Upper mantle SH- and P-velocity structures and compositional models beneath southern Africa

Abstract

We constrain SH and P wave velocity structures in the upper mantle beneath southern Africa, using triplicated phases recorded in the epicentral distance range of 11°–28° for one shallow event. We then explore thermal and compositional models appropriate for explaining the inferred seismic structures in the region. Both SH and P wave data suggest presence of a low velocity zone with velocity reductions of at least − 5% for S waves and − 2% for P waves beneath a 150 km thick high-velocity lithospheric lid. Seismic observations also suggest that the P/S ratio is larger (1.88) in the transition zone than in the lithospheric lid (1.70). The inferred P wave velocity jump across the 660-km discontinuity is small (< 4%), while the inferred SH wave velocity jump across the discontinuity is comparable to that in the Preliminary Reference Earth Model. The low velocity zone can be explained by a high temperature gradient of 6 ° C/km or presence of partial melt. Partial melt would require presence of water or other volatile elements in the depth range of the low velocity zone to lower the mantle solidus. The different P/S velocity ratios between the lithospheric lid and the transition zone can be explained by a difference in aluminum content of mantle composition, with values of 1% in the lithospheric lid and 4% in the transition zone, respectively. The inferred P and SH velocity jumps suggest a bulk sound velocity decrease of 3.4% across the 660-km discontinuity. Such a large decrease of bulk sound velocity is possible only if the bulk modulus of perovskite is similar to those of ringwoodite and garnet, a result remained to be confirmed by experimental or theoretical studies. The presence of the low velocity zone with large velocity reductions beneath southern Africa suggests existence of a low-density anomaly beneath the lithospheric lid, which may provide an explanation to the observed localized uplift in the “African Superswell”.