Low S velocity atop the 410-km discontinuity and mantle plumes

A thin, low-S-velocity layer atop the 410-km discontinuity is an intriguing feature of the upper mantle with important implications for geodynamics, but relevant seismic data are few. By applying S receiver function technique to more than 50 globally distributed stations, in 10 regions we obtain evidence for a negative discontinuity at a depth of about 350 km. In most cases, the low velocity is found beneath Precambrian platforms, in association with either Mesozoic or Cenozoic mantle plumes. This relationship suggests dehydration of water-bearing silicates as a possible reason for the low velocity, but contradicts the predictions of the transition-zone-water-filter model of Bercovici and Karato (Nature 425, 39-44, 2003). The presence of the low velocity beneath some Mesozoic traps, in spite of plate motions, implies the possibility of coupling of the continental lithosphere and the underlying upper mantle up to a depth of ∼ 400 km.