Thermal, hydrous, and mechanical states of the mantle transition zone beneath southern Africa

Observations of P-to-S conversions from the seismic discontinuities near 410 and 660 km depth reveal the shoaling of the 410-km discontinuity and the deepening of the 660-km discontinuity beneath the Archaean cratons in southern Africa; consequently, the mantle transition zone is 20 km thicker than beneath post-Archaean regions and the global average. The discontinuity structure, combined with tomographically imaged seismic velocities, suggests a relatively cold (vTV100 K) and water-enriched (vOHV20^35% saturation) transition zone beneath the cratons. The thickerthan-normal transition zone correlates with surface geology and does not follow the track of African plate motion as numerical simulations of small-scale convection imply. The observations suggest that cold thermal downwelling is not the primary cause of the anomalously thick transition zone beneath the cratons, although it may exist in a confined area near the boundary between the Kheis thrust belt and the Namaqua-Natal belt. If thermal downwelling exists, its temperature anomaly at the 660-km discontinuity must be below the levels detectable by the method used in this study (vTV60 K). The spatial correlation between surface geology, high velocities beneath the cratons, and the transition zone structure indicates that the cratonic keels extend to the base of the upper mantle, substantially deeper than most estimates. Our results demonstrate the need to closely consider the effect of water content on seismic velocities beneath deep continental roots. 6 2003 Elsevier B.V. All rights reserved.