Mid-mantle anisotropy in subduction zones and deep water transport

Andy Nowacki*, J-Michael Kendall, James Wookey, Asher Pemberton

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

28 Citations (Scopus)
347 Downloads (Pure)

Abstract

The Earth's transition zone has until recently been assumed to be seismically isotropic. Increasingly, however, evidence suggests that ordering of material over seismic wavelengths occurs there, but it is unclear what causes this. We use the method of source-side shear wave splitting to examine the anisotropy surrounding earthquakes deeper than 200 km in slabs around the globe. We find significant amounts of splitting (≤2.4 s), confirming that the transition zone is anisotropic here. However, there is no decrease in the amount of splitting with depth, as would be the case for a metastable tongue of olivine which thins with depth, suggesting this is not the cause. The amount of splitting does not appear to be consistent with processes in the ambient mantle, such as lattice-preferred orientation development in wadsleyite, ringwoodite, or MgSiO<inf>3</inf>-perovskite. We invert for the orientation of several mechanisms - subject to uncertainties in mineralogy and deformation - and the best fit is given by updip flattening in a style of anisotropy common to hydrous phases and layered inclusions. We suggest that highly anisotropic hydrous phases or hydrated layering is a likely cause of anisotropy within the slab, implying significant water transport from the surface down to at least 660 km depth.

Original languageEnglish
Pages (from-to)764-784
Number of pages21
JournalGeochemistry, Geophysics, Geosystems
Volume16
Issue number3
Early online date19 Mar 2015
DOIs
Publication statusPublished - 15 Apr 2015

Keywords

  • deep earthquakes
  • DHMS
  • mantle flow
  • shear wave splitting
  • subduction
  • transition zone

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