Constraining Lowermost Mantle Dynamics with Seismic Anisotropy Observations Beneath Australia

Abstract:

Shear wave splitting is a valuable tool to analyze anisotropy throughout the mantle. By combining pairs of shear wave phases such as ScS/S and SKS/SKKS phases, we can constrain seismic anisotropy in the lowermost mantle, specifically the D” region. Several studies have revealed strong shear wave splitting due to lowermost mantle anisotropy in the D” region along the boundaries of the African Large Low Shear Velocity Province (LLSVP). The two LLSVPs, centered beneath Africa and the Pacific, are robust features of global S wave tomographic models of lower mantle structure, and likely play a role in lower mantle dynamics. Here we present shear wave splitting measurements of SKS, SKKS and ScS phases that reflect seismic anisotropy in the lowermost mantle beneath Australia, near the southwestern boundary of the Pacific LLSVP. Data from 8 long-running (>20 years) stations in Australia, New Zealand, and Antarctica provide splitting measurements for approximately 130 SKS-SKKS pairs. A significant minority (25) of these indicate significant discrepancies between SKS and SKKS splitting behavior, reflecting a contribution from the lowermost mantle. Work is ongoing to to combine this data set with differential S-ScS splitting measurements, which can constrain splitting of the ScS phase due to anisotropy in D”. In combination with SK(K)S measurements, such measurements can provide good sampling of individual regions over a range of propagation directions. Future work will include mineral physics based forward modeling in order to constrain the range of mantle flow scenarios that are consistent with our observations.

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