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Deformation of the lowermost mantle from seismic anisotropy

Abstract

The lowermost part of the Earth’s mantle—known as D″—shows significant seismic anisotropy, the variation of seismic wave speed with direction1,2,3,4,5. 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 impinges6 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 assumed2,3,7 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.

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Figure 1: Source–receiver geometry, and explanation of ϕ
Figure 2: Multi-azimuth stacked shear wave splitting results in each region.
Figure 3: Section through study region and compatible shear planes for candidate ppv slip systems.

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Acknowledgements

We thank J. Brodholt and D. Dobson for comments. A.N. was supported by NERC. Seismic data were provided by I. Bastow, D. Thompson, and the IRIS and CNSN data centres.

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Contributions

A.N. analysed the data and produced the manuscript and figures. J.W. wrote the analysis and modelling code and performed the modelling. J.W. and J-M.K. supervised the analysis and commented on the manuscript and figures. All authors discussed the results and implications at all stages.

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Correspondence to Andy Nowacki.

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The authors declare no competing financial interests.

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This file contains Supplementary Results and Discussion, Supplementary Figures 1-14 with legends, Supplementary Tables 1-6 and additional references. (PDF 6965 kb)

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Nowacki, A., Wookey, J. & Kendall, JM. Deformation of the lowermost mantle from seismic anisotropy. Nature 467, 1091–1094 (2010). https://doi.org/10.1038/nature09507

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