TOMOGRAPHIC STUDY OF UPPER-MANTLE STRUCTURE OF THE SOUTH-AFRICAN CONTINENT, USING WAVE-FORM INVERSION

Wave-form inversion has been applied to S wave and surface wave data from a broad-band portable array. The stations were located on the Kaapvaal Craton and adjacent Namaqua mobile zone. Using 10 modes, the vertical-component wave-forms were fitted in the frequency window from 0.05 to 0.02 Hz. Two events located roughly on the array's great circle were processed both forwards and in reverse. The background model for these inversions was the Preliminary Reference Earth Model (PREM). The southern part of the profile is located mainly on the mobile zone, and this has an S wave velocity-depth relation similar to that of the reference model. The velocity-depth distribution of the profile which is entirely within the Kaapvaal Craton has an S wave velocity of about 4.3 km s-1 down to a depth of 80 km; this is followed by a positive velocity gradient and a correspondingly higher velocity of about 4.6 km s-1 between 110 and 220 km. Published results of the structure of cratons within the African continent indicate positive velocity gradients down to depths of 110-120 km and negative gradients down to 200 km depth.