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Seismological evidence for girdled olivine lattice-preferred orientation in oceanic lithosphere and implications for mantle deformation processes during seafloor spreading
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  • Joshua B. Russell,
  • James B. Gaherty,
  • Hannah Mark,
  • Greg Hirth,
  • Lars Hansen,
  • Daniel Lizarralde,
  • John A. Collins,
  • Rob L. Evans
Joshua B. Russell
Brown University, Brown University, Brown University, Brown University

Corresponding Author:[email protected]

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James B. Gaherty
Northern Arizona University, Northern Arizona University, Northern Arizona University, Northern Arizona University
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Hannah Mark
Washington University in St. Louis, Washington University in St. Louis, Washington University in St. Louis, Washington University in St. Louis
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Greg Hirth
Brown University, Brown University, Brown University, Brown University
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Lars Hansen
University of Minnesota, University of Minnesota, University of Minnesota, University of Minnesota
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Daniel Lizarralde
Woods Hole Oceanographic Institution, Woods Hole Oceanographic Institution, Woods Hole Oceanographic Institution, Woods Hole Oceanographic Institution
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John A. Collins
Woods Hole Oceanographic Institution, Woods Hole Oceanographic Institution, Woods Hole Oceanographic Institution, Woods Hole Oceanographic Institution
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Rob L. Evans
Woods Hole Oceanographic Institution, Woods Hole Oceanographic Institution, Woods Hole Oceanographic Institution, Woods Hole Oceanographic Institution
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Abstract

Seismic anisotropy produced by aligned olivine in oceanic lithosphere offers a window into mid-ocean ridge dynamics. Yet, interpreting anisotropy in the context of grain-scale deformation processes and strain observed in laboratory experiments and natural olivine samples has proven challenging due to incomplete seismological constraints and length scale differences spanning orders of magnitude. To bridge this observational gap, we estimate an in situ elastic tensor for oceanic lithosphere using co-located compressional- and shear-wavespeed anisotropy observations at the NoMelt experiment located on ~70 Ma seafloor. The elastic model for the upper 7 km of the mantle, NoMelt_SPani7, is characterized by a fast azimuth parallel to the fossil-spreading direction, consistent with corner-flow deformation fabric. We compare this model with a database of 123 petrofabrics from the literature to infer olivine crystallographic orientations and shear strain accumulated within the lithosphere. Direct comparison to olivine deformation experiments indicates strain accumulation of 250–400% in the shallow mantle. We find evidence for D-type olivine lattice-preferred orientation (LPO) with fast [100] parallel to the shear direction and girdled [010] and [001] crystallographic axes perpendicular to shear. D-type LPO implies similar amounts of slip on the (010)[100] and (001)[100] easy slip systems during mid-ocean ridge spreading; we hypothesize that grain-boundary sliding during dislocation creep relaxes strain compatibility, allowing D-type LPO to develop in the shallow lithosphere. Deformation dominated by dislocation-accommodated grain-boundary sliding (disGBS) has implications for in situ stress and grain size during mid-ocean ridge spreading and implies grain-size dependent deformation, in contrast to pure dislocation creep.
Oct 2022Published in Geochemistry, Geophysics, Geosystems volume 23 issue 10. 10.1029/2022GC010542