Lower mantle seismic scatterers below the subducting Tonga slab

Evidence for slab entrainment of transition zone materials

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18 Citations (Scopus)

Abstract

We show evidence that materials with significantly different elastic properties are juxtaposed in the lower mantle immediately below the subducting Tonga slab (depths ≤1000. km), like the anomalies preferentially located beneath the lower mantle slabs at other Pacific subduction zones (Kaneshima, 2009). Array analyses of wave form data of short period seismic networks at western United States and Japan for deep earthquakes at the Tonga slab reveal S-to-P scatterers with a size less than the wavelengths (~10. km). The scatterers are located mostly outside of the slab by several tens of kilometers. Assuming a locally planar interface for the geometry of the scatterers, the amplitudes and polarities of the S-to-P waves are modeled to constrain the properties of the scatterers. We find that the scatterers are steeply dipping, the Vs increases oceanward across the interface, and the Vs contrasts are at least comparable to that associated with the post-spinel transformation (≥6%). It is unclear at this stage what these subslab scatterers represent, so we discuss about three mechanisms which seem plausible from mantle dynamics viewpoints: (1) they may represent basaltic rocks which were emplaced by partial melting immediately beneath the former oceanic lithosphere-asthenosphere boundary (LAB) before the slab started subducting; (2) alternatively the elastic anomalies of the scatterers may be caused by localized presence of dehydrated water; or (3) the scatterers may correspond to a sharp boundary between fine-grained isotropic rocks in the immediate vicinity of the slab and coarse grained anisotropic rocks more distant from the slab. The presence of pronounced and localized elastic anomalies preferentially beneath the slabs in the shallow lower mantle, whatever its mechanism is, implies that a geophysically observable amount of transition zone material is entrained by the subducting slabs into the lower mantle.

Original languageEnglish
Pages (from-to)35-46
Number of pages12
JournalPhysics of the Earth and Planetary Interiors
Volume222
DOIs
Publication statusPublished - Sep 1 2013

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entrainment
lower mantle
transition zone
slab
Earth mantle
slabs
scattering
rocks
anomalies
anomaly
rock
material
asthenosphere
elastic property
oceanic lithosphere
dipping
P waves
lithosphere
spinel
subduction zone

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Geophysics
  • Physics and Astronomy (miscellaneous)
  • Space and Planetary Science

Cite this

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title = "Lower mantle seismic scatterers below the subducting Tonga slab: Evidence for slab entrainment of transition zone materials",
abstract = "We show evidence that materials with significantly different elastic properties are juxtaposed in the lower mantle immediately below the subducting Tonga slab (depths ≤1000. km), like the anomalies preferentially located beneath the lower mantle slabs at other Pacific subduction zones (Kaneshima, 2009). Array analyses of wave form data of short period seismic networks at western United States and Japan for deep earthquakes at the Tonga slab reveal S-to-P scatterers with a size less than the wavelengths (~10. km). The scatterers are located mostly outside of the slab by several tens of kilometers. Assuming a locally planar interface for the geometry of the scatterers, the amplitudes and polarities of the S-to-P waves are modeled to constrain the properties of the scatterers. We find that the scatterers are steeply dipping, the Vs increases oceanward across the interface, and the Vs contrasts are at least comparable to that associated with the post-spinel transformation (≥6{\%}). It is unclear at this stage what these subslab scatterers represent, so we discuss about three mechanisms which seem plausible from mantle dynamics viewpoints: (1) they may represent basaltic rocks which were emplaced by partial melting immediately beneath the former oceanic lithosphere-asthenosphere boundary (LAB) before the slab started subducting; (2) alternatively the elastic anomalies of the scatterers may be caused by localized presence of dehydrated water; or (3) the scatterers may correspond to a sharp boundary between fine-grained isotropic rocks in the immediate vicinity of the slab and coarse grained anisotropic rocks more distant from the slab. The presence of pronounced and localized elastic anomalies preferentially beneath the slabs in the shallow lower mantle, whatever its mechanism is, implies that a geophysically observable amount of transition zone material is entrained by the subducting slabs into the lower mantle.",
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T1 - Lower mantle seismic scatterers below the subducting Tonga slab

T2 - Evidence for slab entrainment of transition zone materials

AU - Kaneshima, Satoshi

PY - 2013/9/1

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N2 - We show evidence that materials with significantly different elastic properties are juxtaposed in the lower mantle immediately below the subducting Tonga slab (depths ≤1000. km), like the anomalies preferentially located beneath the lower mantle slabs at other Pacific subduction zones (Kaneshima, 2009). Array analyses of wave form data of short period seismic networks at western United States and Japan for deep earthquakes at the Tonga slab reveal S-to-P scatterers with a size less than the wavelengths (~10. km). The scatterers are located mostly outside of the slab by several tens of kilometers. Assuming a locally planar interface for the geometry of the scatterers, the amplitudes and polarities of the S-to-P waves are modeled to constrain the properties of the scatterers. We find that the scatterers are steeply dipping, the Vs increases oceanward across the interface, and the Vs contrasts are at least comparable to that associated with the post-spinel transformation (≥6%). It is unclear at this stage what these subslab scatterers represent, so we discuss about three mechanisms which seem plausible from mantle dynamics viewpoints: (1) they may represent basaltic rocks which were emplaced by partial melting immediately beneath the former oceanic lithosphere-asthenosphere boundary (LAB) before the slab started subducting; (2) alternatively the elastic anomalies of the scatterers may be caused by localized presence of dehydrated water; or (3) the scatterers may correspond to a sharp boundary between fine-grained isotropic rocks in the immediate vicinity of the slab and coarse grained anisotropic rocks more distant from the slab. The presence of pronounced and localized elastic anomalies preferentially beneath the slabs in the shallow lower mantle, whatever its mechanism is, implies that a geophysically observable amount of transition zone material is entrained by the subducting slabs into the lower mantle.

AB - We show evidence that materials with significantly different elastic properties are juxtaposed in the lower mantle immediately below the subducting Tonga slab (depths ≤1000. km), like the anomalies preferentially located beneath the lower mantle slabs at other Pacific subduction zones (Kaneshima, 2009). Array analyses of wave form data of short period seismic networks at western United States and Japan for deep earthquakes at the Tonga slab reveal S-to-P scatterers with a size less than the wavelengths (~10. km). The scatterers are located mostly outside of the slab by several tens of kilometers. Assuming a locally planar interface for the geometry of the scatterers, the amplitudes and polarities of the S-to-P waves are modeled to constrain the properties of the scatterers. We find that the scatterers are steeply dipping, the Vs increases oceanward across the interface, and the Vs contrasts are at least comparable to that associated with the post-spinel transformation (≥6%). It is unclear at this stage what these subslab scatterers represent, so we discuss about three mechanisms which seem plausible from mantle dynamics viewpoints: (1) they may represent basaltic rocks which were emplaced by partial melting immediately beneath the former oceanic lithosphere-asthenosphere boundary (LAB) before the slab started subducting; (2) alternatively the elastic anomalies of the scatterers may be caused by localized presence of dehydrated water; or (3) the scatterers may correspond to a sharp boundary between fine-grained isotropic rocks in the immediate vicinity of the slab and coarse grained anisotropic rocks more distant from the slab. The presence of pronounced and localized elastic anomalies preferentially beneath the slabs in the shallow lower mantle, whatever its mechanism is, implies that a geophysically observable amount of transition zone material is entrained by the subducting slabs into the lower mantle.

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