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

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 wave speed. 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, and within a weak asthenosphere that lubricates large-scale plate-driven flow and accommodates smaller scale convection. 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, but at the global and ocean-basin scales, oceanic lithosphere typically appears weakly anisotropic when compared to the asthenosphere. 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.

In Situ Characterization of the Lithosphere‐Asthenosphere System beneath NW Pacific Ocean Via Broadband Dispersion Survey With Two OBS Arrays

“…The similar intensity reduction with depth is observed in other parts of Pacific from seafloor array experiments (Lin et al, 2016;Takeo et al, 2016). The similar intensity reduction with depth is observed in other parts of Pacific from seafloor array experiments (Lin et al, 2016;Takeo et al, 2016).…”

Constraints on lithospheric mantle and crustal anisotropy in the NoMelt area from an analysis of long-period seafloor magnetotelluric data

“…For the anisotropic lithosphere, the red line shows a most conductive direction that is supposed to be parallel to the fossil spreading direction (y-direction), and the black line shows a least conducive direction that is supposed to be perpendicular to a fossil spreading direction (x-direction). The isotropic 1D model is consistent with the red line for type 1, the dotted line for type 2, and the black line for type 3, respectively, in the anisotropic lithosphere response) corresponds to the fossil spreading direction (~ N80°E-S80°W) and least conductive (x-direction for MT response) is parallel to the strike of the paleo-ridge (~ N10°W-S10°E) (Lin et al 2016). The seismic data show clear evidence for anisotropy in the fossil spreading direction (Lin et al 2016), so we maintain this geometry in assigning anisotropy.…”

“…Strong azimuthal anisotropy in the NoMelt upper mantle that decreases with depth through the lithosphere (Lin et al 2016) suggests a greater degree of shearing and fabric alignment at shallow depths. We also suggest that the lowermost lithosphere (~ 60-90 km) in the NoMelt 70 Ma seafloor area, which is thicker than at the midocean ridge of ~ 60 km, could be damp and sheared and have accreted onto the base of the overlying plate with age.…”

“…Some models, derived from global seismic data sets, have suggested a relatively isotropic lithospheric structure underlain by an asthenosphere with substantial anisotropy resulting from current mantle flow (e.g., Beghein et al 2014). More localized seismic results from the NoMelt experiment, carried out over 70 Ma lithosphere in the central Pacific, show strong fast azimuthal anisotropy within the lithosphere in a direction aligned parallel to that of past spreading at the ridge crest (Lin et al 2016). The NoMelt seismic data also show more modest azimuthal anisotropy in the asthenosphere with the weakest anisotropy seen in the middle of the seismic low-velocity zone (~ 100-150 km depth) before increasing again (~ 200 km depth) (Lin et al 2016).…”

Structure of the oceanic lithosphere and upper mantle north of the Gloria Fault in the eastern mid‐Atlantic by receiver function analysis

“…The lithosphere‐asthenosphere boundary (LAB) beneath the oceans is often imaged using surface waves [e.g., Romanowicz , ; Takeo et al , , ; Lin et al , ], SS waveforms [e.g., Rychert et al , ], RFs employing land stations [e.g., Li et al , ; Kumar and Kawakatsu , ], or OBSs [e.g., Kawakatsu et al , ; Olugboji et al , ]. Most of the discussed models of the LAB [ Kawakatsu et al , ; Fischer et al , ; Olugboji et al , ] include thermal control, changes in rheology, dehydration, anisotropy, or partial melt.…”