Deformation of the lowermost mantle from seismic anisotropy

Andy J Nowacki, J Wookey, J-M Kendall

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

58 Citations (Scopus)

Abstract

The lowermost part of the Earth’s mantle—known as D″—shows significant seismic anisotropy, the variation of seismic wave speed with direction. This is probably due to deformation-induced alignment of MgSiO3-post-perovskite (ppv), which is believed to be the main mineral phase present in the region. If this is the case, then previous measurements of D″ anisotropy, which are generally made in one direction only, are insufficient to distinguish candidate mechanisms of slip in ppv because the mineral is orthorhombic. Here we measure anisotropy in D″ beneath North and Central America, where material from subducting oceanic slabs impinges on the core–mantle boundary, using shallow as well as deep earthquakes to increase the azimuthal coverage in D″. We make more than 700 individual measurements of shear wave splitting in D″ in three regions from two different azimuths in each case. We show that the previously assumed case of vertical transverse isotropy (where wave speed shows no azimuthal variation) is not possible, and that more complicated mechanisms must be involved. We test the fit of different MgSiO3-ppv deformation mechanisms to our results and find that shear on (001) is most consistent with observations and the expected shear above the core–mantle boundary beneath subduction zones. With new models of mantle flow, or improved experimental determination of the dominant ppv slip systems, this method will allow us to map deformation at the core–mantle boundary and link processes in D″, such as plume initiation, to the rest of the mantle.
Translated title of the contributionDeformation of the lowermost mantle from seismic anisotropy
Original languageEnglish
Pages (from-to)1091 - 1094
Number of pages4
JournalNature
Volume467
DOIs
Publication statusPublished - Oct 2010

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