EVOLUTION OF SOUTHERN AFRICAN CRATONS BASED ON SEISMIC IMAGING

The stability of the lithosphere of stable continental cratonic areas is a matter of debate. Geochemical results indicate that the crust and mantle of these areas have remained assembled for periods exceeding 2 Gy, but geophysical data has so far not been able to contribute to the subject. We present a new seismic model for the structure of the crust and lithospheric mantle of the Kalahari Craton, constrained by seismic receiver functions and finite-frequency tomography based on the seismological data from the South Africa Seismic Experiment (SASE). The combination of these two methods provides high vertical and lateral resolution.

The main results obtained are:

(1) the crustal structure is highly heterogeneous in terms of thickness, composition (as indicated by Vp/Vs), and sharpness of the discontinuities [Youssof et al. 2013],

(2) observation of unexpectedly strong crustal azimuthal anisotropy, which may amount to 35-40 % of the total anisotropy in the crust and the mantle as judged by comparison of SkS splitting data and modeling of crustal anisotropy. Similar direction of the fast axes for total and crustal anisotropy indicates that the lithosphere has remained assembled as one unit since formation of the craton, and

(3) seismically fast lithospheric keels are imaged in the Kaapvaal and Zimabwe cratons to depths of 300-350 km. Relatively low velocity anomalies are imaged beneath both the paleo-orogenic Limpopo Belt and the Bushveld Complex down to depths of ~250 km and ~150 km, respectively. The latter observation indicates a deep reaching origin of the Bushveld magmatic structure.