George Zandt

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• Erdman, M. E., Hacker, B. R., Zandt, G., & Seward, G. (2013). Seismic anisotropy of the crust: Electron-backscatter diffraction measurements from the Basin and Range. Geophysical Journal International, 195(2), 1211-1229.
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  Abstract: Crystal preferred orientations were measured in a suite of rocks from three locations in the Basin and Range using electron-backscatter diffraction. Anisotropic velocities were calculated for all rocks using single-crystal stiffnesses, the Christoffel equation and Voigt-Reuss-Hill averaging. Anisotropic velocities were calculated for all three crustal sections using these values combined with rock proportions as exposed in the field. One suite of rocks previously measured in the laboratory was used as a benchmark to evaluate the accuracy of the calculated velocities. Differences in the seismic anisotropy of the Funeral Mountains, Ruby Mountains and East Humboldt Range sections arise because of differences in mineralogy and strain, with the calcsilicate dominated Ruby Mountains section having higher P-wave speeds and VP/VS ratios because of the reduced quartz content. In all cases, the velocities show either transverse isotropy or nearly so, with a unique slow axis normal to the foliation. Velocity anisotropy can thus be used to infer the flow plane, but not the flow direction in typical crustal rocks. Areas with a subhorizontal foliation have minimal shear wave splitting for vertically propagating waves and are thus good places to measure mantle anisotropy using SKS-splitting. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.
• Levandowski, W., Jones, C. H., Reeg, H., Frassetto, A., Gilbert, H., Zandt, G., & Owens, T. J. (2013). Seismological estimates of means of isostatic support of the Sierra Nevada. Geosphere, 9(6), 1552-1561.
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  Abstract: The modern topography of the Sierra Nevada (California, UNITED STATES) has been attributed to rapid uplift following foundering of negatively buoyant lithosphere into the asthenosphere since ca. 10 Ma. Uplift now manifests as ~2 km mean topographic relief between the crest of the southern Sierra Nevada and the western foothills and 1-2 km between the Sierran crest and adjacent Basin and Range. In this study, we use seismic P-wave velocity structures derived from teleseismic tomography to estimate the lithospheric density structure in the region and thus infer the current sources of topographic support. We exploit the different derivatives of crustal density with temperature and wave speed to attempt to identify a single solution for crustal density and temperature that satisfi es fl exural isostasy and the P-wave tomography. This solution yields both temperature variations compatible with observed heat fl ow and Bouguer gravity anomalies concordant with observations. We fi nd that the topographic gradient between the crest and the eastern Great Valley is due to both crustal and mantle sources. Despite a greater thickness, the foothills crust is less buoyant than that beneath the range crest, accounting for ~1 km of the topographic difference. High densities are due principally to composition. High-velocity upper mantle (~50-100 km depth) is also observed beneath the foothills but not the range crest, and this contrast explains an additional 1 km of topographic difference. Miocene or more recent removal of such upper mantle material from the Sierran crest, as inferred from xenoliths, would have triggered rapid uplift of ~1 km. Our fi ndings are consistent with the removal of negatively buoyant material from beneath the Sierra Nevada since the Miocene.
• Worthington, J. R., Hacker, B. R., & Zandt, G. (2013). Distinguishing eclogite from peridotite: EBSD-based calculations of seismic velocities. Geophysical Journal International, 193(1), 489-505.
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  Abstract: Seismic velocities were calculated for 11 eclogites from the Western Gneiss Region, Norway, based on electron-backscatter diffraction (EBSD). The P-wave velocities are 8.0-8.5 kms-1 and the S-wave velocities are 4.5-4.8 kms-1; VP/VS1 (the ratio of P-wave to fast S-wave velocities) is 1.74-1.81. All the eclogites are relatively isotropic, with the higher anisotropies (3- 4 per cent) in micaceous samples. Peridotite is comparatively more anisotropic (4-14 per cent more for P waves and up to 10 per cent more for S waves), and can have anomalously low VP/VS1, which may be useful means of distinguishing it from eclogite. Micaceous eclogite may be modelled using hexagonal anisotropy with a slow unique axis, whereas peridotite is most robustly modelled using orthorhombic anisotropy. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.
• Gilbert, H., Yang, Y., Forsyth, D. W., Jones, C. H., Owens, T. J., Zandt, G., & Stachnik, J. C. (2012). Imaging lithospheric foundering in the structure of the Sierra Nevada. Geosphere, 8(6), 1310-1330.
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  Abstract: Tomographic studies of the mantle of southern California (USA) commonly found evidence for seismically high speed material, known as the Isabella anomaly, extending from near the base of the crust of the southwestern Sierra Nevada foothills into the asthenosphere. This anomaly has been interpreted to mark downwelling lithospheric material that had been removed from the southern Sierra Nevada. Using data from the Sierra Nevada EarthScope Project (SNEP) array, we investigate the lithosphere of the Sierra Nevada and surrounding region to better understand the process by which batholiths form dense lithospheric roots that become unstable and founder. Inverting phase velocities of fundamental mode Rayleigh waves for shear wave speeds provides observations of the distribution of high and low wave-speed anomalies, which correspond to portions of the batholith that formed an intact lithospheric root, and where seismically slower shallow asthenosphere marks areas where lithosphere has been removed. Our results corroborate previous observations that the southern Sierra Nevada has thin crust underlain by shallow asthenosphere. High shear wave velocity (Vs) material in the mantle beneath the southwestern foothills marks the location of the Isabella anomaly, to the east of which is a region of low Vs mantle where asthenosphere has risen to replace the delaminating root. Farther north, near the latitude of Long Valley, low velocities at shallow depths beneath the high elevations of the eastern Sierra indicate the presence of asthenosphere close to the base of the crust. Thicker high-speed material, however, underlies the western foothills of the Sierra Nevada at this latitude and dips to the east where it extends to depths of ~100 km or more, giving it the appearance of a portion of lithosphere that has detached from the east but remains attached to the west as it is currently peeling off. The structure of the Sierra Nevada changes near the latitude of Lake Tahoe, where thinner lithosphere extends between depths of 40 and 80 km, but does not reach greater depths. It appears that the lithospheric material of the Sierra Nevada from latitudes close to Lake Tahoe, and continuing to the north, is not being removed, indicating a change between the structure and evolution of the southern and northern Sierra Nevada. © 2012 Geological Society of America.
• Frassetto, A. M., Zandt, G., Gilbert, H., Owens, T. J., & Jones, C. H. (2011). Structure of the Sierra Nevada from receiver functions and implications for lithospheric foundering. Geosphere, 7(4), 898-921.
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  Abstract: Receiver functions sampling the Sierra Nevada batholith and adjacent regions exhibit signifi cant 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 fi elds 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 (
• Porter, R., Zandt, G., & McQuarrie, N. (2011). Pervasive lower-crustal seismic anisotropy in Southern California: Evidence for underplated schists and active tectonics. Lithosphere, 3(3), 201-220.
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  Abstract: Understanding lower-crustal deformational processes and the related features that can be imaged by seismic waves is an important goal in active tectonics. We demonstrate that teleseismic receiver functions calculated for broadband seismic stations in Southern California reveal a signature of pervasive seismic anisotropy in the lower crust. The large amplitudes and small move-out of the diagnostic converted phases, as well as the broad similarity of data patterns from widely separated stations, support an origin primarily from a basal crustal layer of hexagonal anisotropy with a dipping symmetry axis. We conducted neighborhood algorithm searches for depth and thickness of the anisotropic layer and the trend and plunge of the anisotropy symmetry (slow) axis for 38 stations. The searches produced a wide range of results, but a dominant SW-NE trend of the symmetry axis emerged. When the results are divided into crustal blocks and restored to their pre-36 Ma locations, the regional-scale SW-NE trend becomes more consistent, although a small subset of the results can be attributed to NW-SE shearing related to San Andreas transform motion. We interpret this dominant trend as a fossilized fabric within schists, created from top-to-the-SW sense of shear that existed along the length of coastal California during pretransform, early Tertiary subduction or from shear that occurred during subsequent extrusion. Comparison of receiver-function common conversion point stacks to seismic models from the active Los Angeles Regional Seismic Experiment shows a strong correlation in the location of anisotropic layers with "bright" reflectors, further affirming these results. © 2011 Geological Society of America.
• Zandt, G., & Reiners, P. (2011). Earth science: Lithosphere today.... Nature, 472(7344), 420-421.
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  PMID: 21525917;
• Frassetto, A., Zandt, G., Gilbert, H., Owens, T. J., & Jones, C. H. (2010). Improved imaging with phase-weighted common conversion point stacks of receiver functions. Geophysical Journal International, 182(1), 368-374.
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  Abstract: Broad-band array studies frequently stack receiver functions to improve their signal-to-noise ratio while mapping structures in the crust and upper mantle. Noise may produce spurious secondary arrivals that obscure or mimic arrivals produced by P-to-S conversions at large contrasts in seismic impedance such as the Moho. We use a Hilbert transform to calculate phase-weights, which minimize the constructive stacking of erroneous signal in receiver function data sets. We outline this approach and demonstrate its application through synthetic data combined with different types of noise, a previously published example of signal-generated noise, and a large data set from the Sierra Nevada EarthScope Project. These examples show that phase-weighting reduces the presence of signal-generated noise in receiver functions and improves stacked data sets. © 2010 The Authors Journal compilation © 2010 RAS.
• Bennett, R. A., Fay, N. P., Hreinsdóttir, S., Chase, C., & Zandt, G. (2009). Increasing long-wavelength relief across the southeastern flank of the Sierra Nevada, California. Earth and Planetary Science Letters, 287(1-2), 255-264.
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  Abstract: A high degree of correlation between present-day relative rock uplift measured using continuous GPS geodesy and spatially averaged surface elevations suggests that long-wavelength topographic relief is presently increasing along the southeastern flank of the Sierra Nevada range and within an adjacent portion of the northern Basin and Range province. Current estimates for erosion rate are an order of magnitude smaller than the relative rates determined by geodesy. Thus, although the uplift serves to enhance long-wavelength relief, it cannot be explained entirely as an isostatic response to erosion. If uplift rates have been constant through time, the duration over which the uplift could have been active (
• Mercier, J. -., Bostock, M. G., Cassidy, J. F., Dueker, K., Gaherty, J. B., Garnero, E. J., Revenaugh, J., & Zandt, G. (2009). Body-wave tomography of western Canada. Tectonophysics, 475(3-4), 480-492.
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  Abstract: In this study, we have produced P- and S-wave velocity models for western Canada using 23,420 delay times measured on vertical component seismograms, and 15,805 delay times measured on transverse component seismograms, respectively, from a range of permanent and temporary networks. Resolution is best in southwestern British Columbia, and along the CANOE (northwestern Alberta, southern Yukon and Northwest Territories) and BATHOLITHS (northwestern BC) arrays where the station density is the highest, and fair elsewhere. We focus our attention on two distinct features 1) the transition from Phanerozoic to Cratonic mantle in northwestern Canada, and 2) the complex tectonic environment at the northern terminus of the Cascadia subduction zone where the plate boundary changes from convergent to transform. We find that the main transition from Phanerozoic to Cratonic mantle in northwestern Canada occurs at the Cordilleran deformation front and represents a sharp jump in seismic velocity from - 2% to + 2% over a distance of ~ 50 km. In northern Cascadia, we have imaged and characterized the signature of the subducting Juan de Fuca plate and observed evidence of subduction beyond the northern edge of the slab. We also demonstrate that the Anahim hotspot track is underlain by a - 2% low-velocity zone possibly extending to 400 km beneath Nazko cone that appears to be the source of volcanism in this area. Consequently, we associate the source of magmatism in this area to a mantle-scale rather than lithospheric-scale process. © 2009 Elsevier B.V. All rights reserved.
• Ozacar, A. A., & Zandt, G. (2009). Crustal structure and seismic anisotropy near the San Andreas Fault at Parkfield, California. Geophysical Journal International, 178(2), 1098-1104.
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  Abstract: Receiver functions (RFs) from station PKD located ∼3 km SW of the San Andreas Fault (SAF) samples the Salinian terrane near Parkfield. Crustal multiples indicate a 26-km-thick crust with a VP/ VS of 1.88, which is slightly lower (1.83) for the upper and middle crust in the west. For the mid-crust, arrivals are observed at times corresponding to recently imaged seismic reflectors and may correspond to a layer of metasedimentary rocks below the base of the granitic batholith exposed at the surface. For the lower crust, RFs display strong polarity reversals with backazimuth and a change in Moho amplitude that require strong seismic anisotropy (>15 per cent) in a low velocity, high VP/VS, possibly serpentinite or fluid filled schist layer that has a ENE dipping (∼35°) rock fabric. Similar patterns of amplitude variations and polarity reversal observed in RFs for some southern California stations located west of the SAF support the hypothesis that the cause of these data characteristics is a regionally prevalent lower crustal anisotropy. The orientation of this anisotropic fabric is inconsistent with the recent San Andreas sense of shear and is most likely a fossilized fabric of past eastward-directed (Farallon Plate) subduction. © Journal compilation © 2009 RAS.
• Ozacar, A. A., Gilbert, H., & Zandt, G. (2008). Upper mantle discontinuity structure beneath East Anatolian Plateau (Turkey) from receiver functions. Earth and Planetary Science Letters, 269(3-4), 426-434.
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  Abstract: Imaging of seismic velocity discontinuities with receiver functions suggests a complex mantle structure beneath the East Anatolian Plateau. By stacking all the receiver functions as a function of ray parameter, we examined the moveout characteristics of the data set and tested different velocity models to fit the arrival times of converted phases from the Moho and the lithosphere-asthenosphere boundary and their multiples. The best fitting model shows anomalously thin crust (40 km) and lithosphere (65 km), and low uppermost mantle velocities causing systematic delays on later discontinuity arrivals. These findings are consistent with the high plateau supported by partially molten and buoyant asthenosphere. The upper mantle discontinuities are all visible in different frequencies (0.15 and 0.4 Hz). Especially, the 410 km discontinuity does not show frequency dependence in amplitude and suggests a sharpness of 10 km or less. Spatial variations of the 410 and 660 km discontinuities reveal anticorrelated topography and distinct zones of diminished amplitude associated with a detached slab and delaminated fragments of lithospheric mantle. In the south, the slab becomes deeper and horizontally deflected towards the east suggesting westward migration of slab detachment and resistance to slab penetration at the 660 km discontinuity. In the north, the mantle transition zone is characterized by two spatially limited lithospheric fragments with complex geometry. At the center of the study area, the transition zone is thin (230 km) suggesting excess temperature of 120 °C. This warm mantle anomaly most likely is due to hot material trapped within the upper mantle between slab in the south and delaminated fragments in the north. © 2008 Elsevier B.V. All rights reserved.
• Zandt, G., & Humphreys, E. (2008). Toroidal mantle flow through the western U.S. slab window. Geology, 36(4), 295-298.
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  Abstract: The circular pattern of anisotropic fast-axis orientations of split SKS arrivals observed in the western U.S. cannot be attributed reasonably to either preexisting lithospheric fabric or to asthenospheric strain related to global-scale plate motion. A plume origin for this pattern accounts more successfully for the anisotropy field, but little evidence exists for an active plume beneath central Nevada. We suggest that mantle flow around the edge of the sinking Gorda-Juan de Fuca slab is responsible for creating the observed anisotropy. Seismic images and kinematic reconstructions of Gorda-Juan de Fuca plate subduction have the southern edge of this plate extending from the Mendocino triple junction to beneath central Nevada, and flow models of narrow subducted slabs produce a strong toroidal flow field around the edge of the slab, consistent with the observed pattern of anisotropy. This flow may enhance uplift, extension, and magmatism of the northern Basin and Range while inhibiting extension of the southern Basin and Range. © 2008 The Geological Society of America.
• Gilbert, H., Jones, C., Owens, T. J., & Zandt, G. (2007). Imaging Sierra Nevada lithospheric sinking. Eos, 88(21), 225+229.
• Gilbert, H., Velasco, A. A., & Zandt, G. (2007). Preservation of Proterozoic terrane boundaries within the Colorado Plateau and implications for its tectonic evolution. Earth and Planetary Science Letters, 258(1-2), 237-248.
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  Abstract: During the Proterozoic, the North American continent grew by the accretion of arcs onto the Archean craton. Subsequent tectonic events affected much of the lithosphere of western North American resulting in the destruction of many of the features that reflect continental growth. The Colorado Plateau, which is centrally located within the tectonically active western portion of North America, experienced only minor amounts of volcanism during subsequent magmatic events, and has undergone comparatively little internal deformation during the remainder of the Precambrian and Phanerozic, despite the occurrence of some significant tectonic events. Because the lithosphere of the plateau has not been significantly reworked by tectonic events it may have retained structures since accretion that give insight into processes of continental growth. The crust of the southeastern portion of the Colorado Plateau, revealed here through seismic data recorded by the temporary SPE and LA RISTRA arrays, is greater than 40 km thick and possesses distinct changes in intercrustal layering and the character of the Moho. The Moho appears quite dim towards the center of the plateau and much brighter along its southeastern margins. Connecting changes in crustal features that are common between the two arrays indicates a northeastern structural trend that matches the pattern of gravity anomalies in the region. Differences in crustal structures between terranes extend across much of the crust suggesting that entire distinct crustal columns align with terrane boundaries. It is unlikely that extensive crustal flow, as required by some models of plateau evolution, could have traversed across the entire Colorado Plateau without producing features that cross-cut these boundaries. © 2007.
• Sakaguchi, K., Gilbert, H., & Zandt, G. (2006). Converted wave imaging of the Toba Caldera, Indonesia. Geophysical Research Letters, 33(20).
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  Abstract: Receiver functions calculated from data recorded by the 1995 PASSCAL Toba Seismic Experiment in northern Sumatra, Indonesia and the permanent station PSI reveal the presence of a low seismic velocity zone at between 8 and 14 km depth, which is interpreted to be the top of a magma reservoir. The Moho under the Toba caldera lies near 30 km with a localized thickened region that reaches 39 km beneath the eastern edge of the caldera with an abrupt shallowing to the northeast. This thickened crust may be where hot gabbroic material underplates the base of the crust to serve as the heat source for shallower volcanism. Alternatively, the thickened crust and Moho offset may be due to localized crustal thickening when this region was situated in a sharp restraining bend of a now extinct segment of the Sumatran fault. Copyright 2006 by the American Geophysical Union.
• Silva, S. D., Zandt, G., Trumbull, R., Viramonte, J. G., Salas, G., & Jiménez, N. (2006). Large ignimbrite eruptions and volcano-tectonic depressions in the Central Andes: A thermomechanical perspective. Geological Society Special Publication, 269, 47-63.
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  Abstract: The Neogene ignimbrite flare-up of the Altiplano Puna Volcanic Complex (APVC) of the Central Andes produced one of the best-preserved large silicic volcanic fields on Earth. At least 15 000 km3 of magma erupted as regional-scale ignimbrites between 10 and 1 Ma, from large complex calderas that are typical volcano-tectonic depressions (VTD). Simple Valles-type calderas are absent. Integration of field, geochronological, petrological, geochemical and geophysical data from the APVC within the geodynamic context of the Central Andes suggests a scenario where elevated mantle power input, subsequent crustal melting and assimilation, and development of a crustal-scale intrusive complex lead to the development of APVC. These processes lead to thermal softening of the sub-APVC crust and eventual mechanical failure of the roofs above batholith-scale magma chambers to trigger the massive eruptions. The APVC ignimbrite flare-up and the resulting VTDs are thus the result of the time-integrated impact of intrusion on the mechanical strength of the crust, and should be considered tectonomagmatic phenomena, rather than purely volcanic features. This model requires a change in paradigm about how the largest explosive eruptions may operate. © The Geological Society of London.
• Ozacar, A. A., & Zandt, G. (2004). Crustal seismic anisotropy in central Tibet: Implications for deformational style and flow in the crust. Geophysical Research Letters, 31(23), 1-4.
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  Abstract: Receiver functions obtained at INDEPTH III stations located near the Bangong-Nujiang suture in central Tibet display a weak Moho signal and strong P to S conversions within the first 5 s that vary systematically with back-azimuth. A single station with representative azimuthal variations located at the sharp onset of strong SKS splitting, is modeled for both dipping layers and seismic anisotropy by using a global minimization technique. Inversion results indicate strong anisotropy (>10%) near the surface and in the middle crust separated by a south-dipping (∼25°) layer, possibly related to the earlier phase of crustal shortening. Near-surface anisotropy has a fabric dipping steeply southward and trending WNW-ESE that correlates with the suture and younger strike-slip faults. In contrast, midcrustal anisotropy occurs in a low-velocity zone and has a fabric dipping gently (∼18°) northward that might be related to a well-developed near-horizontal rock fabric induced by crustal flow. Copyright 2004 by the American Geophysical Union.
• Sherrington, H. F., Zandt, G., & Frederiksen, A. (2004). Crustal fabric in the Tibetan Plateau based on waveform inversions for seismic anisotropy parameters. Journal of Geophysical Research B: Solid Earth, 109(2), B02312 1-20.
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  Abstract: The Tibetan Plateau has the thickest continental crust on Earth, and fabrics within the crust that are anisotropic to seismic waves may provide clues to how it reached such extreme proportions and how it is currently deforming. Waveform modeling using a global minimization inversion technique applied to receiver functions computed from 11 stations spanning.the north-south length of the eastern plateau has yielded a suite of crustal models that include anisotropy. These models suggest that the Tibetan crust contains 4-14% anisotropy at different depths that is likely a result of both fossil fabrics and more recent deformation. All models contain anisotropy in the surface layer, and for most stations the alignment of the slow symmetry axis suggests a relationship with crustal fabrics asociated with E-W trending thrust faults or sutures. Middle to lower crustal anisotropy is present at most stations with a fast axis trending N-S to NW-SE in the south, nearly E-W in the central plateau, and N-S to NE-SW in the northern plateau. This pattern appears consistent with recent ductile deformation due to both topographically induced flow and to boundary forces from subducting lithosphere at the northern and southern margins of the plateau. The orientations of crustal anisotropy determined for most stations in this study are significantly different from shear wave splitting fast polarization directions, implying distinct deformation in the crust and mantle. Copyright 2004 by the American Geophysical Union.
• Zandt, G., Gilbert, H., Owens, T. J., Ducea, M., Saleeby, J., & Jones, C. H. (2004). Active foundering of a continental arc root beneath the southern Sierra Nevada in California. Nature, 430(7004), 41-46.
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  Abstract: Seismic data provide images of crust-mantle interactions during ongoing removal of the dense batholithic root beneath the southern Sierra Nevada mountains in California. The removal appears to have initiated between 10 and 3 Myr ago with a Rayleigh-Taylor-type instability, but with a pronounced asymmetric flow into a mantle downwelling (drip) beneath the adjacent Great Valley. A nearly horizontal shear zone accommodated the detachment of the ultramafic root from its granitoid batholith. With continuing flow into the mantle drip, viscous drag at the base of the remaining ∼35-km-thick crust has thickened the crust by ∼7 km in a narrow welt beneath the western flank of the range. Adjacent to the welt and at the top of the drip, a V-shaped cone of crust is being dragged down tens of kilometres into the core of the mantle drip, causing the disappearance of the Moho in the seismic images. Viscous coupling between the crust and mantle is therefore apparently driving present-day surface subsidence.
• Ducea, M. N., Kidder, S., & Zandt, G. (2003). Arc composition at mid-crustal depths: Insights from the Coast Ridge Belt, Santa Lucia Mountains, California. Geophysical Research Letters, 30(13), 36-1.
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  Abstract: The Coast Ridge Belt (CRB, Santa Lucia Mts., central California) comprises mid-crustal rocks (750-800°C and 0.8 GPa) of the California magmatic arc. We estimated the bulk composition of the CRB and converted our results to seismic velocities expected at the observed pressures and temperatures. The bulk composition of this arc section changes abruptly at 25 km depth from a granodiorite to a quartz-diorite or diorite. These data are in agreement with results from other Cordilleran batholiths suggesting 1.5 to 2 times thicker felsic columns than interpreted for modern continental arcs, and a relatively sharp transition from a felsic upper crustal batholith, and a mafic deep crust. This implied rheological boundary may have significant implications for intracrustal faulting or convective removal of the roots of batholiths.
• Frederiksen, A. W., Folsom, H., & Zandt, G. (2003). Neighbourhood inversion of teleseismic Ps conversions for anisotrophy and layer dip. Geophysical Journal International, 155(1), 200-212.
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  Abstract: The inversion of teleseismic receiver functions for lithospheric structure is difficult due to the non-linearity of the problem, which is greatly increased in the presence of dipping interfaces and layer anisotropy. Given an efficient ray-theoretical tool for forward-modelling teleseismic seismograms, we perform a directed Monte Carlo search technique using the neighbourhood algorithm of Sambridge, enabling us to search 20-30 parameters in a reasonable amount of computer time. Tests on synthetic data reveal inherent velocity-depth trade-offs in typical data sets, due to the limited moveout present in teleseismic Ps; the azimuth of the anisotropic symmetry axis and the strike of a dipping interface prove to be well-resolved given adequate backazimuthal coverage. We apply this technique to two single-station data sets. The first, from permanent station PGC, Vancouver Island, British Columbia, displays dipping low-velocity sediment layers in the mid-crust. The second, from a station at the northern end of the Tibetan plateau operating in 1991 and 1992, requires a sequence of thick crustal anisotropic layers to explain the observed pattern of receiver-function arrivals.
• Zandt, G. (2003). The southern Sierra Nevada drip and the mantle wind direction beneath the southwestern United States. International Geology Review, 45(3), 213-224.
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  Abstract: The Miocene-Pliocene convective removal of the southern Sierra Nevada batholithic root and its sinking through the upper mantle provides a natural experiment to estimate the direction and velocity of mantle flow beneath the southwestern United States. Xenolith data, volcanism patterns, and geologic evidence are used to constrain the location and timing of the initial detachment. Seismic tomography images of the upper mantle are used to determine the displacement of the downwelling "tail" by the background mantle flow. The "mantle wind" direction based on this analysis is estimated to be SSW, in contradiction to another recent estimate based on a combination of geodetic data and shear-wave splitting measurements. The conflicting conclusions can be reconciled if the splitting measurements are dominated by anisotropy in the shallow mantle imposed by lithospheric motion. The SSW direction is approximately consistent with global models of asthenospheric counterflow directions, and therefore supports the idea that self-driving plate forces can explain the absolute motion of the North American plate.
• Zandt, G., Leidig, M., Chmielowski, J., Baumont, D., & Yuan, X. (2003). Seismic detection and characterization of the Altiplano-Puna magma body, Central Andes. Pure and Applied Geophysics, 160(3-4), 789-807.
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  Abstract: The Altiplano-Puna Volcanic Complex (APVC) in the central Andes is the product of an ignimbrite "flare-up" of world class proportions (DE SILVA, 1989). The region has been the site of large-scale silicic magmatism since 10 Ma, producing 10 major eruptive calderas and edifices, some of which are multiple-eruption resurgent complexes as large as the Yellowstone or Long Valley caldera. Seven PASSCAL broadband seismic stations were operated in the Bolivian portion of the APVC from October 1996 to September 1997 and recorded teleseismic earthquakes and local intermediate-depth events in the subducting Nazca plate. Both teleseismic and local receiver functions were used to delineate the lateral extent of a regionally pervasive ~20-km-deep, very low-velocity layer (VLVL) associated with the APVC. Data from several stations that sample different parts of the northern APVC show large amplitude Ps phases from a low-velocity layer with Vs ≤ 1.0 km/s and a thickness of ~1 km. We believe the crustal VLVL is a regional sill-like magma body, named the Altiplano-Puna magma body (APMB), and is associated with the source region of the Altiplano-Puna Volcanic Complex ignimbrites (CHMIELOWSKI et al., 1999). Large-amplitude P-SH conversions in both the teleseismic and local data appear to originate from the top of the APMB. Using the programs of LEVIN and PARK (1998), we computed synthetic receiver functions for several models of simple layered anisotropic media. Upper-crustal, tilted-axis anisotropy involving both Vp and Vs can generate a "split Ps" phase that, in addition to the Ps phase from the bottom of a thin isotropic VLVL, produces an interference waveform that varies with backzimuth. We have forward modeled such an interference pattern at one station with an anisotropy of 15%-20% that dips 45° within a 20-km-thick upper crust. We develop a hypothesis that the crust above the "magma body" is characterized by a strong, tilted-axis, hexagonally symmetric anisotropy. We speculate that the anisotropy is due to aligned, fluid-filled cracks induced by a "normal-faulting" extensional strain field associated with the high elevations of the Andean Puna.
• Zandt, G. (2002). Earth science: The slippery slope. Nature, 417(6888), 497-498.
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  PMID: 12037549;Abstract: Seismic readings suggest that a zone of weak, slippery rocks lies beneath the Pacific northwest coast of the United States. These frail layers might be limiting the violence of earthquakes.
• Chmielowski, J., Zandt, G., & Haberland, C. (1999). The central Andean Altiplano-Puna magma body. Geophysical Research Letters, 26(6), 783-786.
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  Abstract: Receiver function analysis of 14 teleseismic events recorded by 6 temporary PASSCAL broadband stations within the Altiplano-Puna volcanic complex (APVC) shows a consistent ∼2 s negative-polarity P-to-S conversion for all stations for all available azimuths. Forward modeling of the largest amplitudes suggests that this conversion is produced by the top of a very low velocity zone at a depth of ∼19 km, with a Vs < 0.5 km/s and a thickness of 750-810 m. We interpret the characteristics of the low-velocity zone (low Vs, areal extent, and flatness) to be consistent with a sill-like magma body. On the basis of additional data from the German ANCORP experiment, the Altiplano-Puna magma body appears to underlie much of the APVC, and it may therefore be the largest known active continental crustal magma body.
• Ruppert, S., Fliedner, M. M., & Zandt, G. (1998). Thin crust and active upper mantle beneath the Southern Sierra Nevada in the western United States. Tectonophysics, 286(1-4), 237-252.
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  Abstract: Analysis of seismic-refraction/wide-angle reflection travel-time and seismic amplitude data collected during the 1993 Southern Sierra Nevada Continental Dynamics (SSCD) protect resolves a small crustal root (40-42 km thick) centered 80 km west of the Sierran topographic crest. The SSCD experiment consisted of a west-east profile across the Sierra Nevada at approximately 36.5°N and a north-south profile extending the length of Owens Valley, located eastward of the Sierran topographic crest. Two-dimensional finite-difference travel-time inversion of Pg, and Pn arrivals resolves upper-crustal velocities of 6.0 to 6.4 km s-1 within the Sierran Batholith and Basin and Range, an upper-crustal west-dipping wedge of higher velocities (6.8-7.2 km s-1) consistent with ophiolitic material underlying the Great Valley sedimentary sequence, and higher velocities in the lower crust beneath the Basin and Range (6.8-7.0 km s-1) than those beneath the Sierran Batholith (6.6 km s-1). Low average Pn velocities (7.6-7.9 km s-1) and a laminated transitional Moho imaged beneath the Sierran Batholith also differ from the higher Pn, velocities (7.9-8.0 km s-1) and sharp first-order Moho observed beneath the Basin and Range. The crust decreases in thickness both westward of the root to 28-34 km beneath the Great Valley and eastward to 35 km beneath the highest Sierran topography and decreases further to 27-30 km beneath the Basin and Range. Crustal thickness also appears to increase southeast to northwest from 29-30 km beneath the Garlock Fault in the south to 38-40 km beneath the north end of Chalfant Valley. Juxtaposition of the crustal model with previous P-wave tomography models of the southernmost Sierra Nevada upper-mantle reveal that the thickest Sierran crust on the west-east profile overlies a pronounced upper-mantle high-velocity anomaly (+5%), whereas the region of laminated Moho overlies a flanking upper-mantle low-velocity region (-3%). The upper-mantle velocity anomalies, relatively low Pn, and relatively flat Moho, observed beneath the Sierran crest suggest that the recent uplift of the Sierra Nevada is due to asthenospheric flow and/or lithospheric thinning beneath the southeastern Sierra Nevada and Basin and Range. © 1998 Elsevier Science B.V. All rights reserved.
• Owens, T. J., & Zandt, G. (1997). Implications of crustal property variations for models of Tibetan plateau evolution. Nature, 387(6628), 37-43.
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  Abstract: Shear-coupled teleseismic P waves sampling the interior of the Tibetan plateau provide evidence of systematic variations in crustal structure. The crust thins by up to 20 km from south to north with a concomitant increase in Poisson's ratio from normal values in the south to unusually high values in the north. This suggests that the crust of the northern plateau is partially melted due to high temperatures. These changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained.
• Baker, G. E., Minster, J. B., Zandt, G., & Gurrola, H. (1996). Constraints on crustal structure and complex moho topography beneath Piñon Flat, California, from teleseismic receiver functions. Bulletin of the Seismological Society of America, 86(6), 1830-1844.
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  Abstract: We use teleseismic P waves recorded at Piñon Flat Observatory (PFO) to constrain the three-dimensional crustal and upper-mantle velocity structure beneath the station. By forward modeling radial receiver function waveforms, we construct a one-dimensional crustal model that includes a significant shear-velocity inversion at 9 km in depth. Arrivals on the tangential components indicate dip of at least the uppermost discontinuity. Complicated Moho topography, deepening to the northwest of PFO, is suggested by azimuthal dependence of travel times and amplitudes of the receiver functions and travel times of crustal P-wave reverberations. Although fine details cannot be resolved, each of those sets of observations plus mislocation vectors provide strong indications of abrupt Moho topography, possibly including step offsets of several kilometers. This is not only consistent with gravity data in implying Airy isostasy with compensation at Moho depth but extends that model to a much finer length scale than had been resolved.
• Liu, M., & Zandt, G. (1996). Convective thermal instabilities in the wake of the migrating Mendocino triple junction, California. Geophysical Research Letters, 23(13), 1573-1576.
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  Abstract: Whether the high seismic-velocity anomalies in the upper mantle under California represent cold downwelling mantle flow or fragments of the subducted Farallon plate has important implications for mantle dynamics related to the tectonic history of western North America. We investigate the former possibility by simulating the time-dependent thermal evolution in the asthenosphere beneath California, in the wake of the migrating Mendocino triple junction (MTJ), using a two-dimensional model that represents a transect parallel to the direction of the MTJ migration. For viscosity within the typical range for the asthenosphere beneath active tectonic regions (1×1018 - 2×1019 Pa s), our model predicts vigorous small-scale convection characterized by periodic formation of cold downwelling flows near the southern edge of the migrating Gorda plate. Within the relevant time frame, the predicted convective thermal structures are comparable to the major mantle features shown by seismic imaging. Copyright 1996 by the American Geophysical Union.
• Protti, M., Schwartz, S. Y., & Zandt, G. (1996). Simultaneous inversion for earthquake location and velocity structure beneath central Costa Rica. Bulletin of the Seismological Society of America, 86(1 SUPPL. A), 19-31.
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  Abstract: We have imaged the complex crustal and upper mantle structure beneath central Costa Rica using P-wave arrival times from locally recorded earthquakes. Thurber's (1983) iterative inversion method is used to simultaneously estimate velocities along a three-dimensional grid and hypocentral parameters of local earthquakes. Our data consist of over 12,000 arrival times from more than 1300 earthquakes recorded by stations of a permanent seismographic network in Costa Rica. Our resulting velocity model correlates well with mapped geologic units at very shallow depth and with tectonic features at greater depth. We find low velocities (4.0 to 4.8 km/sec) in the shallow crust (above 10 km) near the active volcanoes and associated with a NW-SE trending late Cretaceous to late Tertiary sedimentary basin southeast of Herradura peninsula. High velocities (5.4 to 5.7 km/sec) in the shallow crust correlate with outcrops of late Jurassic to early Tertiary ultramafic ophiolitic units and with basic Tertiary volcanic units. At depths between 20 and 30 km, high velocities (6.8 to 7.2 km/sec) are associated with the subducting Cocos plate under Costa Rica and two prominent low-velocity bodies (6.3 to 6.5 km/sec) are present about 30 km trenchward of the volcanic arc and along the projection of the aseismic Cocos Ridge as it subducts beneath Costa Rica. The thickened oceanic crust of the Cocos Ridge is most likely responsible for its low velocities. The deep low-velocity anomaly located trenchward of the axis of the volcanoes may indicate the presence of a low-density intrusive resulting from an earlier phase of magmatism, possibly the late Miocene episode that produced the Talamanca intrusive complex.
• Searcy, C. K., Christensen, D. H., & Zandt, G. (1996). Velocity structure beneath College Station Alaska from receiver functions. Bulletin of the Seismological Society of America, 86(1 SUPPL. A), 232-241.
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  Abstract: Time-domain inversions of radial receiver functions and forward modeling of radial and tangential receiver functions were used to determine the velocity structure beneath DWWSSN station COL (Fairbanks, Alaska). Intermediate-period waveforms from 57 events recorded between February 1982 and November 1985 were used in the final analysis. Receiver functions from similar backazimuths and distances are very uniform and were stacked to increase the signal-to-noise ratio. At some azimuths (particularly from the west), significant energy is observed in the tangential component, indicating the existence of heterogeneous structure. The calculated velocity profiles are complicated and contain several distinct features that are consistent between the different stacks. The most obvious features are two strong velocity increases or boundaries in the lower crust. The first of these velocity gradients is between 27 and 32 km, a depth that is often associated with the crust-mantle transition in this region. Forward modeling of the receiver function arrivals associated with this boundary indicates that it has a strike of ∼280° and a dip of at least 10° to the northeast. The velocity profiles and forward modeling also indicate a low-velocity layer between depths of about 15 and 28 km. This layer is consistent with TACT results that show a layer at approximately this depth, with high electrical conductivity and a lower-than-expected seismic velocity, interpreted to be Mesozoic flysch.
• Zandt, G., & Ammon, C. J. (1995). Continental crust composition constrained by measurements of crustal Poisson's ratio. Nature, 374(6518), 152-154.
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  Abstract: DECIPHERING the geological evolution of the Earth's continental crust requires knowledge of its bulk composition and global variability. The main uncertainties are associated with the composition of the lower crust. Seismic measurements probe the elastic properties of the crust at depth, from which composition can be inferred. Of particular note is Poisson's ratio, σ; this elastic parameter can be determined uniquely from the ratio of P- to S-wave seismic velocity, and provides a better diagnostic of crustal composition than either P- or S-wave velocity alone'. Previous attempts to measure σ have been limited by difficulties in obtaining coincident P- and S-wave data sampling the entire crust2. Here we report 76 new estimates of crustal σ spanning all of the continents except Antarctica. We find that, on average, σ increases with the age of the crust. Our results strongly support the presence of a mafic lower crust beneath cratons, and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions, followed by magmatic underplating, or a model in which crust formation processes have changed since the Precambrian era.
• Zandt, G., Myers, S. C., & Wallace, T. C. (1995). Crust and mantle structure across the Basin and Range-Colorado Plateau boundary at 37°N latitude and implications for Cenozoic extensional mechanism. Journal of Geophysical Research, 100(B6), 10,529-10,548.
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  Abstract: The paper presents new evidence on the seismic velocity and density of the crust and upper mantle along a 200-km-long transect across the eastern Basin and Range and western Colorado Plateau at 37°N latitude. The data and available geophysical constraints are most consistent with a lithosphere that thickens from an average thickness of 60 km beneath the Basin and Range to 100 km beneath the western Colorado Plateau. The crustal and lithospheric thinning across the tectonic boundary occurs over a short distance, suggesting it is a geologically young feature produced by a predominantly mechanical response to late Cenozoic extension. The new lithosphere model is consistent with the existence, in early Cenozoic time, of a flat subducted slab at 100 km depth and a relict Sevier-Laramide 50-60 km thick crustal welt, and 60-100% pure shear extension (β values of 1.6-2.0) during the late Cenozoic. -from Authors
• Verdonck, D., & Zandt, G. (1994). Three-dimensional crustal structure of the Mendocino triple junction region from local earthquake travel times. Journal of Geophysical Research, 99(B12), 23,843-23,858.
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  Abstract: The large-scale, three-dimensional geometry of the Mendocino Triple Junction at Cape Mendocino, California, was investigated by inverting nearly 19 000 P wave arrival times from over 1400 local earthquakes to estimate the three-dimensional velocity structure and hypocentral parameters. The evidence is interpreted as tectonic underplating of the Pacific oceanic crust beneath the coastal Franciscan Complex, similar to the situation seismically imaged in central coastal California. This geometry indicates that the westernmost portion of the slab window has been filled by tectonic underplating and crustal thickening, while deep-seated mantle upwelling is confined to an area centered about 75 km north of Clear Lake. -from Authors
• Ammon, C. J., & Zandt, G. (1993). Receiver structure beneath the southern Mojave Block, California. Bulletin - Seismological Society of America, 83(3), 737-755.
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  Abstract: An inversion of a receiver function recorded at LNN Station LAC reveals a range of 1-D velocity structures that are consistent the observed waveform. The shear-velocity structure southeast of LAC is relatively simple, consisting of an approximately 30 to 34km thick crust. All models contain relatively sharp crust-mantle boundaries. The range of models can be roughly divided into two groups that differ mostly in the mid-to-lower crust. In contrast with the simplicity of the southeast receiver function, the receiver functions corresponding to the northwest and southwest show the influence of lateral velocity heterogeneity and can at best be discussed qualitatively. -from Authors
• Mangino, S. G., Zandt, G., & Ammon, C. J. (1993). The receiver structure beneath Mina, Nevada. Bulletin - Seismological Society of America, 83(2), 542-560.
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  Abstract: Receiver functions obtained from broadband teleseismic P waveforms recorded at seismic station MNV, near Mina, Nevada, are inverted for the vertical shear-wave velocity structure southeast of the station. The time-domain inversion procedure for vertically heterogeneous and laterally homogeneous one-dimensional models includes smoothness constraints to minimize model roughness and a pseudo-Monte-Carlo approach to investigate the nonuniqueness of the solution models. The preferred southeast solution models show a smooth positive gradient in the mid- and lower crust with the top of the crust-mantle boundary at 34- to 36-km depth, reaching an upper-mantle P velocity of 7.8 to 7.9 km sec-1 between 38 and 40 km. These results suggest that the crust beneath MNV may be representative of a transition zone between typical Basin and Range province crust to the east and a thicker Sierran block to the west of the station. -from Authors
• Zandt, G., & Carrigan, C. R. (1993). Small-scale convective instability and upper mantle viscosity under California. Science, 261(5120), 460-463.
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  PMID: 17770025;Abstract: Thermal calculations and convection analysis, constrained by seismic tomography results, suggest that a small-scale convective instability developed in the upper 200 kilometers of the mantle under California after the upwelling and cooling of asthenosphere into the slab window associated with the formation of the San Andreas transform boundary. The upper bound for the upper mantle viscosity in the slab window, 5 × 1019 pascal seconds, is similar to independent estimates for the asthenosphere beneath young oceanic and tectonically active continental regions. These model calculations suggest that many tectonically active continental regions characterized by low upper mantle seismic velocities may be affected by time-dependent small-scale convection that can generate localized areas of uplift and subsidence.
• Benz, H. M., Zandt, G., & Oppenheimer, D. H. (1992). Lithospheric structure of northern California from teleseismic images of the upper mantle. Journal of Geophysical Research, 97(B4), 4791-4807.
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  Abstract: Teleseismic P wave trave time residuals from 120 earthquakes recorded across the U.S. Geological Survey California seismic network were used to determine the lithosphere P wave velocity structure beneath northern California. Inversion results for the crust show strong correlations to volcanic features. Results from this study provide improved constraints on Gorda plate subduction, evolution of the San Andreas fault system, and development of the lithosphere beneath western North America. -from Authors
• Goldstein, P., Walter, W. R., & Zandt, G. (1992). Upper mantle structure beneath central Eurasia using a source array of nuclear explosions and waveforms at regional distances. Journal of Geophysical Research, 97(B10), 14,097-14,113.
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  Abstract: Consistent broadband, regional, three-component P waveforms from a set of 11 explosions that occurred at the former Soviet test site in Kazakhstan during 1988 and 1989 have been observed and most of the prominent features in these waveforms are modelled to determine upper mantle structure beneath central Eurasia. It is argued that most of the prominent features in these waveforms can be explained by reflections at or refractions near discontinuities or large velocity gradients in the upper mantle. A model with discontinuities of approximately 3.0% and 6.5% near 200 km and 400 km, respectively, produces a better fit to the broadband data at ARU and GAR than previous models for this region. This model also produces a good match to waveforms recorded at OBN and NORESS, however, this model is not unique. Comparison of our results with other upper mantle studies shows that the central Eurasian upper mantle is similar to the upper mantle of the central and eastern United States. -from Authors
• Patton, H. J., & Zandt, G. (1991). Seismic moment tensors of western US earthquakes and implications for the tectonic stress field. Journal of Geophysical Research, 96(B11), 18,245-18,259.
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  Abstract: Using a linear inversion method applied to regional surface wave data, we have determined seismic moment tensors for 50 earthquakes in the western US. The direction of maximum horizontal compressive stress is mainly N-S in northern California and western Oregon. Our results, along with the results of Zoback et al. (1981) for central California, indicate a 60° rotation of the maximum horizontal compressive stress between northern and central segments of the San Andreas fault system. A change in stress state along the northern segment could be related to the thermal and mechanical evolution of the Pacific-North American plate boundary. -from Authors
• Ammon, C. J., Randall, G. E., & Zandt, G. (1990). On the nonuniqueness of receiver function inversions. Journal of Geophysical Research, 95(B10), 15,303-15,318.
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  Abstract: To study the resolving power of teleseismic P waveforms for receiver structure, we model synthetic waveforms using a time domain waveform inversion scheme beginning with a range of initial models to estimate the range of acceptable velocity structures. We present the results of more than 235 waveform inversions for one-dimensional velocity structures that indicate that the primary sensitivity of a receiver function is to high wavenumber velocity changes, and a depth-velocity product, not simply velocity. The range of slownesses in a typical receiver function study does not appear to be broad enough to remove the depth-velocity ambiguity; the inclusion of a priori information is necessary. -from Authors
• Furlong, K. P., Hugo, W. D., & Zandt, G. (1989). Geometry and evolution of the San Andreas fault zone in northern California. Journal of Geophysical Research, 94(B3), 3100-3110.
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  Abstract: The evolution of the San Andreas fault system is controlled by thermal-mechanical processes associated with the development and evolution of a narrow "slabless window' formed beneath the western edge of North America. This fault zone evolution begins after initiation of transform motion along the plate boundary with the northward migration of the Mendocino triple junction. As a consequence of initial lithospheric structure and the shallow emplacement of asthenospheric mantle, the plate boundary separating the North American and Pacific plates follows a complex three-dimensional geometry which varies through time. Seismic velocity structure, heat flow, seismicity, surface deformation, uplift, and fault development are controlled by the evolving thermal structure in the region after triple junction passage. Thermal-mechanical models have been used to evaluate the fault system's time-varying three-dimensional dynamical behavior, simulating the principal processes involved in the thermal-mechanical evolution of the San Andreas fault system. -from Authors
• Priestley, K. F., Zandt, G., & Randall, G. E. (1988). Crustal structure in eastern Kazakh, USSR from teleseismic receiver functions.. Geophysical Research Letters, 15(6), 613-616.
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  Abstract: Broadband receiver functions determined from teleseismic P-waveforms recorded at 2 seismic stations in E Kazakh, USSR, are inverted for the vertical velocity structure beneath the stations. The detailed broadband receiver functions are obtained by stacking source-equalized radial components of teleseismic P-waveforms. A time-domain inversion of the radial receiver function is used to determine the structure assuming a crustal model parameterized by flat-lying, homogeneous layers. The results indicate that the crust in this region is relatively uniform, however, the nature and depth of the Moho changes significantly across the region.-from Authors
• Zandt, G., & Owens, T. J. (1986). Comparison of crustal velocity profiles determined by seismic refraction and teleseismic methods. Tectonophysics, 128(1-2), 155-161.
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  Abstract: Recently, two diverse seismic techniques were applied independently to the study of the crustal structure of the Cumberland Plateau, eastern Tennessee. One involved a reinterpretation of a refraction experiment performed in 1965 by the U.S. Geological Survey, consisting of two 400 km long, reversed refraction lines. The other entailed the inversion of broadband teleseismic P waveforms recorded at a single three-component broadband station, RSCP, located at the intersection of the two refraction profiles. A comparison of the two sets of velocity profiles revealed many similarities and some significant differences. Both sets of velocity models consist of three major crustal layers: (1) an upper crust (Vp = 6.1-6.4 km/s) down to about 17 km, (2) a mid-crust (Vp = 6.7-6.9 km/s) between 17 and 40 km depth, (3) a lower crust (Vp = 7.2-7.4 km/s) from 40 to 51 km depth. The refraction models have linear transition zones up to 11 km thick at the base of each layer, whereas the teleseismic models have more irregular transition zones at the base of the mid- and lower crust. The differences in the results of these studies are attributed to the differing frequency bandwidths of the data sets; the predominant sensitivity of the teleseismic data to shear velocities, compared to compressional velocities for the refraction data; and the different analysis procedures involved in each method. Nevertheless, the similarities indicate that the teleseismic waveform method with broadband data is capable of retreiving comparable crustal information as the Cumberland Plateau refraction survey. In addition, it provides the kind of complementary information required to constrain the composition of the continental lower crust and uppermost mantle. © 1986.
• Owens, T. J., & Zandt, G. (1985). The response of the continental crust- mantle boundary observed on broadband teleseismic receiver functions ( N America).. Geophysical Research Letters, 12(10), 705-708.
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  Abstract: Examines receiver functions from 3 stations (RSCP, RSNY, and RSON in E N America) to illustrate the variability in reponse of the continental 'Moho' and to demonstrate the utility of broadband data in studying these variations. Of particular interest is the clear evidence for frequency-dependence of the amplitudes of converted phases arriving from the crust-mantle boundary. The shear velocity structures for these stations obtained by inverting the radial component of the receiver functions in the time domain indicate that this frequency-dependence is a measure of the 'sharpness' of the crust-mantle boundary region. -after Authors
• Zandt, G., & Randall, G. E. (1985). Observations of shear-coupled P waves.. Geophysical Research Letters, 12(9), 565-568.
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  Abstract: We observed several shear-coupled P waves on seismograms of deep earthquakes recorded at broadband seismic stations (RSTN) in N America. For these paths, large amplitudes relative to SV characterize the lithospheric multiple near 50o, with amplitudes decreasing rapidly beyond 6o. Calculation of synthetic seismograms indicate that a post-critical reflection from a discontinuity at 70-80 km depth where compressional velocity increases abruptly to about 8.5 km/s produce the largest amplitude P arrival. The observations of these shear-coupled P waves presents a previously untapped source of information about the fine structure of the upper mantle.-from Authors
• Owens, T. J., Zandt, G., & Taylor, S. R. (1984). SEISMIC EVIDENCE FOR AN ANCIENT RIFT BENEATH THE CUMBERLAND PLATEAU, TENNESSEE: A DETAILED ANALYSIS OF BROADBAND TELESEISMIC P WAVEFORMS.. Journal of Geophysical Research, 89(B9), 7783-7795.
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  Abstract: Broadband receiver functions developed from teleseismic P waveforms are inverted for vertical velocity structure beneath the Cumberland Plateau, Tennessee. The detailed broadband receiver functions are obtained by stacking source-equalized horizontal components of teleseismic P waveforms. The resulting receiver functions are most sensitive to the shear velocity structure near the station. A time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin, flat-lying, homogeneous layers. Lateral changes in structure are identified by examining azimuthal variations in the vertical structure. The results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the East Continent Gravity High located northeast of the Cumberland Plateau.
• Zandt, G., & Furlong, K. P. (1982). Evolution and thickness of the lithosphere beneath coastal California.. Geology, 10(7), 376-381.
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  Abstract: Many of the tectonic features of the California Coast Ranges are directly related to the N migration of the Mendocino triple junction along the W edge of N America during the past 20-30Ma. At the triple junction, the W edge of the N American plate slides off the (relative) N moving and subducting Gorda plate, leaving the thin W edge in direct contact with asthenosphere upwelling to fill the space vacated by the underlying Gorda plate. 2-D time-dependent thermal modeling of this process, constrained by teleseismic delay studies is used to construct a map of lithospheric thickness of coastal California. Among the implications of this map are that 1) the high heat flow in the Coast Ranges can be almost entirely accounted for by the asthenospheric upwelling associated with the migrating Mendocino triple junction; 2) the general elevation and the late Cenozoic volcanism in the California Coast Ranges are responses to a zone of unusually thin (20-45km) lithosphere that extends S behind the Mendocino triple junction; and 3) the course of some segments of the San Andreas fault (in both central and S California) appear to deviate from a deeper, more fundamental, transform boundary separating the Pacific and N American plates.-Authors
• Zandt, G., McPherson, L., Schaff, S., & Olsen, S. (1982). Seismic baseline and induction studies: Roosevelt Hot Springs, Utah and Raft River, Idaho ( USA).. Array.
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  Abstract: Local seismic networks were established to monitor the background seismicity prior to initiation of geothermal power production. Results of the studies are described. It is concluded that the occurrence of natural earthquake swarms indicates a potential for induced seismicity at Roosevelt Hot Springs after major production operations are initiated. -from STAR, 21(17), 1983
• Hawley, B. W., Zandt, G., & Smith, R. B. (1981). Simultaneous inversion for hypocenters and lateral velocity variations: an iterative solution with a layered model.. Journal of Geophysical Research, 86(B8), 7073-7086.
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  Abstract: An iterative inversion technique has been developed that uses the direct P and S wave arrival times from local earthquakes to compute simultaneously a three-dimensional velocity structure and relocated hypocenters. Crustal structure is modeled by subdividing flat layers into rectangular blocks. An interpolation function is used to smoothly vary velocities between blocks, allowing ray trace calculations of travel times in a three-dimensional medium. Tests using synthetic data from known models show that solutions are reasonably independent of block size and spatial distribution but are sensitive to the choice of layer thicknesses. -Authors
• Iyer, H. M., Evans, J. R., Zandt, G., Stewart, R. M., Coakley, J. M., & Roloff, J. N. (1981). A deep low-velocity body under the Yellowstone caldera, Wyoming: Delineation using teleseismic P-wave residuals and tectonic interpretation. Bulletin of the Geological Society of America, 92(11 PART2), 1471-1646.
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  Abstract: In previously published papers (Iyer, 1975; Iyer and Stewart, 1977; Iyer, 1979), evidence was presented for the presence of a large body of anomalous material, with compressional velocities considerably lower than normal from the surrounding rock, in the crust and upper mantle under Yellowstone National Park, Wyoming. In this paper, we give a detailed analysis of P-residual data from the 26-element telemetered seismic array and three groups of portable stations operated by the U.S. Geological Survey in the Yellowstone region. Using a variety of analytical techniques, we delineate the shape of the low-velocity body, determine the velocity structure inside the body, and interpret the nature of the material that constitutes the body. We will also outline some tectonic implications of our model. The spectacular surface geothermal phenomena for which Yellowstone is famous result from deep-seated volcanism. The volcanic evolution of Yellowstone is discussed by Christiansen and Blank (1972) and summarized by Eaton and others (1975). Briefly, the geothermal manifestations in Yellowstone are results of three cycles of Quaternary volcanism, each culminating in a devastating pyroclastic eruption and resulting in large collapse calderas and predominantly rhyolitic flows. The oldest cycle ended about 1.9 m.y. ago, forming a caldera from Island Park to, perhaps, the central part of the present Yellowstone caldera. The second cycle climaxed about 1.2 m.y. ago and was confined to the Island Park caldera. The third cycle began soon after the second and is responsible for the 75-by 45-km Yellowstone caldera as it is outlined today. This caldera is the result of a catastrophic eruption approximately 600,000 yr ago. The most recent episode of volcanism probably began about 150,000 yr ago in the western half of the caldera near Old Faithful geyser, and the youngest flows in Yellowstone (about 70,000 yr old) are probably associated with this episode. © 1981 The Geological Society of America, Inc.
• Zandt, G. (1981). Seismic images of the deep structure of the San Andreas fault system, Central Coast Ranges, California.. Journal of Geophysical Research, 86(B6), 5039-5052.
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  Abstract: Three-dimensional inversion of teleseismic P wave travel time residuals recorded at the US Geological Survey central California array has resolved small-scale (?? tens of kilometers) crustal and upper mantle heterogeneity down to depths of 90 km beneath the California coast ranges. Upper crustal lateral velocity variations of +-8% correlate closely with surface geology. Lower-than-average velocities are associated with thick Tertiary sedimentary fill and higher-than-average velocities with basement exposures. Lower crustal velocity heterogeneity of +-4% appear to reflect crustal thickness variations. A thinner crust is indicated southwest of the San Andreas fault and northwest of San Pablo Bay. A linear zone of low-velocities (0 to 4%) subparallel to the San Andreas fault was resolved in the upper mantle. The preferred interpretation is that the low-velocities indicate a narrow upwarp of asthenosphere to unusually shallow depths (??45 km) beneath the coast ranges. Such an unusual mantle structure may have been produced by the northwestward migration along the California coast of a transiently unstable Mendocino triple junction. The inversion results also indicate the possibility of partial decoupling of the crust from the upper mantle. -Author