Seismic anisotropy above and below the subducting Nazca lithosphere in southern South America

MacDougall, J.; Fischer, K. M.; Anderson, M. L.

doi:10.1029/2012jb009538

Cited by 22 publications

(23 citation statements)

References 110 publications

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“…This is in direct contrast to what is observed both to the north and south of our study region [Anderson et al, 2004;MacDougall et al, 2012;Calixto et al, 2014] but is consistent with previous observations above the flat slab [Anderson et al, 2004] (Figure 3). This is strong supporting evidence for the assertion Anderson et al [2004] andMacDougall et al [2012]. This signal persists far eastward through the ESP stations.…”

Section: Discussionsupporting

confidence: 91%

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

Lynner

Anderson

Portner

et al. 2017

Geophysical Research Letters

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A tear in the subducting Nazca slab is located between the end of the Pampean flat slab and normally subducting oceanic lithosphere. Tomographic studies suggest mantle material flows through this opening. The best way to probe this hypothesis is through observations of seismic anisotropy, such as shear wave splitting. We examine patterns of shear wave splitting using data from two seismic deployments in Argentina that lay updip of the slab tear. We observe a simple pattern of plate‐motion‐parallel fast splitting directions, indicative of plate‐motion‐parallel mantle flow, beneath the majority of the stations. Our observed splitting contrasts previous observations to the north and south of the flat slab region. Since plate‐motion‐parallel splitting occurs only coincidentally with the slab tear, we propose mantle material flows through the opening resulting in Nazca plate‐motion‐parallel flow in both the subslab mantle and mantle wedge.

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Section: Discussionsupporting

confidence: 91%

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

Lynner

Anderson

Portner

et al. 2017

Geophysical Research Letters

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“…For comparison, MacDougall et al . [] studied shear wave splitting from local S events above the Pampean flat slab and found delay times similar in magnitude to ours (~0.26 s), as well as variable fast directions that resemble our observations south of the ridge. The source of anisotropy there was inferred to reside mainly in the mantle wedge, resulting from two‐dimensional corner flow that is partially deflected by pressure gradients due to the decrease in slab dip.…”

Section: Discussionsupporting

confidence: 89%

Response of the mantle to flat slab evolution: Insights from local S splitting beneath Peru

Eakin

Long

Beck

et al. 2014

Geophysical Research Letters

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The dynamics of flat subduction, particularly the interaction between a flat slab and the overriding plate, are poorly understood. Here we study the (seismically) anisotropic properties and deformational regime of the mantle directly above the Peruvian flat slab. We analyze shear wave splitting from 370 local S events at 49 stations across southern Peru. We find that the mantle above the flat slab appears to be anisotropic, with modest average delay times (~0.28 s) that are consistent with~4% anisotropy in a~30 km thick mantle layer. The most likely mechanism is the lattice-preferred orientation of olivine, which suggests that the observed splitting pattern preserves information about the mantle deformation. We observe a pronounced change in anisotropy along strike, with predominately trench-parallel fast directions in the north and more variable orientations in the south, which we attribute to the ongoing migration of the Nazca Ridge through the flat slab system.

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“…Although most of these publications are generally based on more measurements at single frequencies than results of event‐station pairs at different filter frequencies, frequency dependence of local measurements seems to be a relatively robust feature. Fewer studies find no frequency dependence (MacDougall et al, ). This frequency dependence has been explained either with the presence of small‐scale heterogeneities or with depth‐dependent changes of anisotropy geometry (e.g., multiple layers).…”

Section: Discussion and Interpretationmentioning

confidence: 99%

Seismic Anisotropy Beneath the Pamir and the Hindu Kush: Evidence for Contributions From Crust, Mantle Lithosphere, and Asthenosphere

et al. 2018

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We use local and teleseismic earthquakes to analyze shear wave splitting within the Pamir‐Hindu Kush region, north of the western syntaxis of the India‐Asia collision zone. These two data sets allowed us to map the distribution of azimuthal anisotropy, to put constraints on the depth range where it is accumulated, and to deduce characteristics of ongoing deformation. From 1,073 SKS (core‐mantle refracted phases) measurements at 107 stations, we derived time delays of 0.7–2.25 s and dominantly ENE‐WSW oriented fast polarization directions. Fast polarization directions only deviate adjacent to the subducting slabs and major strike‐slip faults, aligning parallel to these structures. From 461 direct S measurements along a transect perpendicular to the Pamir seismic zone, we obtain fast directions parallel to those from SKS measurements but smaller delay times (average 0.4 s), which vary depending on depth. Time delays exhibit 0.1–0.3 s crustal contribution and increase to 0.8 s in a narrow domain coinciding with the inferred subcrustal contact of the two colliding plates. We find measurements from the same event‐station paths at different filter frequencies to be frequency‐independent, allowing a comparison with SKS results along the studied profile. The smaller average time delays of local events imply that the crust and uppermost mantle only make a minor contribution to the SKS splitting. Thus, the coherent fast direction pattern suggests a strain field dominated by the indentation of India and the escape of sublithospheric material north of the indenter. Crustal anisotropy is likely also controlled by this regional deformation pattern with locally highest strain rates closest to the continental subduction front.

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Seismic anisotropy above and below the subducting Nazca lithosphere in southern South America

Cited by 22 publications

References 110 publications

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

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

Response of the mantle to flat slab evolution: Insights from local S splitting beneath Peru

Seismic Anisotropy Beneath the Pamir and the Hindu Kush: Evidence for Contributions From Crust, Mantle Lithosphere, and Asthenosphere

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