| Summary: | A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2015. === In this thesis the crustal structure beneath two areas of Africa is investigated. In Sudan, the new constraints on the crustal structure beneath the northern part of the Khartoum basin have been obtained. In East Africa, the size of the Tanzania Craton, and the differences between the Eastern and Western branches of the East African Rift System (EARS) have been determined. In southern Tanzania, the debate on the secular variation between Proterozoic and Archean crust has been investigated. The approach used in this thesis involves different data sets and methods.
In first part of the thesis, the crustal structure of the northern part of the Mesozoic Khartoum basin is investigated by using two modelling methods: H-k stacking of receiver functions, and a joint inversion of receiver functions and Rayleigh wave group velocities. H-k stacking indicated that the crust is 33-37 km thick with an average of 35 km, and the crustal Vp/Vs ratio is 1.74-1.81 with an average of 1.78. Similar results were obtained from the joint inversion for Moho depth, as well as an average shear wave velocity of 3.7 km/s for the crust. These results provide the first seismic estimates of Moho depth for a basin in Sudan. When compared to average crustal thickness for unrifted Proterozoic crust in eastern Africa, our results indicate that only a few kilometers of crustal thinning may have occurred beneath the Khartoum basin. This finding is consistent with estimates of effective elastic plate thickness, which indicate little modification of the Proterozoic lithosphere beneath the basin, and suggests that there may be insufficient topography on the lithosphere-asthenosphere boundary beneath the Sudanese basins to channel plume material westward from Ethiopia.
In the second part of the thesis, the uppermost mantle structure beneath East Africa is investigated by using a standard singular value decomposition algorithm model. Results reveal fast Pn velocities beneath the Mozambique Belt to the east of the craton, the Kibaran Belt west of the craton, and beneath the northern half of the Ubendian Belt to the southwest of the craton. These results indicate that the cold, thick lithosphere of the Tanzania Craton extends beneath the Proterozoic mobile belts and the areal extent of the cratonic lithosphere is much larger than is indicated
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by the mapped boundaries of the craton. The results also show that the Pn velocities beneath the volcanic provinces along the Western Branch are not anomalously slow, which indicates little, if any, perturbation of the uppermost mantle beneath them. This is in contrast to the upper mantle structure at depths ≥ 70 km beneath the volcanic regions, which is clearly perturbed. The fast Pn velocities beneath the Western Branch contrast with the slow Pn velocities (7.5-7.8 km/s) beneath the Eastern Branch in Kenya, indicating that the upper mantle beneath the Eastern Branch has been altered more than beneath the Western Branch.
In the third part, the crustal structure beneath two Proterozoic mobile belts, the Usagaran and the Ubendian belts, is investigated by using the Non-Dominated Genetic Algorithm method. In the Usagaran belt, results show an average Moho depth of 35 km for station MAFI and 41 and 42 km for stations MOGR and MIKU, respectively. In the Ubendian Belt, results showed an average Moho depth of 43 km beneath the Ufipa sub-terrane compared to 39 km for Wakole sub-terrane. These results indicate localized thickening in the Ufipa sub-terrane, but not beneath the entire Ubendian Belt. These results indicate that is no clear evidence that Paleoproterozoic crust in East Africa is substantially thicker than Archaean crust.
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