EVIDENCE FOR A STAGNANT PLUME IN THE TRANSITION ZONE

The structure of the mantle transition zone (MTZ) beneath the Pacific ocean is investigated by using Pds waves, converted from P to S from s eismic discontinuities at depths d in the receiver region. At stations within the South Pacific superswell, the pulse of P660s is anomalousl y broad. To explain this effect, we assume that the 660 km phase bound ary may present a barrier for a thermochemical plume ascending from th e lower mantle. Then the low-velocity material of the plume accumulate s just below this boundary, and forms a large-scale lens, which broade ns the pulse of P660s. The phase boundary is uplifted by heating, but the related travel time anomaly is cancelled by the increased effectiv e width of the transition.