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To clarify the deep structure beneath active back-arc basins and the causal mechanism of back-arc spreading, we conduct shear-wave azimuthal anisotropy tomography to image mantle flow beneath active back-arc basins in Kamchatka, Mariana and Tonga. We apply an updated joint-inversion method to teleseismic fundamental mode Rayleigh-wave phase and amplitude data at periods of 25–80 s, as well as teleseismic S-wave travel times. Our results clearly reveal similar anisotropic patterns at a depth of ∼100 km in the mantle wedge beneath the active back-arc basins in the three regions. The anisotropic patterns may reflect subduction-driven mantle flow related to the formation of the back-arc basins, which were suggested by geodynamic modeling but have not been well revealed by seismic imaging. Toroidal mantle flow may exist beneath Kamchatka and Tonga, but it is probably absent beneath Mariana. We propose that the subduction-driven mantle flow may play an important role in the back-arc spreading processes and influence the structure of a back-arc basin.
Zhao et al. (Mon,) studied this question.