Reduced oceanic seismic anisotropy by small-scale convection

Abstract

Amplitudes of azimuthal seismic anisotropy below the Pacific plate are observed to decrease with lithospheric age, whereas an increase is expected due to ongoing shearing between the plate and the underlying mantle. We illustrate that the convective disturbance of this laminar flow field by sub-lithospheric small-scale convection (SSC) provides an explanation for this anisotropy reduction. By combining numerical flow models with forward seismic anisotropy calculations we show that small-scale convection can disturb the anisotropy pattern significantly. When such disturbed signal is smoothed over > ~ 500 km length scale (the approximate present-day seismic resolution for azimuthal anisotropy within the Pacific lithosphere and upper mantle), our results show a significant reduction in anisotropy amplitudes of about a factor two, similar to observed reductions. Perturbations of the fast polarization direction, however, remain relatively small (< ~ 20°). This suggests that, despite the influence of any SSC, observed long-wavelength LPO orientations still represent the large-scale shearing between the lithosphere and the underlying mantle. For higher resolution LPO data, SSC can significantly complicate the interpretation of sub-lithospheric mantle flow from azimuthal seismic anisotropy data.