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

Rayleigh waves recorded with an ocean-bottom seismograph array in the central Pacific Ocean constrain the seismic anisotropy within the oceanic lithosphere–asthenosphere system: seafloor-spreading-induced lithospheric fabric generates the strongest anisotropy, while density- and/or pressure-driven flow produces a secondary peak in anisotropy at the base of the asthenosphere. Observing the oceanic plate and asthenosphere Pei-Ying Lin and co-authors use Rayleigh waves, recorded across an ocean-bottom seismograph array in the central Pacific, to provide localized constraints on seismic anisotropy within the oceanic lithosphere–asthenosphere system in the middle of a tectonic plate. They find that azimuthal anisotropy is strongest within the high-velocity seismic lid, with a fast direction consistent with seafloor spreading, that a minimum occurs within the middle of the seismic low-velocity zone and that the anisotropy then increases with depth below the weakest portion of the asthenosphere. In no depth range does the fast direction correlate with the apparent plate motion. The authors conclude that the highest strain deformation in the shallow oceanic mantle occurs during corner flow at the ridge axis, and via pressure- or buoyancy-driven flow within the asthenosphere. 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 wavespeed 1 , 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 flow 4 , and within a weak asthenosphere that lubricates large-scale plate-driven flow and accommodates smaller-scale convection 5 , 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 scales 7 , 8 , but at the global and ocean-basin scales, oceanic lithosphere typically appears weakly anisotropic when compared to the asthenosphere 9 , 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