Letter | Published:

High-resolution seismic constraints on flow dynamics in the oceanic asthenosphere

Nature volume 535, pages 538541 (28 July 2016) | Download Citation

Abstract

Convective flow in the mantle and the motions of tectonic plates produce deformation of Earth’s interior, and the rock fabric produced by this deformation can be discerned using the anisotropy of the seismic wavespeed1,2,3. This deformation is commonly inferred close to lithospheric boundaries beneath the ocean in the uppermost mantle, including near seafloor-spreading centres as new plates are formed via corner flow4, and within a weak asthenosphere that lubricates large-scale plate-driven flow and accommodates smaller-scale convection5,6. Seismic models of oceanic upper mantle differ as to the relative importance of these deformation processes: seafloor-spreading fabric is very strong just beneath the crust–mantle boundary (the Mohorovičić discontinuity, or Moho) at relatively local scales7,8, but at the global and ocean-basin scales, oceanic lithosphere typically appears weakly anisotropic when compared to the asthenosphere9,10. Here we use Rayleigh waves, recorded across an ocean-bottom seismograph array in the central Pacific Ocean (the NoMelt Experiment), to provide unique localized constraints on seismic anisotropy within the oceanic lithosphere–asthenosphere system in the middle of a plate. We find that azimuthal anisotropy is strongest within the high-seismic-velocity lid, with the fast direction coincident with seafloor spreading. A minimum in the magnitude of azimuthal anisotropy occurs within the middle of the seismic low-velocity zone, and then increases with depth below the weakest portion of the asthenosphere. At no depth does the fast direction correlate with the apparent plate motion. Our results suggest that the highest strain deformation in the shallow oceanic mantle occurs during corner flow at the ridge axis, and via pressure-driven or buoyancy-driven flow within the asthenosphere. Shear associated with motion of the plate over the underlying asthenosphere, if present, is weak compared to these other processes.

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Acknowledgements

We thank the scientific party, captain, crews, and technical teams of RV Marcus G. Langseth and RV Melville for work that made this study possible. The OBS were provided and supported by Scripps Institution of Oceanography’s facility as part of the US Ocean Bottom Seismograph Instrument Pool (http://www.obsip.org). This work was funded by the US National Science Foundation. P.-Y.P.L. thanks the Institute of Earth Science, Academia Sinica, Taipei, Taiwan and Institute of Undersea Technology, National Sun Yat-sen University, Kaohsiung, Taiwan for support during completion of this work.

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Author notes

    • Pei-Ying Patty Lin

    Present address: Taiwan Ocean Research Institute, National Applied Research Laboratories, Kaohsiung, Taiwan (P.-Y.P.L.).

Affiliations

  1. Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, USA

    • Pei-Ying Patty Lin
    • , James B. Gaherty
    •  & Ge Jin
  2. Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA

    • John A. Collins
    • , Daniel Lizarralde
    •  & Rob. L. Evans
  3. Geological Sciences Department, Brown University, Providence, Rhode Island, USA

    • Greg Hirth

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Contributions

P.-Y.P.L. and J.B.G. collaborated in developing the concept of this paper and writing the first draft. All authors contributed to the scientific discussion, including presentation of results, interpretation, and implications.

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The authors declare no competing financial interests.

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Correspondence to Pei-Ying Patty Lin.

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https://doi.org/10.1038/nature18012

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