Constraints on Flow Dynamics within the Oceanic Asthenosphere from a High-Resolution Estimate of Seismic Anisotropy

Abstract:

Convective flow in the mantle and the motions of tectonic plates produce deformation of the Earth’s interior, and the rock fabric produced by this deformation can be discerned using anisotropy of seismic wavespeed. This deformation is particularly prevalent within the oceanic asthenosphere, including near seafloor-spreading centers as new plates are formed via corner flow, and within a weak asthenosphere that lubricates large-scale plate-driven flow and/or accommodates smaller-scale convection. Seismic models of oceanic upper mantle are conflicting regarding the relative importance of these deformation processes. Seafloor-spreading fabric is very strong just beneath the Moho at relatively local scales. At ocean-basin scales, the strongest fabric in the asthenosphere, and the relative importance of density-driven flow and plate-induced shear is ambiguous. Using Rayleigh waves recorded across the NoMelt ocean-bottom seismograph (OBS) array in the central Pacific, we provide a unique high-resolution constraint on seismic anisotropy within the oceanic lithosphere-asthenosphere system in the middle of a plate. Shear-velocity and conductivity profiles delineate a dry, high-velocity lid overlying a damp, weak asthenosphere. Azimuthal anisotropy is strongest within the lid, with fast direction coincident with seafloor spreading, consistent with Pn observations. Minimum azimuthal anisotropy occurs within the lowest-velocity (weakest) portion of the asthenosphere, and below which it increases to a secondary maximum. In no depth range does the fast direction correspond to apparent plate motion. The results suggest that the dominant deformation in the oceanic mantle occurs during corner flow at the ridge axis, and via pressure- and/or buoyancy-driven flow within the asthenosphere, possibly within a non-Newtonian low-viscosity channel. Shear associated with motion of the plate over the underlying asthenosphere, if present, is weak compared to these processes.