, and S. W. Roecker, Missing roots and mantle "drips:" Regional Pn and teleseismic arrival times in the Southern Sierra Nevada and vicinity, California, J. Geophys. Res., 99 (B3), 4567-4601, 1994.
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.
, and S. W. Roecker, Missing roots and mantle "drips:" Regional Pn and teleseismic arrival times in the Southern Sierra Nevada and vicinity, California, J. Geophys. Res., 99 (B3), 4567-4601, 1994.
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
, Li Li, J. P. Eaton, C. H. Jones, and J. N. Brune, Earthquake refraction profiles of the root of the Sierra Nevada, Tectonics, 13 (4), 803-817, 1994.
, R. Clayton
, M. Ducea
, C. H. Jones
, S. Park
, S. Ruppert
, J. Saleeby
, J. K. Snow
, L. Squires
, M. Fliedner
, G. Jiracek
, R. Keller
, S. Klemperer
, J. Luetgert
, P. Malin
, K. Miller
, W. Mooney
, H. Oliver
, and R. Phinney, Origin of high mountains in the continents: The southern Sierra Nevada, Science, 271, 190-193, 1996 (12 January)The CU page for the 1993 SSCDP project has many links relevant to this paper.
View official Science abstract
See S. Ruppert write-up of SSCD from his point of view at LLNL
You can see a newspaper article from San Francisco Examiner on this paper as well as a commentary.
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). (Additional SSCD info can be found at Princeton)
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).
,
Constraints on the seismic structure of the lithosphere from teleseismic converted
arrivals observed at small arrays in the southern Sierra Nevada and vicinity,
California, J. Geophys. Research,v. 103, 10065-10090, 1998.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, 2004 .
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.
Abstract at Science and full text at Science and data supplement
Press links (will die over time):
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.
,
and C.
B. Parker, Lithospheric structure from teleseismic P to S conversions at
small arrays, Southern Sierra Nevada, California (abstract), EOS,
75 (44, supl.), 584, 1994.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
,
C. H. Jones, and C.
B. Parker, Reflection and refraction imaging of the crust under the southern
Sierra Nevada using local and regional earthquakes recorded by small arrays
(abstract), EOS, 75 (44, supl.), 584, 1994. 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
,
Lithospheric structure from teleseismic P to S conversions at small arrays,
Southern Sierra Nevada, California (abstract), EOS,77 [46,
suppl.], F464 , 1996. 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
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. [1995]. 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
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.
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.
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