Deep structure and seismic anisotropy beneath the East Pacific Rise

Seismic velocities in the upper mantle beneath mid-ocean ridges are known to exhibit anisotropy, especially at depths shallower than 200 km. In this study, we present evidence for deeper anisotropy beneath fast-spreading ridges based on simultaneous inversion of body wave travel times, waveforms, and surface wave phase velocities. The 'vertical' (BSV) shear wave speed is systematically faster (by more than 2%) than the 'horizontal' (BSH) one at depths of 200-300 km beneath the East Pacific Rise, a result that is consistent with vertical alignment of the fast crystallographic axis of olivine. This anomaly is especially coherent beneath the northern East Pacific Rise, where the observations also support the presence of a modest mantle reflector near 260-km depth. The anisotropy may be associated with deep vertical flow or finite strain, and the presence of this reflector could signal a rheological transition near the bottom of the asthenosphere, reflecting changes in flow direction, development of partial melt, or the persistence of a deep pre-existing strain regime.