ETD Collection

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Author: search.filters.author.Kgaswane, Eldridge MaungweSubject: search.filters.subject.Earth - Mantle.

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    Geophysical studies of the crust and uppermost mantle of South Africa.
    (2014-03-05) Kgaswane, Eldridge Maungwe
    The general aim of this thesis is to investigate heterogeneity in the structure of the crust and uppermost mantle of Archaean and Proterozoic terrains in southern Africa and to use the findings to advance our understanding of Precambrian crustal genesis. Teleseismic, regional and local seismic recordings by the broadband stations of the Southern African Seismic Experiment (SASE), Kimberley array, South African National Seismograph Network (SANSN) and the Global Seismic Network (GSN) are used in the inversion procedures to address the aim of this thesis. In the first part of the thesis, the nature of the lower crust across the southern African shield is investigated by jointly inverting receiver functions and Rayleigh wave group velocities. The resultant Vs models show that much of southern Africa has a lower crust that is mafic in composition, whereas the western parts of the Kaapvaal and Zimbabwe Cratons have a lower crust that is intermediate-to-felsic in composition probably due to rifting. The second part of the thesis evaluates the “dipping-sheet” and “continuous-sheet” models of the Bushveld Complex using better-resolved seismic models derived in a two-step approach, employing high-frequency Rayleigh wave group velocity tomography and the joint inversion of high-frequency receiver functions and 2–60 sec Rayleigh wave group velocities. The resultant seismic models favor a “continuous-sheet” model of the Bushveld Complex, although detailed modelling near the centre of the Complex shows that the subsurface mafic layering could be disrupted. The third part of the thesis, is focused on jointly inverting high-frequency teleseismic receiver functions and 10–60 sec Rayleigh wave group velocities to place shear wave velocity constraints on the source of the Beattie Magnetic Anomaly (BMA) at depth and to evaluate existing geophysical models of the BMA source. The resultant Vs models across the BMA suggest the BMA source to be at upper to middle crustal depths (5–20 km) with high velocity layers (≥ 3.5 km/s). Further to this, is a lower crust that is highly mafic (Vs ≥ 4.0 km/s) and a crust beneath the BMA that is on average thicker than 40 km. Plausible models of the BMA source are massive sulphide ore bodies and/or mineralized granulite-facies mid-crustal rocks and/or mineralized Proterozoic anorthosites. v Overall, the findings in this research project are consistent with the broad features of a previous model of Precambrian lithospheric evolution but allows for refinements of that model.