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Development of anisotropic structure in the Earth's lower mantle by solid-state convection

Abstract

Seismological observations reveal highly anisotropic patches at the bottom of the Earth's lower mantle, whereas the bulk of the mantle has been observed to be largely isotropic1,2,3,4. These patches have been interpreted to correspond to areas where subduction has taken place in the past or to areas where mantle plumes are upwelling, but the underlying cause for the anisotropy is unknown—both shape-preferred orientation of elastically heterogenous materials5 and lattice-preferred orientation of a homogeneous material6,7,8 have been proposed. Both of these mechanisms imply that large-strain deformation occurs within the anisotropic regions, but the geodynamic implications of the mechanisms differ. Shape-preferred orientation would imply the presence of large elastic (and hence chemical) heterogeneity whereas lattice-preferred orientation requires deformation at high stresses. Here we show, on the basis of numerical modelling incorporating mineral physics of elasticity and development of lattice-preferred orientation, that slab deformation in the deep lower mantle can account for the presence of strong anisotropy in the circum-Pacific region. In this model—where development of the mineral fabric (the alignment of mineral grains) is caused solely by solid-state deformation of chemically homogeneous mantle material—anisotropy is caused by large-strain deformation at high stresses, due to the collision of subducted slabs with the core–mantle boundary.

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Figure 1: Snapshot of a slab impinging on the core-mantle boundary (CMB) in the bottom-heated model.
Figure 2

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Acknowledgements

We thank M. Gurnis, D. Yamazaki, R. Wenk, T. Lay and B. Kiefer for discussions. This research was supported by the CSEDI program of the National Science Foundation.

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Correspondence to Allen K. McNamara.

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McNamara, A., van Keken, P. & Karato, SI. Development of anisotropic structure in the Earth's lower mantle by solid-state convection. Nature 416, 310–314 (2002). https://doi.org/10.1038/416310a

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