Horizontal transition from fast to slow structures at the core¯mantle boundary; South Atlantic

Recent tomographic studies of the deep mantle have revealed large-scale low-velocity zones beneath Africa and relatively fast velocity zones beneath South America. Here we conduct a concentrated core¯mantle boundary (CMB) study across this transition zone from fast to slow beneath the South Atlantic. Deep South American earthquakes recorded in Africa provide S and ScS waveform data at core-grazing distances of 85¯105°. The waveform interference pattern produced at these ranges provides an excellent sample of the CMB structure in the vicinity of the ScS bounce point which can be used to map out regions of low-velocity zones (<5% reductions) and the more extreme ultra-low-velocity zones (>10% reductions). The strongest anomalies occur at the western edge roughly beneath the Tristat Plateau with weaker structures to the east beneath the so-called `Great Africa Plumeʹ. These results agree with dynamic models where strong local upwellings develop at the edges of Dʹʹ (fast) structures.