Seismological constraints on deep mantle structures

The lowermost hundreds of kilometers of the mantle is a stronly heterogeneous region. This region appears to be characterized by two large provinces of distinctive slow velocities, one beneath the Pacific and one beneath Africa. Besides these two large slow provinces,we map a distinctive small province of slow velocity beneath Russia through direct waveform evidence. The pile is less than 1000 km across, and thus much smaller than the Pacific and African provinces. Its height is hundreds of kilometers and its velocity reduction suggests it is composed of the same material as the large provinces. Within these large provinces, there are smaller patches with ultra-slow velocities. Beneath Hawaii, at the northern edge of the Pacific province we find a widespread thin zone of ultra-low velocities. This is the first time the three-dimensional extent of such a zone is mapped. The constraints on morphology come from strong postcursors to seismic phases, delayed by 30 to 50 seconds. Given its location, one can speculate on this zone being an anchor to a whole-mantle plume beneath the Hawaiian hotspot and the source of geochemical anomalies in Hawaiian basalts. The lowermost mantle is also a region where strong seismic anisotropy is observed. Around the southern margin of the African province, we map strong variations of anisotropic velocity structure. These could be interpreted as evidence for strong flow outside the region of slow velocity, and insignificant flow within.