Mantle transition-zone structure beneath the South Pacific Superswell and evidence for a mantle plume underlying the Society hotspot

Underside reflections of shear waves from the discontinuities at 410 and 660 km depth are used to map lateral variations in the thickness of the mantle transition zone beneath the South Pacific Superswell and surrounding regions. Differential travel times of reflected waves indicate that the transition zone is about 25 km thinner than normal, and thus hotter than normal, over an area 500 km or less in diameter beneath the Society hotspot. There is no general difference, however, in transition-zone thickness between the Superswell area and its surroundings. Our observations support the inference that the thermal flux from lower to upper mantle beneath the South Pacific Superswell occur on the lateral scale of a mantle plume (several hundred kilometers) rather than that of a superplume (several thousand kilometers).