A complex pattern of mantle flow in the Lau backarc.

Shear-wave splitting analysis of local events recorded on land and on the ocean floor in the Tonga arc and Lau backarc indicate a complex pattern of azimuthal anisotropy that cannot be explained by mantle flow coupled to the downgoing plate. These observations suggest that the direction of mantle flow rotates from convergence-parallel in the Fiji plateau to north-south beneath the Lau basin and arc-parallel beneath the Tonga arc. These results correlate with helium isotopes that map mantle flow of the Samoan plume into the Lau basin through an opening tear in the Pacific plate.