Browsing by Autor "G. Zandt"

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Ambient noise tomography across the Central Andes
(Oxford University Press, 2013) Kevin M. Ward; Ryan Porter; G. Zandt; S. L. Beck; L. S. Wagner; E. Minaya; Hernando Tavera
The Central Andes of southern Peru, Bolivia, Argentina and Chile (between 12°S and 42°S) comprise the largest orogenic plateau in the world associated with abundant arc volcanism, the Central Andean Plateau, as well as multiple segments of flat-slab subduction making this part of the Earth a unique place to study various aspects of active plate tectonics. The goal of this continental-scale ambient noise tomography study is to incorporate broad-band seismic data from 20 seismic networks deployed incrementally in the Central Andes from 1994 May to 2012 August, to image the vertically polarized shear wave velocity (<it>V</it><inf>sv</inf>) structure of the South American Cordillera. Using dispersion measurements calculated from the cross-correlation of 330 broad-band seismic stations, we construct Rayleigh wave phase velocity maps in the period range of 8–40 s and invert these for the shear wave velocity (<it>V</it><inf>sv</inf>) structure of the Andean crust. We provide a dispersion misfit map as well as uncertainty envelopes for our <it>V</it><inf>sv</inf> model and observe striking first-order correlations with our shallow results (∼5 km) and the morphotectonic provinces as well as subtler geological features indicating our results are robust. Our results reveal for the first time the full extent of the mid-crustal Andean low-velocity zone that we tentatively interpret as the signature of a very large volume Neogene batholith. This study demonstrates the efficacy of integrating seismic data from numerous regional broad-band seismic networks to approximate the high-resolution coverage previously only available though larger networks such as the EarthScope USArray Transportable Array in the United States.
Anomalous crust of the Bolivian Altiplano, central Andes: Constraints from broadband regional seismic waveforms
(American Geophysical Union, 1996) G. Zandt; S. L. Beck; S. R. Ruppert; Charles J. Ammon; Don Rock; E. Minaya; Terry C. Wallace; Paul G. Silver
A one‐year deployment of broadband seismographs in the Bolivian Altiplano recorded numerous intermediate‐depth earthquakes at near‐regional distances. We modeled the associated broadband waveforms of two earthquakes to estimate an average crustal structure for the Altiplano. The resulting model is characterized by an anomalously low mean P velocity of 6.0 km/s, a low Poisson's ratio of 0.25, and a crustal thickness of 65 km. The combination of the low mean velocity and low Poisson's ratio can be explained only by a predominantly quartz‐rich, felsic bulk composition. This constraint precludes significant volumes of magmatic addition from the mantle contributing to the great thickness of the Altiplano crust, but is consistent with thickening by compressive shortening concentrated in a weak felsic layer.
Central Andean crustal structure from receiver function analysis
(Elsevier BV, 2016) Jamie Ryan; S. L. Beck; G. Zandt; L. S. Wagner; E. Minaya; Hernado Tavera
Crustal-thickness variations in the central Andes
(Geological Society of America, 1996) S. L. Beck; G. Zandt; Stephen C. Myers; Terry C. Wallace; Paul G. Silver; Lawrence A. Drake
Research Article| May 01, 1996 Crustal-thickness variations in the central Andes Susan L. Beck; Susan L. Beck 1SASO and Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar George Zandt; George Zandt 2IGPP, Lawrence Livermore National Laboratory, Livermore, California, and SASO and Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Stephen C. Myers; Stephen C. Myers 1SASO and Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Terry C. Wallace; Terry C. Wallace 1SASO and Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Paul G. Silver; Paul G. Silver 3Carnegie Institution of Washington, Washington, D.C. Search for other works by this author on: GSW Google Scholar Lawrence Drake Lawrence Drake 4Observatorio San Calixto, La Paz, Bolivia Search for other works by this author on: GSW Google Scholar Author and Article Information Susan L. Beck 1SASO and Department of Geosciences, University of Arizona, Tucson, Arizona 85721 George Zandt 2IGPP, Lawrence Livermore National Laboratory, Livermore, California, and SASO and Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Stephen C. Myers 1SASO and Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Terry C. Wallace 1SASO and Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Paul G. Silver 3Carnegie Institution of Washington, Washington, D.C. Lawrence Drake 4Observatorio San Calixto, La Paz, Bolivia Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1996) 24 (5): 407–410. https://doi.org/10.1130/0091-7613(1996)024<0407:CTVITC>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Susan L. Beck, George Zandt, Stephen C. Myers, Terry C. Wallace, Paul G. Silver, Lawrence Drake; Crustal-thickness variations in the central Andes. Geology 1996;; 24 (5): 407–410. doi: https://doi.org/10.1130/0091-7613(1996)024<0407:CTVITC>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We estimated the crustal thickness along an east-west transect across the Andes at lat 20°S and along a north-south transect along the eastern edge of the Altiplano from data recorded on two arrays of portable broadband seismic stations (BANJO and SEDA). Waveforms of deep regional events in the downgoing Nazca slab and teleseismic earthquakes were processed to isolate the P-to-S converted phases from the Moho in order to compute the crustal thickness. We found crustal-thickness variations of nearly 40 km across the Andes. Maximum crustal thicknesses of 70–74 km under the Western Cordillera and the Eastern Cordillera thin to 32–38 km 200 km east of the Andes in the Chaco Plain. The central Altiplano at 20°S has crustal thicknesses of 60 to 65 km. The crust also appears to thicken from north (16°S, 55–60 km) to south (20°S, 70–74 km) along the Eastern Cordillera. The Subandean zone crust has intermediate thicknesses of 43 to 47 km. Crustal-thickness predictions for the Andes based on Airy-type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation. In contrast, at the boundary between the Eastern Cordillera and the Subandean zone and in the Chaco Plain, the crust is thinner than predicted, suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere. With additional constraints, we conclude that the observation of Airy-type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting Nazca plate and the cratonic lithosphere of the Brazilian craton. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Erratum: Ambient noise tomography across the Central Andes
(Oxford University Press, 2013) Kevin M. Ward; Ryan Porter; G. Zandt; S. L. Beck; L. S. Wagner; E. Minaya; Hernando Tavera
Kevin M. Ward,1 Ryan C. Porter,2 George Zandt,1 Susan L. Beck,1 Lara S. Wagner,3 Estela Minaya4 and Hernando Tavera5 1Department of Geosciences, The University of Arizona, 1040 E. 4th Street Tucson, AZ 85721, USA. E-mail: wardk@email.arizona.edu 2Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015-1305, USA 3Department of Geological Sciences, University of North Carolina at Chapel Hill, 104 South Rd., Mitchell Hall, CB #3315, Chapel Hill, NC 27599-3315, USA 4El Observatorio San Calixto, Calle Indaburo 944, Casilla 12656, La Paz, Bolivia 5Instituto Geofisico Del Peru, Calle Badajo No. 169, Urb. Mayorazgo IV Etapa, Lima, Peru
Imaging the transition from flat to normal subduction: variations in the structure of the Nazca slab and upper mantle under southern Peru and northwestern Bolivia
(Oxford University Press, 2015) A. C. Scire; G. Zandt; S. L. Beck; Maureen D. Long; L. S. Wagner; E. Minaya; Hernando Tavera
Two arrays of broad-band seismic stations were deployed in the north central Andes between 8° and 21°S, the CAUGHT array over the normally subducting slab in northwestern Bolivia and southern Peru, and the PULSE array over the southern part of the Peruvian flat slab where the Nazca Ridge is subducting under South America. We apply finite frequency teleseismic P- and S-wave tomography to data from these arrays to investigate the subducting Nazca plate and the surrounding mantle in this region where the subduction angle changes from flat north of 14°S to normally dipping in the south. We present new constraints on the location and geometry of the Nazca slab under southern Peru and northwestern Bolivia from 95 to 660 km depth. Our tomographic images show that the Peruvian flat slab extends further inland than previously proposed along the projection of the Nazca Ridge. Once the slab re-steepens inboard of the flat slab region, the Nazca slab dips very steeply (∼70°) from about 150 km depth to 410 km depth. Below this the slab thickens and deforms in the mantle transition zone. We tentatively propose a ridge-parallel slab tear along the north edge of the Nazca Ridge between 130 and 350 km depth based on the offset between the slab anomaly north of the ridge and the location of the re-steepened Nazca slab inboard of the flat slab region, although additional work is needed to confirm the existence of this feature. The subslab mantle directly below the inboard projection of the Nazca Ridge is characterized by a prominent low-velocity anomaly. South of the Peruvian flat slab, fast anomalies are imaged in an area confined to the Eastern Cordillera and bounded to the east by well-resolved low-velocity anomalies. These low-velocity anomalies at depths greater than 100 km suggest that thick mantle lithosphere associated with underthrusting of cratonic crust from the east is not present. In northwestern Bolivia a vertically elongated fast anomaly under the Subandean Zone is interpreted as a block of delaminating lithosphere.
Implications of spatial and temporal development of the aftershock sequence for the <i>M<sub>w</sub></i> 8.3 June 9, 1994 Deep Bolivian Earthquake
(American Geophysical Union, 1995) Stephen C. Myers; Terry C. Wallace; S. L. Beck; Paul G. Silver; G. Zandt; J. C. VanDecar; E. Minaya
On June 9, 1994 the M w 8.3 Bolivia earthquake (636 km depth) occurred in a region which had not experienced significant, deep seismicity for at least 30 years. The mainshock and aftershocks were recorded in Bolivia on the BANJO and SEDA broadband seismic arrays and on the San Calixto Network. We used the joint hypocenter determination method to determine the relative location of the aftershocks. We have identified no foreshocks and 89 aftershocks ( m > 2.2) for the 20‐day period following the mainshock. The frequency of aftershock occurrence decreased rapidly, with only one or two aftershocks per day occuring after day two. The temporal decay of aftershock activity is similar to shallow aftershock sequences, but the number of aftershocks is two orders of magnitude less. Additionally, a m b ∼6, apparently triggered earthquake occurred just 10 minutes after the mainshock about 330 km east‐southeast of the mainshock at a depth of 671 km. The aftershock sequence occurred north and east of the mainshock and extends to a depth of 665 km. The aftershocks define a slab striking N68°W and dipping 45°NE. The strike, dip, and location of the aftershock zone are consistent with this seismicity being confined within the downward extension of the subducted Nazca plate. The location and orientation of the aftershock sequence indicate that the subducted Nazca plate bends between the NNW striking zone of deep seismicity in western Brazil and the N‐S striking zone of seismicity in central Bolivia. A tear in the deep slab is not necessitated by the data. A subset of the aftershock hypocenters cluster along a subhorizontal plane near the depth of the mainshock, favoring a horizontal fault plane. The horizontal dimensions of the mainshock [ Beck et al., this issue; Silver et al., 1995] and slab defined by the aftershocks are approximately equal, indicating that the mainshock ruptured through the slab.
Large ignimbrite eruptions and volcano-tectonic depressions in the Central Andes: a thermomechanical perspective
(Geological Society of London, 2006) Shanaka L. de Silva; G. Zandt; Robert B. Trumbull; J. Viramonte; G. Salas; Néstor Jiménez
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 km 3 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.
Lowermost mantle anisotropy near the eastern edge of the Pacific LLSVP: constraints from SKS–SKKS splitting intensity measurements
(Oxford University Press, 2017) Jie Deng; Maureen D. Long; Neala Creasy; L. S. Wagner; S. L. Beck; G. Zandt; Hernando Tavera; E. Minaya
Seismic anisotropy has been documented in many portions of the lowermost mantle, with particularly strong anisotropy thought to be present along the edges of large low shear velocity provinces (LLSVPs). The region surrounding the Pacific LLSVP, however, has not yet been studied extensively in terms of its anisotropic structure. In this study, we use seismic data from southern Peru, northern Bolivia and Easter Island to probe lowermost mantle anisotropy beneath the eastern Pacific Ocean, mostly relying on data from the Peru Lithosphere and Slab Experiment and Central Andean Uplift and Geodynamics of High Topography experiments. Differential shear wave splitting measurements from phases that have similar ray paths in the upper mantle but different ray paths in the lowermost mantle, such as SKS and SKKS, are used to constrain anisotropy in D″. We measured splitting for 215 same station-event SKS–SKKS pairs that sample the eastern Pacific LLSVP at the base of the mantle. We used measurements of splitting intensity(SI), a measure of the amount of energy on the transverse component, to objectively and quantitatively analyse any discrepancies between SKS and SKKS phases. While the overall splitting signal is dominated by the upper-mantle anisotropy, a minority of SKS–SKKS pairs (∼10 per cent) exhibit strongly discrepant splitting between the phases (i.e. the waveforms require a difference in SI of at least 0.4), indicating a likely contribution from lowermost mantle anisotropy. In order to enhance lower mantle signals, we also stacked waveforms within individual subregions and applied a waveform differencing technique to isolate the signal from the lowermost mantle. Our stacking procedure yields evidence for substantial splitting due to lowermost mantle anisotropy only for a specific region that likely straddles the edge of Pacific LLSVP. Our observations are consistent with the localization of deformation and anisotropy near the eastern boundary of the Pacific LLSVP, similar to previous observations for the African LLSVP.
Overriding plate, mantle wedge, slab, and subslab contributions to seismic anisotropy beneath the northern Central Andean Plateau
(Wiley, 2016) Maureen D. Long; C. B. Biryol; C. M. Eakin; S. L. Beck; L. S. Wagner; G. Zandt; Estella Minaya; Hernando Tavera
Abstract The Central Andean Plateau, the second‐highest plateau on Earth, overlies the subduction of the Nazca Plate beneath the central portion of South America. The origin of the high topography remains poorly understood, and this puzzle is intimately tied to unanswered questions about processes in the upper mantle, including possible removal of the overriding plate lithosphere and interaction with the flow field that results from the driving forces associated with subduction. Observations of seismic anisotropy can provide important constraints on mantle flow geometry in subduction systems. The interpretation of seismic anisotropy measurements in subduction settings can be challenging, however, because different parts of the subduction system may contribute, including the overriding plate, the mantle wedge above the slab, the slab itself, and the deep upper mantle beneath the slab. Here we present measurements of shear wave splitting for core phases ( SKS, SKKS, PKS , and sSKS ), local S , and source‐side teleseismic S phases that sample the upper mantle beneath southern Peru and northern Bolivia, relying mostly on data from the CAUGHT experiment. We find evidence for seismic anisotropy within most portions of the subduction system, although the overriding plate itself likely makes only a small contribution to the observed delay times. Average fast orientations generally trend roughly trench‐parallel to trench‐oblique, contradicting predictions from the simplest two‐dimensional flow models and olivine fabric scenarios. Our measurements suggest complex, layered anisotropy beneath the northern portion of the Central Andean Plateau, with significant departures from a two‐dimensional mantle flow regime.
Seismicity and state of stress in the central and southern Peruvian flat slab
(Elsevier BV, 2016) Abhash Kumar; L. S. Wagner; S. L. Beck; Maureen D. Long; G. Zandt; Bissett Young; Hernando Tavera; Estella Minaya
Synthesis: PLUTONS: Investigating the relationship between pluton growth and volcanism in the Central Andes
(Geological Society of America, 2018) M. E. Pritchard; Shanaka L. de Silva; Gary Michelfelder; G. Zandt; Stephen R. McNutt; Joachim Gottsmann; M. E. West; Jon Blundy; D. H. Christensen; N. J. Finnegan
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