Seismic Structure of the Tectosphere beneath Southern Africa
The Southern Africa Seismic Experiment was designed to image the deep structure of cratonic roots. It is part of a multidisciplinary research project by Carnegie Institution, MIT, southern African academic institutions and industry collaborators to study the structure, composition, and evolution of the cratons and adjacent mobile belts of southern Africa. An array of fifty five portable broadband REFTEK/STS-2 seismic stations was deployed in April/May 1997 along a NNE-SSW transect about 1800 km long by 600 km wide in southern Africa. Approximately half the station were redeployed to new sites in April/May, 1998. A total of eighty two stations have been occupied in a swath extending from the Cape Fold Belt and Namaqua-Natal mobile belt in South Africa, through the Kaapvaal craton, across the Limpopo Belt, and into the cratons of Zimbabwe and Botswana. P-wave and S-wave delay times have been analyzed to obtain tomographic images of mantle structure beneath the array to depths in excess of 1000 km. Receiver function and surface wave inversions have been used to constrain crustal thickness and mean velocity across the array. Results based on the first several months of data show that mantle velocities beneath the craton are significantly higher than those of the adjacent Proterozoic belts, with shear velocity constrasts disproportionately large. Mantle velocities vary across the craton itself at about the 1 percent level. Highest mantle velocities are found in the southern part of the Kaapvaal craton and in the region of the Zimbabwe craton of NE Botswana and SW Zimbabwe. Clear evidence of cratonic root structures is seen to extend to at least 200--250 km depth, perhaps substantially more, with the most prominent root structures generally underlying diamondiferous regions. Preliminary results indicate a prominent low velocity feature in the mantle below about 700 km in the general region beneath the Bushveld province. The Bushveld province within the Kaapvaal craton is marked by relatively lower velocities in the mantle root. While the velocity contrast with adjacent craton is comparatively small, it does correlate also with slightly greater crustal delay times and with a zone of null SKS splitting (Gao et al., this session). The seismic results coupled with evidence of younger Re/Os model ages of mantle nodules suggests partial cratonic disruption during formation of the Bushveld.
D. E. James et al., "Seismic Structure of the Tectosphere beneath Southern Africa,", vol. 79 American Geophysical Union (AGU), Dec 1998.
AGU Fall Meeting (1998: Dec. 1, San Francisco, CA)
Geosciences and Geological and Petroleum Engineering
Article - Conference proceedings
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01 Dec 1998