2:15 PM - 2:30 PM
[SIT08-03] Constraints on composition and flow in the oceanic mantle from a high-resolution estimate of seismic velocities and electrical conductivity in the central Pacific
Keywords:seismic anisotropy, electrical conductivity, lithosphere, asthenosphere, ocean-bottom seismology
The combined results from MT, surface-wave, and P-wave refraction data suggest that the central Pacific upper mantle can be characterized by a cold, dry lithosphere overlying a damp asthenosphere, with no melt required. P-wave velocity increases rapidly in the shallow mantle, with evidence for a distinct, high-velocity reflector at mid-lithosphere depths suggestive of a possible phase change. Seismic anisotropy is extremely strong in the lithosphere with fast direction aligned with fossil spreading. Strength of the fabric increases with depth in the shallow lithosphere, before systematically decreasing with depth into the asthenosphere. Minimum azimuthal anisotropy occurs within the middle of the low-velocity zone, and then it increases with depth, achieving a secondary maximum at about 250 km depth, below the weakest portion of the asthenosphere. Fast directions rotate from fossil-spreading direction within the lithosphere, to a more east-west direction at depth. In no depth range does the direction correspond to apparent plate motion. We interpret the anisotropy as arising from the combination of two processes: shear-strain during corner flow at the ridge axis, and pressure- and/or buoyancy-driven flow within the asthenosphere, perhaps in a non-Newtonian viscous channel. Shear associated with motion of the plate over the underlying asthenosphere, if present, is weak compared to these processes.