The structure of the mantle transition zone (MTZ) beneath the Pacific ocean is investigated by using Pds, waves, converted from P to S from seismic discontinuities at depths d in the receiver region. At stations within the South Pacific superswell, the pulse of P660s is anomalously broad. To explain this effect, we assume that the 660 km phase boundary may present a barrier for a thermochemical plume ascending from the lower mantle. Then the low-velocity material of the plume accumulates just below this boundary, and forms a large-scale lens, which broadens the pulse of P660s. The phase boundary is uplifted by heating, but the related travel time anomaly is cancelled by the increased effective width of the transition.¿ 1997 American Geophysical Union