, as previously mentioned,
so the instruments could capture earthquakes occurring anytime. 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, in the vicinity of Paso
Robles, CA, directly on strike with transect, providing a reversal to the Barrick
GoldStrike blasts.
The two earthquakes, M 2.8 Paso Robles, CA and M 3.8 Lake Nacimiento, have
first arrivals visible on our records out 750 km from their epicenters. The Lake
Nacimiento earthquake, M 3.8, although visible, the larger earthquake provided more
difficult first-arrival Pn picks. The complicated nature of the earthquake provided more
emergent Pn arrivals. The Paso Robles, CA 233 earthquake record is pickable across the
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entire record, and remarkably continuous. Although the earthquake 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. Because this earthquake occurred
during the last day of the experiment, like the Friday Barrick 233 record, many of the
instruments suffered battery failure, making the picking through the Sierra Nevadas
slightly more difficult. All of the picks on the Paso Robles 233 record are Pn first arrivals
because the earthquake had an epicenter of 7800 km depth and an offset of 151 km from
the first Texan instrument. Back calculating from the first picks, the Pn cross-over
distance should be approximately 150-250 km from the earthquake’s epicenter. This
calculated cross-over distance is in agreement with a refraction study just south of the
INC transect, who recorded a cross-over distance of 150-175 km from their surface shot
point (Ruppert et al., 1998).
Tomography models
The initial tomography inversion, Model 1, used one blast from Barrick
GoldStrike and the 2.8 M Paso Robles, CA earthquake. The inversion 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 reveals a typical 30-32 km crust in northern Nevada,
deepening 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. A recent refraction experiment extending
west from the NV-ID state border, shows deeper, 35-40 km thick crust in this area, thus
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the reason for the deepening of the Moho on this paper’s initial model interpretation
(Klemperer et al., 2007). 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 a re-picked
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 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 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. In this experiment’s model,
and in the models by other workers (Fliedner et al.,1996, Ruppert et al., 1998, Fliedner et
al., 2000), we see a laminated Moho beneath the Sierra Nevada. This paper uses
convention from more recent papers (Fliedner et al.,1996, Ruppert et al., 1998, Fliedner
et al., 2000), and employs a velocity of 7.6 km/s, the top of the laminated Moho as the
velocity for the Sierra Nevada root. Within northern Nevada, we observe 30-32 +/- 2 kmthick crust. In agreement with the Model 1 as well as the 1986 PASSCAL and the
COCORP 40 N transect, this gives confidence in the picks and inversion. There is little
transition between the steep-sided root on it’s eastern side and the Basin and Range. At
the latitude of Long Valley, we immediately observe 30 km-thick crust. Assuming a root
velocity of 7.6 km/s and a crustal velocity in Nevada of 7.8 km/s, this forces the Moho
interpretation to cut across the model’s velocity boundaries. Where the change from 7.6
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to 7.8 km/s actually takes place is undeterminable from this study alone. We assume that
the velocity changes at the corner of the root given that there is most likely upwelling
asthenosphere against the recently delaminated mantle lithosphere (**Side note-if there is
upwelling asthenosphere wouldn’t it be hotter, therefore slower?) (Zandt et al., 2004,
Wernicke et al., 1996). On the north side of Barrick GoldStrike, where we have no Pn
first arrival picks, we have extrapolated the Moho across to a depth of 35-38 km based on
recent refraction experiment to the north of the INC transect (Klemperer 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 refraction experiments
within the Great Valley and coast ranges (Fliedner et al., 2000, Mooney and Weaver,
1991).
Differences and Problems within the models
On the whole, the picks were similar except for a few major differences. These
changes led to the differences in the results, Models 1 and 2, from the inversion. The first
obvious difference is that the first model has a complete Sierran root, and the second
model doesn’t provide a clear maximum depth, and the two models roots varying in
depth by 5-7 km. Despite the lacking a bottom to the Sierra Nevada root, Model 2 is an
improvement for several reasons. Although the depth of the root is not defined, we
assume 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 were 2.5 seconds later. After picking the first arrivals from the
Paso Robles earthquake for Model 1, there seemed to be a consistent 2.5 second delay
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across the entire record. This delay was interpreted as a network error in the time that the
earthquake was reported, and so 2.5 seconds was added to each pick time. After
reevaluating the Paso Robles earthquake record, as well as re-picking the record, this
standard error was not present. The picks used for the inversion producing Model 2 did
not include any added time. 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 doesn’t account for in the large
increase in the velocity of the crustal root. Theoretically, it ought to be a trade-off: deeper
root with low velocity, or fast shallow crust. Yet, the earlier pick times alone account for
the change in depth. This lends 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
previously the 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 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 midcrustal velocities extend down to the Moho. The high velocities are most likely an artifact
from the inversion due to a lack of Pg arrivals within the Sierras. Fliedner et al., 2000 was
able to determine 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
velocity to the base of the root from the same experiment, also in more agreement with
Model 1. However, Model 2 and Model 1 have comparable velocities and depths just east
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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
entire velocity within the Sierras to be faster.
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 an epicenter at 7800 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,
causing the entire inversion to adopt these fast arrivals. Also, 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 low velocity layer of metasedimentary rocks,
like the Franciscan, between the ophiolites and Moho.
On a similar note, because the crustal root in the INC model is partly controlled
by Pn arrivals from the Barrick GoldStrike blast, which we mainly attribute to the cause
of the high velocity in the root, it is possible that rays traveling through the volcanic
laden Long Valley caldera and vicinity had faster arrivals, and again possibly forcing the
inversion to interpolate based on these fast arrivals. As mentioned, the first inversion for
Model 1 did not include picks as far west as the inversion for Model 2. Both Ruppert et
al. (1998) and Fliedner et al. (2000) agree that the Panamint Valley to the east of the
Sierras has high lower crustal velocities of 6.8-7.0 at a depth 19-22 km, and with a Moho
near 30 km. Ruppert et al. (1998) attributes this to Basin and Range underplating. This
underplating most likely extends further north beneath the INC transect. Future work of
19
producing synthetics, or purposely picking later and inverting the data may prove useful
in determining this error.
For several reasons, despite the high velocity within the root, we assert that Model
1 is still a better model for interpreting the depth of the Moho. One, the depth of the
Sierran root and the crustal thickness within Northern Nevada is consistent with other
data in the study area, as will be discussed (Fliedner et al., 1996, Ruppert et al., 1998,
Zandt et al., 2000, PASSCAL 1986). Second, we attribute the high velocity within the
crustal root to an interpolation artifact of the inversion and not to any first arrival picks
from the data, as the Hitplot (Figure 9) confirms. Third, the lack definition at the bottom
of the Sierra Nevada crustal root could be caused by the tunneling of rays through the
root, or more likely, the diffraction of seismic rays off of 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, or small
crustal root (Zandt et al., 2004, Fliedner et al., 1996). Zandt et al. (2004) attributes this
missing Moho to possible scattering of reflections and Ps conversions along a boundary
that may have some topography due to deformation caused by delamination. However,
small-scale topography along the Moho, shouldn’t affect Pn arrivals other than a
smoothing over the region. Also, 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 preferment of Model 2 to 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
20
delays beneath the 55 km root (Louie et al., 2004). A deeper root within this paper’s
study area 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 cross-over distance for the Paso
Robles earthquake is only approximately 150-200 km, and as previously mentioned in
agreement with Ruppert et al. (1998) Who also saw a cross-over of 150 km, but from a
surface shot. Ruppert et al. (1998) also found a lack of delay in their PmP arrivals.
Crustal thickness maps
We have complied a contoured crustal thickness map (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 create 1 km thick contours. 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. The map doesn’t contain any data sets that may have scientifically of
methodology issues. In the compilation map, Mooney and Weaver 1989 and Fliedner et
al., 2000, served as base maps. Mooney and Weaver (1989) is an interpretation of the
crustal thickness within the Sierra Nevada root and surrounding region utilizing most
geophysical work before 1989. Their map relies heavily on result from Eaton et al. (1963)
and Pakiser and Brune (1982) for crustal control within the Sierra Nevada. Fliedner et al.
21
(2000) expanded on the original Mooney and Weaver (1989) map by inverting the SSCD
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 data sets, specifically
Mooney and Weaver (1989), over estimate the depth of the Sierra Nevada root simply by
semantics. As previously mentioned in this paper, current convention is to use the top of
a laminated Moho, which for the Sierras is about 7.6 km/s, as the depth to calculate
crustal thickness. Previously, a velocity of 7.8 km/s was used to describe the Moho
beneath the Sierras (Pakiser and Brune, 1982), which is most likely the bottom of any
lamination.
From this compilation map, a preferred model was generated (Figure 11). The
preferred model eliminates most of these velocity discrepancies, except where no other
data exists. By compiling these data sets it allows better visualize of key features, and
allows us to better address discrepancies and how proposed models fit the actual data.
Several key features to note: the crustal root that is centered west of the topographic
crest; the deepening of the root north of Fresno, California; the steep sided walls of the
crustal root; and the dramatic shift to normal, 30-km thick Basin and Range crust.
Discussion
It has been show that results from the INC refraction experiment are in agreement
with results and modeling from the SCCD experiement (Ruppert et al., 1998, Fliedner et
al., 1996, Fliedner et al., 2000), as well as other work (Eaton et al., 1963, Mooney and
Weaver, 1989, Zandt et al., 2004). However, these experiments were slightly south of the
INC transect. Because the model for the southern Sierras is well developed, using the
22
INC and NWL refraction experiments, we have tied in major geologic events and
supporting evidence for the entire Sierra Nevda. Between 12-10 Ma, the ecologized
section of the root delaminated across entire Sierra Nevada. This coincides and is
probably related to extension within the Death Valley region (Jones et al., 2002, and
Jones et al., 1987). The plus of Miocene volcanism also occurs at this time across the
entire Sierras. We suggest that the root was removed acorss the entire Sierras for several
explanations. First, the Cenozoic uplift of the Sierras began around 12 Ma, and has been
linked to the upwelling asthenosphere (Jones et al., 2002, Wernicke et al., 1998, Lui and
Shen, 1996). For both the southern and northern parts of the Sierras, calculations show
that for the gravity anomolies present in the Sierras, a thick, Airy-type crustal root, at any
reasonable depth, cannot accommodate the measurements. Some compensation from the
mantle is required (Mavko and Thompson, 1988 and Fliedner et al., 1996). The Sierra
Nevada root should have a root of approximatley 75 km depth (Fliedner et al., 2000), but
as discussed in the beginning of this paper the root only has a depth of 35 km in the
south, thickening to only 55 km in the north, as visable on the preferred crustal thickness
map (Figure 11). With the incorporation of results from the INC and NWL transect, this
interpration is made possible.
After a period of quiet, the pulse of potassic volcanism erupted in a localized area
within the Sierra Nevada and east of the range (see outline in Figure 1) (Manley et al.,
2000 and Farmer et al., 2003). The composition of the xenoliths entrained in the
volcanics imply the sinking of the eclogite facies, the mafic lower crust, as well as the
underlying mantle lithosphere (Manley et al., 2000). Between the latitude of 35-38 N, the
slightly thinner root, approx. 42 km thick, is centered west of the highest topography (ie
23
Mt. Whitney, see Figure 1). This suggests that more of the root must have foundered in
the southern and central Sierras than in the north. From reciever 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 fact the crustal root is dripping into the mantle, and so the
Moho is actual “missing.” Because of the evidence from the thickness of the root, the
lack of PmP arrivals, and the localized potassic volcanics are interpted that the entire
mantle lithosphere was removed in this 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 al.
(2004). The INC results fall just north of Zandt et al. (2004) proposed mantle drip, but the
drip maybe pulling areas the north, south.
As previously mentioned it is unclear as to how much of the root to the north was
removed. There are no large volcanic fields like in the south-central Sierras, but as
Manley et al. (2003) points out, there has been little analysis in the northern region. But
because of the location of the proposed Sr line (see Figure 1), the compositions of the
xenoliths may be different, and the same type of eruptions cannot be expected. 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 fact that typically in old
continental crust PmP reflections aren’t clear, as opposed to recently extended crust that
typically have clear, continuous reflections as in the Basin and Range. At this time, there
is no evidence that the mantle lithoshopere has been removed in the northern Sierras, and
the depth of the root as determined by the NWL transect, 55 km-thick, suggests that it
hasn’t, but this cannot be confirmed without further work. Because the root along the
24
entire Sierra Nevada is centered west of the topographic, we assert that the delamination
of the root is in conjuction with deformation in the Basin and Range Province.
Future work
For the INC experiment, there is future work that could add valuable data to the
region. Currently there are deep earthquakes within Fresno, CA that are not recorded on
the seismic networks. Because there is no recorded time, we need to look at all 96 hours
or recording, and at this time to do not have an effective method for doing this. A
Fresno, CA earthquake would give better depth and velocity control in the Sierra Nevada.
The instrument’s recorded several small, M 1-2 earthquakes in Tom’s Place and
Mammoth, NV that would provide crustal velocity and depth control in the Long Valley
area. The earthquakes were processed and inverted, but the start time of the earthquakes
were off, resulting in an unrealistic geologic and geophysical model. By having the
opportunity of re-locating these earthquakes, a more complete model would be achieved.
In addition, to the earthquakes and blasts in-line with the INC transect. The
experiment recorded off-line earthquakes in Tokop, NV and at Round Mountain mine.
The sources will provide important fan-type shots, as well as PmP reflection data for
further analysis.
Conclusions
1. Some root, not as deep as ought to be
2. No model is confirmed, but can show that models showing no root are wrong
i) Negative tests
3. Adds data to community in the area where no data previously existed
25
This material is based upon work supported by the U.S.Department of Energy under
instruments numbered DEFG07-02ID14311 and DE-FG36-02ID14311, managedthrough
the DOE Golden Field Office. The instruments used in the field program were provided
by the PASSCALfacility of the Incorporated Research Institutions for Seismology (IRIS)
through the PASSCAL InstrumentCenter at New Mexico Tech. Data collected during this
experiment will be available through the IRIS DataManagement Center. The facilities of
the IRIS Consortium are supported by the National Science Foundation underCooperative
Agreement EAR-0004370 and the Department of Energy National Nuclear Security
Administration. TheCalifornia Integrated Seismic Network (CISN) and the Western
Great Basin Seismic Network (USGS CooperativeAgreement 04HQAG0004) provided
earthquake locations used in this experiment. We would like to thank BarrickGoldStrike
Round Mountain and Cortez mines for their cooperation and kind help.
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