Geophysical studies of the craton will provide a larger-scale view of the structure of the deep cratonic mantle and its relation to underlying convecting mantle. Compared to the mantle beneath the ocean basins, the mantle beneath Archean crustal blocks has anomalously fast seismic velocities extending to depths as great as 200 to 400 km [Jordan, 1975]. The cratons of southern Africa show the high velocity signature in the global maps, but the horizontal resolving power of the best global data sets is only on the order of 3000 km, which is too poor to allow relationships to be established between mantle and crustal structure, or between mantle structure and the mantle xenolith suite. Some preliminary results on the reflectivity and anisotropic structures of the Kaapvaal craton have been obtained from the analysis of digital seismic recordings at a few permanent and temporary sites. Vinnik et al. [1995] propose that the observed anisotropy is a response to present day plate motions, but an alternative interpretation is that the anisotropy is ancient and was produced during craton assembly [Silver and Chan, 1988, 1991]. The essential features of results from these studies include significant topography on the base of the lithosphere, a "cratonic" low velocity zone beneath the base of the mechanical lithosphere, and a pronounced seismic velocity reversal at about 350 km depth.
An array of portable broadband seismic stations was deployed in April/May 1997 along a NNE-SSW transect about 1800 km long by 600 km wide in southern Africa. The array extends in a swath from the Cape Fold Belt and Namaqua-Natal mobile belt in South Africa, through the heart of the Kaapvaal craton, across the Limpopo Belt, and into the Zimbabwe craton of Zimbabwe and Botswana. Seismic observations continued for a period of two years. About half the stations were redeployed at the end of the first year, for a total observing network of 82 stations.
The seismic array of REFTEK/STS-2 seismograph systems is designed to image the crust and mantle beneath southern Africa at a resolution of better than 100 km. A major focus of the studies will be on the structure of the deep mantle keel of the cratons the nature of the contact between the cratonic keel and the underlying mantle to depths of several hundred km. The studies include body and surface wave tomography, shear-wave anisotropy, discontinuity structure from converted phase analysis, and travel time analysis of local and regional events. Of particular interest is the degree of vertical coherence between surface geology and mantle structure. The seismic results will be integrated with detailed petrologic, geochemical, and petrofabric studies of mantle and crustal nodules rafted to the surface in diatremes.
