Mantle flow through a tear in the Nazca slab inferred from shear wave splitting

Lynner, Colton; Anderson, M. L.; Portner, D. E.; Beck, S. L.; Gilbert, H. J.

doi:10.1002/2017gl074312

Cited by 25 publications

(27 citation statements)

References 47 publications

(99 reference statements)

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“…Our hypothesized tear is consistent with previous work that identified a slab hole downdip of our proposed tear (Lynner et al, ; Portner et al, ). The slab hole can be explained by two end‐member hypotheses.…”

Section: Discussionsupporting

confidence: 92%

S‐Wave Receiver Function Analysis of the Pampean Flat‐Slab Region: Evidence for a Torn Slab

Haddon

Porter

2018

Geochem Geophys Geosyst

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Flat‐slab subduction is an atypical form of subduction where the downgoing plate assumes a low‐angle or subhorizontal geometry as it descends beneath the overriding plate. These systems have profound impacts on upper‐plate deformation and volcanism, yet there are outstanding questions regarding the causes, initiation, and termination of flat‐slab subduction. The Pampean flat‐slab region, located in the South American Cordillera, is an ideal locale to study the evolution of these systems due to its well‐constrained geologic history and the continuity of subduction along the western margin of the continent. In this work, we utilize S wave receiver functions (SRFs) to measure the thicknesses and geometry of the subducting Nazca plate lithosphere and the overriding South American plate within the region. Results from this study show overthickened Nazca Plate crust (~15–20 km thick) in the flat‐slab region. Prior to the slab steepening, SRFs indicate a velocity increase rather than a decrease at the top of slab crust, which is consistent with a transition from basalt to eclogite within the crust as it dehydrates. Margin parallel cross sections are consistent with differing transitions between flat‐subduction and more‐typical subduction occurring to the north and south. In the north, the transition appears to be gradual, consistent with slab bending. To the south, SRFs indicate an abrupt change, consistent with slab tearing. These differences are consistent with significant variations in slab rheology along the margin, which may reflect differing thermal regimes. This work has global implications for the three‐dimensional effects of flat‐slab subduction.

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Section: Discussionsupporting

confidence: 92%

S‐Wave Receiver Function Analysis of the Pampean Flat‐Slab Region: Evidence for a Torn Slab

Haddon

Porter

2018

Geochem Geophys Geosyst

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“…In Cascadia, pervasive entrainment of the subslab mantle is thought to be aided by the fragmented nature of the Juan de Fuca slab in the upper mantle and transition zone. This relationship can likely be further tested by combining mantle flow indicators with high‐resolution images of tears, gaps, and holes in subducting slabs worldwide (e.g., Lynner et al, ). The outcome of this work will have implications for the degree of coupling between slab fragments and the surrounding mantle flow field and, in general, for subducting slabs as drivers of the global mantle convection system.…”

Section: Resultsmentioning

confidence: 99%

SKS Splitting Beneath Mount St. Helens: Constraints on Subslab Mantle Entrainment

Eakin

Wirth

Wallace

et al. 2019

Geochem Geophys Geosyst

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Observations of seismic anisotropy can provide direct constraints on the character of mantle flow in subduction zones, critical for our broader understanding of subduction dynamics. Here we present over 750 new SKS splitting measurements in the vicinity of Mount St. Helens in the Cascadia subduction zone using a combination of stations from the iMUSH broadband array and Cascades Volcano Observatory network. This provides the highest density of splitting measurements yet available in Cascadia, acting as a focused “telescope” for seismic anisotropy in the subduction zone. We retrieve spatially consistent splitting parameters (mean fast direction Φ: 74°, mean delay time ∂t: 1.0 s) with the azimuthal occurrence of nulls in agreement with the fast direction of splitting. When averaged across the array, a 90° periodicity in splitting parameters as a function of back azimuth is revealed, which has not been recovered previously with single‐station observations. The periodicity is characterized by a sawtooth pattern in Φ with a clearly defined 45° trend. We present new equations that reproduce this behavior based upon known systematic errors when calculating shear wave splitting from data with realistic seismic noise. The corrected results suggest a single layer of anisotropy with an ENE‐WSW fast axis parallel to the motion of the subducting Juan de Fuca plate; in agreement with predictions for entrained subslab mantle flow. The splitting pattern is consistent with that seen throughout Cascadia, suggesting that entrainment of the underlying asthenosphere with the subducting slab is coherent and widespread.

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“…The Central Andes are composed of several major morphotectonic provinces including: the fore‐arc region (FA), the Eastern Cordillera (EC), the Western Cordillera (WC), the Altiplano plateau (AP), the Puna plateau (PN), the Sub‐Andes (SA), the Los Frailes Volcanic Complex (LF), and the Altiplano‐Puna Volcanic Complex (APVC; Figure ). The entire region owes its formation to the convergence of the Nazca and South American plates at a rate of ~60–80 mm/year (e.g., Allmendinger et al, ; Isacks, ; Kendrick et al, ; Lamb & Hoke, ; Lynner et al, ). In the CAP region, a significant portion of the convergence has been accommodated by deformation of the western margin of the overriding South American plate (~15 mm/year; Norabuena et al, ).…”

Section: Introductionmentioning

confidence: 99%

Midcrustal Deformation in the Central Andes Constrained by Radial Anisotropy

et al. 2018

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The Central Andes are characterized by one of the largest orogenic plateaus worldwide. As a result, they are home to some of the thickest continental crust observed today (up to ~75‐km thick). Understanding the response of the crust to such overthickening provides insights into the ductile behavior of the midcrust and lower crust. One of the best tools for examining crustal‐scale features is ambient noise tomography, which takes advantage of the ambient noise wavefield to sample crustal depths in great detail. We extract Love and Rayleigh wave phase velocities from ambient noise data to invert for Vsh, Vsv, and radial anisotropy throughout the Central Andes. We capture detailed crustal structure, including pronounced along‐strike isotropic velocity heterogeneity and substantial (up to 10%) radial anisotropy that varies with depth. This crustal anisotropy may have several origins, but throughout the majority of the Central Andes, particularly beneath the Altiplano, we interpret radial anisotropy as the result of mineral alignment due to ductile crustal deformation. Only in the strongly volcanic Altiplano‐Puna Volcanic Complex is radial anisotropy likely caused by magmatic intrusions.

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Mantle flow through a tear in the Nazca slab inferred from shear wave splitting

Cited by 25 publications

References 47 publications

S‐Wave Receiver Function Analysis of the Pampean Flat‐Slab Region: Evidence for a Torn Slab

S‐Wave Receiver Function Analysis of the Pampean Flat‐Slab Region: Evidence for a Torn Slab

SKS Splitting Beneath Mount St. Helens: Constraints on Subslab Mantle Entrainment

Midcrustal Deformation in the Central Andes Constrained by Radial Anisotropy

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