Imaging geometry, velocity, and anisotropy of the 'African Anomaly'

The "African Anomaly" is a prominent low velocity province in the lower mantle beneath Africa. I have determined the geometry and geographic distribution of a very low velocity province (VLVP) at the base of the "African Anomaly" near the core mantle boundary (CMB). The VLVP exhibits an "L-shaped" form stretching from the South Atlantic Ocean to the Indian Ocean, occupying an area of about 1.8 x 107 km2 at the CMB. Waveform modeling analyses with the SH hybrid method suggest that the VLVP has rapidly varying geometries and sharp borders as well as a linear gradient of shear velocity reduction from -2% (top) to -9% - -12% (bottom) relative to the Preliminary Reference Earth Model. These seismic characteristics unambiguously indicate that the VLVP is compositionally distinct, and can best be explained by partial melt driven by a compositional change, possibly produced early in the Earth's history. I have imaged the geometry and P- and S-velocity structures for the "African Anomaly" along the great arc from the East Pacific Rise to the Japan Sea. The "African Anomaly" exhibits a "cusplike" shape with both flanks tilting toward the apex beneath southern Africa, and it continuously extends about 1300 km upward into the mid-lower mantle. The average Vs reductions are about -5% in the base above the CMB and about -2% - -3% in the mid-lower portion above the base. A uniform Vs to Vp perturbation ratio of 3:1 can best explain the P wave data. The geometry and seismic features indicate that the mid-lower mantle portion of the "African Anomaly" is an integrated component of the VLVP at the base, and might also be compositionally distinct and geologically stable. After the geometry and seismic structure were imaged for the "African Anomaly", I have studied the anisotropy associated with the VLVP at the base of the "African Anomaly". I measure the apparent splitting parameters (the fast polarization direction and the delay time) for high-quality SKS and SKKS waveforms of deep earthquakes. The medium in the interior of the VLVP may be isotropic or vertically transverse isotropic due to the good station correlation of apparent splitting parameters and the consistency of apparent splitting parameters for SKS and SKKS waves of the same earthquake when seismic data sample in the interior of the VLVP. However, the medium near the borders of the VLVP has to be anisotropic in order to account for the lack of station correlation of apparent splitting parameters and the inconsistency of apparent splitting parameters for SKS and SKKS waves of the same earthquake when seismic data sample near the borders of the VLVP. The anisotropy near the borders of the VLVP can be generated by the lattice-preferred orientation of anisotropic aggregates, revealing a complex mantle flow in the surrounding areas.