Browsing by Autor "Hernando Tavera"

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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.
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.
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