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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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.
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Power spectral densities (PSDs) of raw, tilt and compliance noise-removed seismograms.
The PSD values are calculated from vertical data recorded by one NoMelt station, B13, located at 9.25° N, 145.55° W). The black curves show the spectrum of the raw vertical component. The red and blue curves show the spectrum after removing the tilt noise and compliance noise, respectively. The PSD is reported in decibels (dB) referenced to 10log[AAS (m−2 s−4 Hz−1)], where AAS is the acceleration-amplitude spectrum. The new high- and low-noise models of ref. 45, NHNM and NLNM (thin dashed lines), are shown as references.
a, Broadband vertical-component seismograms filtered in the 20–100-s period band from an event (green circle) in the southwestern Pacific (see inset map). The red inverted triangle denotes the NoMelt location. b, Ambient noise cross-correlation waveforms, filtered between 10 s and 25 s in the time domain.
19 events (red stars) with high-quality Rayleigh waves were recorded during NoMelt one-year deployment. White lines represent the corresponding great-circle paths. Grey contours show the epicentral distances at 10° intervals.
The upper panels show four input models of G parameters (green solid, magenta solid, orange solid and blue dashed-dotted lines). The lower panels show the forward-calculated peak-to-peak amplitude A and fast direction ϕ as functions of period from the four models. Our observations are shown as blue circle symbols with error bars of 1 s.d. as references. The black and grey dashed lines indicate the orientation of the FSD and apparent plate motion (APM) at the NoMelt location.
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Lin, PY., Gaherty, J., Jin, G. et al. High-resolution seismic constraints on flow dynamics in the oceanic asthenosphere. Nature 535, 538–541 (2016). https://doi.org/10.1038/nature18012
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