Deep seismic structure of the Kaapvaal craton

The Kaapvaal craton is remarkable for the intensive Jurassic-Cretaceous kimberlite magmatism. There are indications that the hot-spot heating of the upper mantle in southern Africa continued in the Cenozoic. However, surface wave data, in spite of their sensitivity to temperature, failed to provide any evidence of thermal agitation at depths less than 350 km beneath the Kaapvaal craton (Bloch et al., 1969). We investigated the S velocity structure in the depth range of the mantle transition zone, with the aid of teleseismic P-to-S converted phases. Our analysis suggests that a layer of anomalously low S velocity is present in the 370-470 km depth interval beneath the Kaapvaal craton. The anomalous layer is most likely of thermal origin; its upper boundary may separate the deep mantle root of the craton from the underlying mantle. The anomaly could arise beneath the craton already in the Mesozoic, and melting within this layer could contribute to the kimberlite magmatism.