Ubiquitous lower-mantle anisotropy beneath subduction zones

Abstract

Seismic anisotropy provides key information to map the trajectories of mantle flow and understand the evolution of our planet. While the presence of anisotropy in the uppermost mantle is well established, the existence and nature of anisotropy in the transition zone and uppermost lower mantle are still debated. Here we use three-dimensional global seismic tomography images based on a large dataset that is sensitive to this region to show the ubiquitous presence of anisotropy in the lower mantle beneath subduction zones. Whereas above the 660 km seismic discontinuity slabs are associated with fast SV anomalies up to about 3%, in the lower mantle fast SH anomalies of about 2% persist near slabs down to about 1,000–1,200 km. These observations are consistent with 3D numerical models of deformation from subducting slabs and the associated lattice-preferred orientation of bridgmanite produced in the dislocation creep regime in areas subjected to high stresses. This study provides evidence that dislocation creep may be active in the Earth’s lower mantle, providing new constraints on the debated nature of deformation in this key, but inaccessible, component of the deep Earth.

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Fig. 1: Comparison of 1D averages of radial anisotropy beneath various subduction zones.
Fig. 2: Cross-sections of perturbations in Voigt average and anisotropic structure.
Fig. 3: K-means clustering analysis of the radially anisotropic structure in SGLOBE-rani.
Fig. 4: Cross-sections of perturbations in anisotropic structure and corresponding geodynamic models.

Code availability

The large scale subduction models were built using the code I3MG, which is not freely available and was kindly provided by T. Gerya.

The mantle fabric calculations used a modified version of the code D-REX available at http://www.ipgp.fr/~kaminski/web_doudoud/DRex.tar.gz.

Data availability

The data that support the findings of this study are available from the corresponding author on request. The tomography model SGLOBE-rani used in this study is available in the IRIS Data Services Products (http://ds.iris.edu/ds/products/emc-earthmodels/).

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Acknowledgements

This research was initially supported by the Leverhulme Trust (project no. F/00 204/AS), followed by support from NERC project NE/K005669/1 and the Korea Meteorological Administration Research and Development Program under grant no. KMI2018-09312. A.M.G.F. also acknowledges discussions supported by COST Action ES1401-TIDES. M.F. was supported by the Progetto di Ateneo FACCPTRAT12 granted by the Università di Padova and by the ERC StG #758199 NEWTON. We acknowledge the availability of global seismograms from the IRIS Data Services and the II, IU, GEOSCOPE and GEOFON networks. The inversions were carried out initially on the High Performance Computing Cluster supported by the Research and Specialist Computing Support services at the University of East Anglia followed by the national UK supercomputing facilities HECToR and Archer. Geodynamic simulations were performed on the Galileo Computing Cluster, CINECA, Italy, thanks to the computational time assigned to M.F. under the NUMACOP and NUMACOP2 projects. We thank J. Brodholt for fruitful discussions and for his valuable suggestions. We also thank our colleagues D. Dobson and A. Song for fruitful discussions, and we are grateful to C. Lithgow-Bertelloni and L. Stixrude for providing HeFESTo’s results. We are grateful to Z. Zhang for providing bridgmanite’s full elastic constants from ab-initio calculations.

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A.M.G.F. designed the study, performed analyses of the tomography images, interpreted the results and wrote the first draft of the manuscript. M.F. contributed to the design of the study, performed and analysed geodynamic models, mantle fabric and SPO calculations, and wrote the text on the geodynamics part of the study. W.S. performed geodynamic models and mantle fabric calculations and prepared some of the tomography and geodynamics figures. S.-J.C. performed analyses of the tomography images and statistical tests of the seismic images. L.S. assisted the analyses of the tomography images and composing the figures.

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Correspondence to Ana M. G. Ferreira.

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Supplementary Video 1

Simulation of slab stagnation at the 660-km discontinuity.

Supplementary Video 2

Simulation of slab penetration to the lower mantle.

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Ferreira, A.M.G., Faccenda, M., Sturgeon, W. et al. Ubiquitous lower-mantle anisotropy beneath subduction zones. Nat. Geosci. 12, 301–306 (2019). https://doi.org/10.1038/s41561-019-0325-7

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