Letter | Published:

Trench-parallel anisotropy produced by serpentine deformation in the hydrated mantle wedge

Nature volume 461, pages 11141117 (22 October 2009) | Download Citation

Abstract

Seismic anisotropy is a powerful tool for detecting the geometry and style of deformation in the Earth’s interior, as it primarily reflects the deformation-induced preferred orientation of anisotropic crystals1,2. Although seismic anisotropy in the upper mantle is generally attributed to the crystal-preferred orientation of olivine3, the strong trench-parallel anisotropy (delay time of one to two seconds) observed in several subduction systems4,5 is difficult to explain in terms of olivine anisotropy, even if the entire mantle wedge were to act as an anisotropic source. Here we show that the crystal-preferred orientation of serpentine, the main hydrous mineral in the upper mantle, can produce the strong trench-parallel seismic anisotropy observed in subduction systems. High-pressure deformation experiments reveal that the serpentine c-axis tends to rotate to an orientation normal to the shear plane during deformation; consequently, seismic velocity propagating normal to the shear plane (plate interface) is much slower than that in other directions. The seismic anisotropy estimated for deformed serpentine aggregates is an order of magnitude greater than that for olivine6, and therefore the alignment of serpentine in the hydrated mantle wedge results in a strong trench-parallel seismic anisotropy in the case of a steeply subducting slab. This hypothesis is also consistent with the presence of a hydrous phase in the mantle wedge, as inferred from anomalously low seismic-wave velocities7.

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Acknowledgements

We thank T. Watanabe and S. Karato for comments and discussions. We also thank D. Mainprice and A. Tommasi for providing the crystallographic data for serpentine. This study was supported by the Japan Society for the Promotion of Science (JSPS).

Author Contributions I.K. and K.-i.H. planned the project and performed the experiments. K.M. has responsibility for the EBSD analysis, and J.-i.A. for the experimental procedures. All authors discussed the results and implications.

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  1. Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan

    • Ikuo Katayama
    • , Ken-ichi Hirauchi
    •  & Jun-ichi Ando
  2. Institute of Geosciences, Shizuoka University, Shizuoka 422-8529, Japan

    • Katsuyoshi Michibayashi

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Correspondence to Ikuo Katayama.

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    Supplementary Information

    This file contains Supplementary Figure S1 and Legend and Supplementary Table S1.

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https://doi.org/10.1038/nature08513

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