The structure of the upper mantle and transition zone beneath southern Africa from broad-band body waves : research article

Average one-dimensional P and S wavespeed models from the surface to depths of 800 km were derived for the southern African region using earthquake seismograms recorded at stations of the Kaapvaal and South African seismic networks. The use of the phase-weighted stack to infer the presence of later P and S arrivals masked by noise was a novel feature of the data analysis. The presence of a low wavespeed zone starting at 210 to ~345 km depth in the S wave model that is not observed in the P wave model was inferred; the depth of the low wavespeed zone is not significantly different from that inferred below northern Australia using S body waves. The 410 km discontinuity shows similar characteristics to that in other continental regions, but occurs slightly deeper at 420 km. Depletion of iron and/or enrichment in aluminium relative to other regions is the preferred explanation. There is no evidence for a discontinuity at 520 km depth for either P or S waves. The 660 km discontinuity also appears to be slightly deeper than average (668 km), although the estimated thickness of the transition zone is 248 km, similar to the global average of 241 km; the discontinuity is also poorly defined for P waves, but clear for S waves, suggesting that its interpretation of the discontinuity as the transformation of the spinel form of olivine to perovskite and magnesiowüstite may require modification.