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1 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. 2 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 3 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). 4 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. 5 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 6 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. 7 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 8 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. 9 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 10 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 11 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 12 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 13 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 14 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 15 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 16 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. 17 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 18 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. 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