Mantle Transition Zone Structure
(1) Beneath the South Pacific Superswell
We have worked on the transition zone structure beneath the South Pacific Superswell and surrounding regions. We have modified the stacking technique used in the previous studies and found 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 thermal and mass fluxes 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). We are currently applying the same technique to the other tectonic regions.
Figure 3. Map view of (A) mantle transition-zone thickness, (B), the depth to the 410-km discontinuity, and (C) the depth to the 660-km discontinuity. Locations of surface hotspots and the South Pacific Superswell are indicated by circles and dashed box. The blue box indicates an area of anomalously thin transition zone. The short line segments on (C) indicate azimuths of the S660S wave paths at the underside reflection points.
D. Suetsugu, T. Saita, H. Tanaka, and F. Niu , Thickness of the mantle transition zone beneath the South Pacific as inferred from analyses of ScS reverberated and Ps converted waves, PEPI, submitted, 2002.
F. Niu, S. C. Solomon, P. G. Silver, D. Suetsugu, H. Inoue, Mantle Transition-zone Structure beneath the South Pacific Superswell and Evidence for a Mantle Plume underlying the Society Hotspot, Earth Planet. Sci. Lett. , 198, 371-380, 2002. [PDF]
F. Niu , H. Inoue, D. Suetsugu, and K. Kanjo, Seismic evidence for a thinner mantle transition zone beneath the South Pacific Superswell, Geophys. Res. Lett., 27, 1981-1984, 2000. [PDF]