In addition, see numerous abstracts listed in SNEP and SN Drips project pages.
The extension in the crust of the California Basin and Range Province and the thinning of the mantle lithosphere under the High Sierra appear to share the same bounds in time and space. The uplift of the High Sierra occurred over the past 9 m.y., which coincides with most of the extension that occurred in the California Basin and Range Province. Because the orientation of extension in the California Basin and Range Province is inferred to be approximately N60°W from geologic, geodetic, and in situ stress measurements, the northern and southern edges of the Death Valley extensional subprovince may extend N60°W from the inferred northern and southern limits of west dipping low-angle normal faults of the Death Valley region. Pronounced changes in the averaged topography and Bouguer gravity anomaly across these two bounds both in the Basin and Range Province and in the Sierra Nevada support a connection between the tectonics of both regions. The geomorphic history of the southern Sierra suggests an up-to-the-north warp of the Sierra across this southern bound during latest Cenozoic time. Hence extension near Death Valley may be localized in the crust and may be laterally connected to thinning of the mantle lithosphere beneath the Sierra Nevada. This geometry requires extended crust to overlie unextended mantle lithosphere near Death Valley and virtually unextended crust to overlie tectonically thinned mantle lithosphere in the High Sierra Nevada.
Abstract. Previous seismological studies have placed the source of the uplift of the Sierra either within the crust, suggesting a Mesozoic age for the source of the uplift, or in the upper mantle, consistent with late Cenozoic creation of the buoyant material producing the uplift of the range. We deployed 16 temporary seismometers in the high part of the southern Sierra Nevada to augment the permanent Southern California Seismic Network and record arrivals from regional and teleseismic earthquakes. Arrival times of P waves from 54 teleseisms recorded at these stations are advanced by over a second by a high-velocity body in the upper mantle west and northwest of Lake Isabella. Inversion of the arrival times indicates that this "Isabella anomaly" is of limited north-south extent (about 40-60 km), has compressional velocities about 4-5% higher than its surroundings, and probably extends from about 100 to 200 km depth. The limited north-south extent of the "Isabella anomaly" indicates that it is unrelated to the Sierra; we speculate that it is the downgoing part of a small scale convection system similar to that inferred beneath Southern California. This inversion does not clearly reveal either a large crustal root or a substantial low-velocity body in the upper mantle beneath the Sierra. Although the presence of either degrades the fit to the arrival times and requires high-velocity material beneath the low-velocity material of either root, Bouguer gravity anomalies require low-density material under the Sierra. Assuming that arrival times from earthquakes 150-350 km north and south from the southern Sierra come from a common refractor (the one-layer structure), the upper mantle P wave velocity (Pn) beneath the High Sierra is about 7.6-7.65 km/s; if the arrivals from north and south are from different refractors (the two-layer structure), material with a P wave velocity greater than ~7.2 km/s (the "7.x" layer) would lie under a nearly flat interface from more normal crustal velocities and be separated by a north-dipping interface from underlying mantle with velocities about 7.9-8.1 km/s. The Pn velocity beneath the region immediately to the east is significantly greater (7.9-8.0 km/s) than that of material at equal depths under the Sierra. For the one-layer structure, further assuming that mean crustal velocities are uniform along north-south lines, we find little dip on the Moho in the area; using the arrival times from earthquakes to the south, we infer a depth of 33 +/- 5 km for the Moho beneath the southern High Sierra. This structure of a thin to normal crust over a low-velocity mantle can be reconciled with earlier observations that were used to infer a thick crust under the Sierra. By considering the Bouguer gravity anomaly, the surface geology, refraction profiles in this region, and our own observations, we suggest that 1/3 to 1/2 of the modern elevation in the range is supported by lateral (east-west) density contrasts in the crust; the remainder is supported by density contrasts in the uppermost mantle or lateral variations in the thickness of the "7.x" layer. Our interpretation is that the southern Sierra overlies mantle lithosphere that has been thinned and warmed in response to regional lithospheric extension in Neogene time. This part of the upper mantle might have provided the melt that migrated to the east and produced volcanics in the southwestern Great Basin; depletion of the upper mantle might have increased the seismic velocity and decreased the density of material about 60-100 km beneath the southern Sierra.
PDF direct from AGU (large); requires login
The CU page for the 1993 SSCDP project has many links relevant to this paper.
View official Science abstract
Location map of 1993 Southern Sierra Continental Dynamics Project; Sierra is near center (between 118 and 119 degrees west); Owens lakebed is just east of crossing of refraction lines. Death Valley is deep (darkest green) valley in the eastern part of the map. (Clicking on map will get you bigger, full color JPEG image).
Synthetic view from the NNE of Mineral King area, showing topography possibly responsible for reflected/converted phases visible on MK seismic array. (Clicking on picture gets high res JPEG file).
Synthetic view north along the Sierran crest over Horseshoe Meadow array, with array seismometers located by red (CMG3) and orange (L4) dots. (Clicking on picture gets high res JPEG file).
At DP the simple, strong Moho event is associated with an earlier intracrustal negative-polarity event defining the top of an S-wave low velocity zone probably associated with flow of lower crust into the Basin-and Range. This feature can be followed under the Sierran crest at HM, but is absent further west at MK. Sub-Moho energy is absent under the Basin-and-Range (DP), but is conspicuous under the high Sierra at positive arrivals ~7.3 s (MK) and ~9 s (HM) after the P. These arrivals might be from the base of a low-velocity, low-density upper mantle body supporting the Sierra. Excellent Moho Ps arrivals demonstrate the general lack of large pods of crustal magma except under the Golden Trout volcanic field.
The full text may be found here
Additional figures related to this paper are above and at the SSCD Project page
We use the apparent change in mantle structure beneath the Sierra Nevada since ~10 Ma, which suggests convective removal of eclogite-rich mantle lithosphere, and scaling laws developed for Rayleigh-Taylor instability to place constraints on the average viscosity coefficient of the mantle lithosphere. By treating the lithosphere as a non-Newtonian fluid obeying power-law creep with an exponent of n = 3.5, we may compare the inferred values of viscosity coefficient with those obtained from laboratory experiments on olivine and eclogite. The values that we obtain overlap those predicted by laboratory-based flow laws for the range of geotherms implied by heat flux measurements within the Sierra Nevada and by metamorphic geothermometry and geobarometry of xenoliths in volcanic rock erupted in the Sierra Nevada at ~10 Ma. Thus, this comparison offers support for laboratory-derived flow laws, and specifically for the high stress limit suggested by Evans and Goetze (1979). Conversely, this agreement shows that the high strength of cold mantle minerals does not prohibit its removal by convective instability.
pdf of preprint of manuscript. (PDF of published manuscript available on request)
Boyd, O.S., C. H. Jones, and A. F. Sheehan, Foundering
Lithosphere Imaged Beneath the Southern Sierra Nevada, California, USA, Science,
305, 660-662, doi: 10.1126/science.1099181, 2004 . (Correction to a caption is at a separate location)
Seismic tomography reveals garnet-rich crust and mantle lithosphere descending into the upper mantle beneath the southeastern Sierra Nevada. The descending lithosphere consists of two layers: an iron-rich eclogite above a magnesium-rich garnet peridotite. These results place descending eclogite above and east of high P wave speed material previously imaged beneath the southern Great Valley, suggesting a previously unsuspected coherence in the lithospheric removal process.
Access article at Science and data supplement
Pliocene (~3.5 Ma) removal of dense eclogitic material under the Sierra Nevada has been proposed from variations in the petrology and geochemistry of Neogene volcanic rocks and their entrained xenoliths from the southern Sierra. The replacement of eclogite by buoyant, warm asthenosphere is consistent with present-day seismologic and magnetotelluric observations made in the southern Sierra. A necessary consequence of replacing eclogite with peridotite is that mean surface elevations and gravitational potential energy both increase. An increase in potential energy should increase extensional strain rates in the area. If these forces are insufficient to significantly alter Pacific-North America plate motion, then increased extensional strain rates in the vicinity of the Sierra must be accompanied by changes in the rate and style of deformation elsewhere. Changes in deformation in California and westernmost Nevada agree well with these predictions. Existing geologic evidence indicates that a period of rapid uplift along the Sierran crest of >~1 km occurred between 3-8 Ma, most likely as a consequence of removal of lithosphere. About this same time, extensional deformation initiated within ~50 km of the eastern side of the Sierra (3-5 Ma) and regional shortening began to produce the California Coast Ranges (3-5 Ma). We suggest that these events were induced by the > 1.2•1012 N/m increase of gravitational potential energy generated by the Sierran uplift. Evidence for Pliocene uplift, adjoining crustal extension, and contraction in directly opposing portions of the Coast Ranges is found along the entire length of the Sierra Nevada and implies that lithosphere was removed beneath all of the present-day mountain range. The uplifted area lies between two large, upper-mantle, high-P-wave-velocity bodies under the south end of the San Joaquin Valley and the north end of the Sacramento Valley. These high-velocity bodies plausibly represent the present position of material removed from the base of the crust. Lithospheric removal may also be responsible for shifting of the distribution of transform slip from the San Andreas fault system to the Eastern California Shear Zone, a prediction that awaits better defined slip histories on both faults. Overall, the Late Cenozoic deformational history of the Sierra Nevada and vicinity illustrates that locally derived forces can influence deformation kinematics within plate boundary zones.
pdf of accepted manuscript
Frassetto, A., G. Zandt, H. Gilbert, T. J. Owens, and C. H. Jones, Structure of the Sierra Nevada from receiver functions and implications for lithospheric foundering, Geosphere, 7(4), 898-921, doi: 10.1130/GES00570.1, 2011.
Receiver functions sampling the Sierra Nevada batholith and adjacent regions exhibit significant variations in the structure of the crust and upper mantle. Crustal Vp/Vs values are lower in the core of the batholith and higher in the northern Sierra Nevada, portions of the Basin and Range, and near young volcanic fields in the eastern Sierra Nevada and Owens Valley. P- to S-wave conversions from the Moho vary from high amplitude and shallow (>25% of the direct P-arrival amplitude, 25–35 km depth) along the eastern Sierra Nevada to low amplitude and deep (<10%, 45–55 km) beneath the western batholith. We propose that dense mafic-ultramafic residue has foundered in the east-central and southern Sierra Nevada but still resides beneath its western portion. The central and northern Sierra Nevada shows inherited, prebatholithic structure at the Moho that was not completely overprinted by emplacement of the massive end-stage batholith. Evidence for the development and/ or loss of substantial residue in the northern Sierra Nevada is equivocal. The asymmetric structure of the lithosphere beneath the central Sierra, which we model using constraints from petrophysical analyses, suggests that foundering progresses from southeast to northwest. This process sharpens the seismic response of the Moho by removing its under- lying lithospheric mantle and allows upwelling asthenosphere to replace the detached material. Deep crustal seismicity and recent volcanism observed to 38° N appear linked to this process and correlate spatially with the change in the character of the Moho, measurements of high crustal Vp/Vs, and pres- ence of prominent negative conversions in the crust and uppermost mantle.
The CU page for the 1993 SSCDP project has many links relevant to this paper.
See other Southern Sierra Continental Dynamics Project info at Princeton
The CU page for the 1993 SSCDP project has many links relevant to this paper.
See other Southern Sierra Continental Dynamics Project info at Princeton
AGU web version of this abstract
Recent petrologic and isotopic analysis of both xenoliths from the seismic upper mantle and the basalts bearing those xenoliths have been interpreted as showing that an eclogitic mass perhaps about 30 km in depth extent existed up to about 4-8 Ma (other abstracts in this session). Removal of this body by 4 Ma, and its replacement with hotter peridotites, would have greatly increased the buoyancy of the mantle. The fate of this mass and any possible lateral equivalents remains unknown. To try and determine the extent of any eclogite under the Sierra and its environs, 24 broadband seismometers were deployed in 1997 from the westernmost foothills to the ranges of the Basin and Range to the east. Many seismological parameters of eclogite and peridotite overlap, including P and S velocities, although S velocities are likely to be somewhat higher in eclogite. One outstanding difference is the seismic anisotropy of the two rocks: eclogites tend to remain seismically isotropic while peridotites are easily made anisotropic. Thus, we are attempting to image variations in S velocity and anisotropy under the Sierra. Raw observations indicate that profound variations do exist: SKS splits vary from ~0.7 s in the western foothills to 1.5-2.5s along the Sierran crest, and SKS arrival time residuals vary by about 3 s from early in the west to late in the east. The fast orientation for S propagation appears to be nearly uniformly N70E across the region, parallel to the Garlock fault, much Neogene extension in the central Basin and Range, and modern North America-hot spot motion. A possible interpretation is that isotropic eclogitic rocks remain under the western Sierran foothills, possibly to very great depth, and are absent to the east. Alternatively a vertically-varying or lower-amplitude anisotropy under the foothills could be juxtaposed with stronger anisotropy under the Sierran crest. Measurements of S-waves will be made to try and provide depth control on this variation in S velocity and anisotropy.
Some of the data presented at GSA is viewable on the web
Abstract at GSA
Sierran Paradox web page has additional info
Previous studies in the Southern Sierra and Southwestern Great Basin [e.g., Jones and Phinney, 1998] have inferred the presence of a large-scale extensional feature that dips west from the Great Basin underneath the Sierran crest. An ongoing passive seismic experiment in and around the Coso Geothermal Area seeks to define the relationship of this large scale feature with the Geothermal Area and identify any other features related to the geothermal resources of this region. We use teleseisms recorded by five seismic arrays of 6-11 mostly short-period seismometers deployed for 2-8 months. The arrays allow us to examine and remove the complications from scattering from topography or strong subsurface variations. The response of all instruments in the arrays was transferred to the instrument response of a L4 seismometer with a free period of about 1s. Approximately twenty teleseismic events from varying backazimuths were chosen for each array to get the best azimuthal coverage possible. For each chosen event, beams of all three components from all stations within an array are formed. We determine P-to-S conversions through receiver function analysis of the beamed seismograms using the least squares time domain deconvolution processing developed by Abers et al. . Preliminary results indicate that the Moho passes through the heart of the geothermal area with little variation from the depth of 30 km b.s.l. from published results to the north. From a very preliminary examination of the data, the conversion from the middle crust associated with an extensional shear system is identified to the east of the geothermal zone but appears to be absent within the geothermal field. Some possible explanations include an upwarping of the extensional system consistent with a developing core complex, shadowing of the extensional shear by fluids in the middle crust, or obliteration of the shear by younger magmatism. Results of a more complete analysis will be used to address these issues, which should illuminate the relationship of the geothermal field to
Abstract at AGU
Advances in passive seismic data collecting and processing have produced higher resolution images of the crust and mantle than have been previously obtainable. The Earth is appearing to be more heterogeneous than was thought when only rougher scale observations were available. Here we present results from a dense array of passive seismometers that show considerable variations in the crust over distances as short as 10 km and compare these results to what is observable with less densely spaced instruments. We utilize data collected during the 18-month deployment of 16 dense mini-arrays in the region of the China Lake geothermal field near Ridgecrest, CA. We image the crustal structure within the geothermal field, its relationship to regional tectonic features, and search for an indication of mantle influence on volcanism. The mini-arrays consist of mostly short period instruments arranged in orthogonal line arrays with 1/2-km station spacing. The average distance between each array is approximately 5 km. We calculate 375 good quality mini-array beamed receiver functions for teleseismic events. Using array-processing techniques, we mitigate the effects of near surface scattered energy. Mini-arrays of seismometers allow for imaging of small-scale crustal structures, as scattered energy will decorrelate across the array while arrivals from converted phases stack coherently. Combining data from all arrays we process the data set as an array of mini-arrays and stack the data into CCP bins. Processing the data in this manner allows us to observe lateral variations in subsurface structures such as mid-crustal features and the Moho within the nearly 40 by 40 km area of sampling. We find extremely complex crustal structure in this region, including many converters dipping nearly 15 degrees and over 8 km of topography on the Moho. It is likely that complex, non-planar interfaces produce artifacts in our CCP stacks, and to accurately image complex crustal structure we perform backprojection migration. By migrating the data recorded by the group of mini-arrays we produce a fine scale image of the crust that is minimally contaminated by scattering artifacts.
Abstract at AGU
Volcanic rocks and associated xenoliths from the Sierra Nevada of California indicate that the entire mantle lithosphere was removed about 3.5 Ma, including about 30 km of eclogites and garnet pyroxenites. Such removal is surprising in that the Sierra has long been noted for its low surface heat flow, which, when combined with temperature information from xenoliths, indicates that this mantle lithosphere was very cold at the time of removal. One means of exploring the magnitude of this event is to examine the tectonic consequences. Replacing such a thick, dense body with more buoyant asthenosphere should drive uplift, which is consistent with uplift of the Sierran crest by more than 1 km between 3 and 8 Ma. Removal will also increase the gravitational potential energy of the Sierran lithosphere by at least 1.2 * 1012 N/m, which is capable of inducing extension. Such extension within 50 km of the east edge of the modern Sierra initiated between 5 and 3 Ma. If there are no changes in Pacific-North American plate motions [e.g., Atwater and Stock, 1998], then new extension must shut down extension elsewhere or increase compression. The California Coast Ranges date to about 3-5 Ma and largely have been created through shortening normal to the Sierran axis. Potentially this could influence San Andreas rates, as narrowing of the rigid Sierran block permits strike-slip motion to increase on the east side. A decrease of 12 mm/yr on the San Andreas at $\sim4$ Ma [Dickinson, 1996] suggests that slip on the Eastern California Shear Zone became viable about this time. All of these effects extend the length of the Sierra, indicating that removal affected the entire Sierra. If the removal occurred as a Rayleigh-Taylor instability, existing models can be reconciled with the cold temperatures if the high stress limit of Evans and Goetze (1979) is used and, probably, the top boundary of the lithosphere weakened prior to removal. However, the removal of the entire lithosphere is unexpected and suggests that the physics of these systems, including the lithospheric rheology, need additional study.
Abstract at AGU
H. Reeg., C. H. Jones, H. Gilbert, T. J. Owens, and G. Zandt, Tomographic observations connecting convective downwellings with lithospheric source regions, Sierra Nevada, California, Eos Trans. AGU, 89(53), Fall Meet. Suppl., Abstract S32B-06. 2008
Considerable speculation has focused on the possible existence of convective downwellings associated with the Sierra Nevada, California. The 2005-2007 Sierra Nevada Earthscope Project (SNEP) occupied ~100 sites within the broader EarthScope Transportable Array using EarthScope FlexArray equipment. We observed 2000 events at 95 SNEP stations and 164 TA, permanent, and pre-SNEP Sierran experiment stations, yielding over 81,000 teleseismic P-wave arrival times picked with G. Pavlis's dbxcor waveform picking algorithm. We selected 27,000 arrivals for inversion both to equalize representation of different backazimuths and accommodate computational limitations. Using a teleseismic inversion code developed by S. Roecker that uses wavespeed gradients between nodes and calculates 3-D raypaths using a finite- difference algorithm, we find that we can recover lateral variations in wavespeed with very high resolution but the extent of sharp anomalies can become smeared vertically as far as one node spacing (~50 km). As expected, we image the large high-velocity anomalies previously seen in California, including the Isabella Anomaly (San Joaquin Valley) between about 70 and 250 km depth, the Redding anomaly under the eastern Sacramento Valley above about 200 km depth, and a Foothills Anomaly near the Moho under much of the western Sierra. The Foothills anomaly extends between the Redding and Isabella anomalies. At each end of the Foothills anomaly, the high-velocity body bends down to connect with the deeper, more vertical anomaly at its end. This is most striking at the north end, where a peculiar convex-upward portion of the anomalies appears to represent interaction of a convective downwelling like that at the south end of the Sierra with the clearly visible Gorda plate. This suggests that some active foundering of lithospheric material occurs in these locations. The eastern, high Sierra are underlain by lower velocity mantle; this mantle increases in velocity from south to north, suggesting more vigorous upwelling to the south. Whether or not the Foothills Anomaly represents material that formed in situ or has been thickened with material originally under the eastern Sierra remains unclear. These results strongly indicate that convective processes under continents are asymmetric and prone to complex interactions with other geologic entities.
Jones, C. H., Volume balancing seismic tomography: Quantitative tests of the origin of the Isabella Anomaly and the Sierra Nevada, GSA Annual Meeting, Paper 316-3, GSA Abstracts with Programs,45 (7), p.727 2013
Levandowski, W, C. H. Jones, G. C. Oliver, Constraining age of delamination with thermal models: a multidisciplinary view of the Sierra Nevada, CA , AGU Fall Meeting, abstract T31D-2550, 2013
Geomorphic, xenolith and seismic evidence suggest that the Sierra Nevada has risen ~1 km since the Miocene in response to removal of cold, dense lower lithosphere. A high wavespeed body beneath the Tulare Basin, southwest of the Sierra, has been proposed to be the downwelling lithospheric root, and isopachs demonstrate latest Pliocene acceleration of subsidence. Nevertheless, a removed mass sufficient to cause 1 km of Sierran uplift would generate ~250% of observed subsidence, potentially vitiating the lithospheric removal hypothesis, and redating of xenoliths has called temporal constraints into question. We first estimate upper mantle densities from seismic velocities, finding that the modern load on the Tulare lithosphere accounts for observed post-Pliocene subsidence. Next, we couple a characterization of material that may have been removed from the Sierra with a 3-dimensional, finite Nusselt number thermal model. We constrain the amount of time required for sufficient thermal equilibration, such that the mass anomaly of this removed material and the modern anomaly responsible for Tulare subsidence are equal; 4-10 Ma suffice. Finally, new seismic anisotropy images from joint analysis of split direct-S and SKS waves show the convective wake of material delaminating from the southern Sierra SSW to beneath the Tulare basin. Taken together, our results support the existing hypothesis that a lithospheric root has been removed from beneath the Sierra and is now found beneath the southern Great Valley.
Bernardino, M. J., C. H. Jones, Understanding complex teleseismic wave propagation in the Sierra Nevada through vertical-component P-wave receiver functions, AGU Fall Meeting, abstract S31C-2360, 2013
Past seismic studies attempting to image the lithosphere underneath the Sierra Nevada and to constrain the geometry of the upper mantle Isabella anomaly, a high wave-speed body underneath the western foothills of the range, have observed complex behavior in teleseismic and regional waveforms recorded at stations within the range. Notably, a 1993 teleseismic mini-array recorded multipath P-wave arrivals, topographic reflections, and scattered energy ~25 km west of the Sierran crest. These effects suggest wave propagation through strongly heterogeneous lithosphere complicated by near-surface phenomena. Multipathing and other complex wave propagation are indicative of strong variations in wavespeed, which in turn reflect structural complexity important in understanding the genesis of the Isabella anomaly. However, determining the extent of such propagative behavior in and underneath the Sierra Nevada has not been studied. We investigate the behavior of teleseismic P-waves using vertical-component receiver functions in an effort to better understand the extent of complex waveforms as a first tool in better constraining the geographic region(s) where sufficiently complex lithospheric structure exists. We expect that the presence of sufficiently high velocity gradients should result in P-wave multipath arrivals from events that skirt the perimeter of the Isabella anomaly from certain backazimuths. We deconvolve regionally beamed vertical P-waveforms from individual vertical component P-waves. This effectively recovers variability in the P waveforms that is normally lost in typical single-station radial- and transverse-component receiver function analyses. Vertical P-wave beams are constructed using dbxcor, a waveform correlation algorithm developed by G. Pavlis. Seismic data for the northern and central Sierra Nevada are from the 2005-2007 Sierra Nevada Earthscope Project (SNEP) and further supplemented by many permanent and temporary stations including the Earthscope Transportable Array. Data for the southern Sierra Nevada comes from the 1997 Sierran Paradox Experiment (SPE).
Jones, C. H., H. Reeg, G. Zandt, H. Gilbert, T. J. Owens, and J. Stachnik, Slab, drip, or peeling lithosphere: Teleseismic P-wave tomography and the Isabella anomaly of the southwestern Sierra Nevada, California, AGU Fall meeting, abstract S21A-2381, 2013
The high-wavespeed Isabella anomaly in the upper mantle at the southwestern edge of the Sierra Nevada has been interpreted as a convective (Rayleigh-Taylor) instability (or drip), the remains of a fragment of the Farallon plate (a slab) or the product of delamination of lithosphere from the east or south. P-wave tomography using 29,186 picks from portable deployments from 1988, 1997, and the SNEP deployment of 2005-7 and surrounding TA and permanent broadband stations was run from a variety of starting models. Some models started from a 1-D earth model, some from the Moschetti et al. (JGR 2010) 3-D S-wave model, some from the Gilbert et al. (Geosphere, 2012) 3-D SV-wave model. S-wave models were converted to P-wavespeeds using the regression suggested by Brocher (BSSA 2005). In some cases the upper levels of the 3-D models were fixed and only wavespeeds below ~55 km were allowed to change. Because of the relatively poor vertical resolution of the teleseismic body wave tomography and the bias towards a minimal model variance, the resulting images in the upper lithosphere vary considerably between models, producing results resembling slabs, drips, and delaminations for the Isabella anomaly. Thus the shape of the shallow part of the anomaly does not reliably determine its origin. Deeper (100-250 km) parts of the anomaly are consistent between different inversions with a ≥4% fast body dipping 60-70° east. Vertical integrals of wavespeed anomaly are relatively insensitive to the models; use of such an integral over the body from 95 to 245 km depth yields an equivalent volume of 7 ± 1 x106 km3 at a mean anomaly of 1%. We expect from geological considerations that a volume on average 5% fast of 0.9-1.6 x 106 km3 was removed from under the southern Sierra, equivalent to a volume of 4.4-8.4 x 106 km3 at 1% fast, in close agreement with the equivalent volume of the Isabella anomaly. We prefer some kind of 3-D convective removal for the Isabella anomaly, noting that the volumes are appropriate and the geometry compatible with such an origin. A slab origin is troubled if the slab is neutrally buoyant because of the odd coincidence of subsidence in the past 3-5 Ma, it is troubled if antibuoyant as the position so far east and a 60-70° dip ~20 Ma after subduction would seem difficult to maintain, and in either event, the Isabella anomaly is north of a position consistent with the subducted remains of the Monterey subplate. True delamination (i.e., peeling with minimal internal deformation) is unlikely as the volume of material is probably greater than what was present due east and no scar in the lithosphere has been preserved; however, some variation on delamination allowing considerable internal deformation might be possible. Identification of the robust and not-so-robust elements of tomography allows for a better test of hypotheses.
Jones, C. H., W. Levandowski, M. Bernardino, and S. Roecker, Resolving the character of the cryptic crust of the western Sierra Nevada, California. Geological Society of America Abstracts with Programs. Vol. 47, No. 7, p.463, 2015.
Although seismology has been a key discipline in understanding the modern framework of the Sierra Nevada, application of seismology alone revealed a dilemma. The 1993 active source experiment traversing the Sierra (with Randy Keller as a participant) showed that the crust was thicker under the western foothills than under the higher eastern part of the range, a sort of anti-Airy situation confirmed by receiver function studies. The confusion is further compounded by teleseismic body waves, which arrive earlier in the foothills and later in the High Sierra, more in keeping with a thicker crust to the east than to the west. The identity of this material requires more information. Here we review constraints from the surface geology and gravity to try and understand the geology of this material and place it within the context of the evolution of the Sierra Nevada. Using seismological models derived in part from surface waves and teleseismic body waves, we have shown that the crust of the western Sierra probably has a temperature-corrected density approaching 3.0 g/cc; thus the apparent conflict between the thick crust, early teleseismic arrivals and low elevation of the western foothills is largely resolved by this crust being unusually thick, dense, and high-wavespeed. There are still puzzles in the area, though. The original observations of a thick Sierran crust, made by Perry Byerly in the 1930s, have yet to be fully explained. These were observations of delayed arrivals of regional (Pn) waves from earthquakes west of the Sierra observed on the eastern side of the range. Waveform simulations of these waves with the seismic structure used in the density and gravity analysis fails to reproduce the observed delays. Intriguingly, published magnetotelluric work and unpublished seismic attenuation results suggest that part of the lower crust under the western Sierran foothills is warm and not cold. The geometry of these results resembles a westward pointing wedge of warm material; this might be a product of ongoing deformation or, perhaps, some poorly understood characteristic dating to the Mesozoic. Resolving this structure and interpreting its tectonic implications will continue to require the use of multiple kinds of geophysical and geologic information.
Bernardino, M., C. H. Jones, W. Levandowski, S-wave tomographic model of the Sierra Nevada, California: Constraining thermal and compositional effects through Vp/Vs, anisotropy, and attenuation, AGU Fall meeting, abstract S23C-2712, 2015.
The lithospheric seismic structure of the Sierra Nevada, California has long been recognized as an important tool for determining the uplift mechanism(s) of this range and its effect on the tectonic evolution of the western United States. Past studies have generally observed that at upper mantle depths, the Sierran crest is characterized by slower wavespeeds, suggestive of buoyant material while the western foothills are characterized as regions of faster, more dense material. Although there exists many different tomographic models of the Sierra Nevada, its exact geometry and structure are difficult to constrain and vary. For instance, the high wavespeed Isabella anomaly has been imaged in many different studies and has been interpreted as delaminated lithosphere, convectively downwelling lithosphere, and a remnant slab. Challenges in estimating a tomographic model include discrepancies in data coverage, resolution, and accounting for the effects of anisotropy and attenuation. It is still not well understood whether seismic velocity variations in the Sierra Nevada are thermal or compositional in origin. To address this question, we will use a three-dimensional P- and S-wave tomographic model of the Sierra Nevada and vicinity to characterize its lithospheric thermal and compositional structure. Travel times for direct Sfast and Sslow using regional-to-teleseismic S- and SKS- phases will be measured to invert for Vp, Vp/Vs, and percent anisotropy. Teleseismic P-wave travel times used to calculate Vp and Vp/Vs were measured from a previous study. Vp/Vs and percent anisotropy will be used to evaluate regions where compositional effects are prevalent. Regions with higher anisotropy could be indicative of olivine-rich, strained mantle. However, lower anisotropy could be indicative of isotropic minerals such as eclogite. High Vp/Vs could suggest more garnet and pyroxene or a decrease in Mg. We will also calculate Sfast and Sslow wave differential attenuation, dt*, to evaluate regions where thermal or scattering effects dominate.
Bernardino, M., C. H. Jones, W. Levandowski, tectonic underpinings of the Sierra Nevada and surroundings: Resolving the physical state of the upper mantle using teleseismic shear waves, Geological Society of America Abstracts with Programs. Vol. 48, No. 7, paper 267-9, doi: 10.1130/abs/2016AM-286826, 2016a.
We refine our understanding of the upper mantle processes driving tectonism in the Sierra Nevada and volcanism to its east by incorporating teleseismic shear waves in a tomographic inversion of the Sierra Nevada and vicinity. Because variations in composition and melt content are expected to have shear-wave signatures differing from a purely thermal interpretation possible with just P-waves, we can explore for garnet-rich lithologies and high-melt regions in the upper mantle. We also investigate the interplay between lithology, melt, and mantle deformation by accounting for anisotropy in our measurements. S-waveforms were rotated into the Sierran SFast and SSlow directions as observed from SKS-splitting measurements. Teleseismic P-, SFast-, and SSlow- arrival times were then inverted for 3-D perturbations in Vp, Vp/VsMean, and percent anisotropy using three surface wave starting models. We observe the highest Vp/Vs anomalies in regions marked by young volcanism. These include the Long Valley-Mono Lake magmatic system and the Clear Lake and Coso volcanic fields. The largest of these anomalies is the Long Valley-Mono Lake magmatic system extending to depths greater than 100 km. Peak Vp/Vs perturbations of +4% are found at 40 km depth. High velocities related to the Isabella and Gorda anomalies are characterized by low Vp/Vs values with peak perturbations of -2% and low anisotropy. For the Isabella anomaly, there is a profound difference between the velocity and the Vp/Vs geometries. The velocity results image the Isabella anomaly as an eastward plunging singular body. However, Vp/Vs results image the Isabella anomaly as two separate bodies whose grouped geometry is also plunging to the east. This dichotomy might indicate that the material making up the P-wave Isabella anomaly is actually compositionally heterogeneous. Within the Sierra Nevada, the highest anisotropy anomalies are largely contained within the central portion of the range and the adjacent section of the Great Valley. Such features suggest that the tectonic and deformational regime beneath the Sierra Nevada is diverse and segmented.
Bernardino, Melissa, C. H. Jones, and G. Monsalve, Upper mantle anisotropic attenuation of the Sierra Nevada and surroundings, Am. Geophys. Union Fall Meeting, T13C-2724, 2016b
We investigate the contribution of anelasticity in the generation of seismic velocity variations within the upper mantle of the Sierra Nevada and surrounding regions through teleseismic shear-wave attenuation. Given that anelastic effects are most sensitive to temperature and hydration and less to composition and small degrees of partial melt, we aim constrain the thermal structure beneath this region and identify locations where elevated upper mantle temperatures dominate. We also investigate the dependence of shear-wave attenuation on direction by accounting for seismic anisotropy in our measurements. S-wave t* values are determined from teleseismic S- and SKS- phases recorded on permanent and temporary deployments within the California region with particular focus on the Sierra Nevada Earthscope Project (SNEP) and the Sierran Paradox Experiment (SPE) stations. S-waveforms are rotated into the Sierran SFast, N75°E, and SSlow, N15°W, components. Following the method of Stachnik et al., (2004), S-wave spectra for each event are jointly inverted for a single seismic moment, M0k, and corner frequency, fck, for each event, and separate t* for each ray path. The resulting t*Fast and t*Slow measurements are then inverted for three-dimensional variations in (1/QFast) and (1/QSlow). Results are compared with previous magnetotelluric, surface heat flow, and body-wave velocity inversion studies.
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Last modified at October 15, 2016