Geophysical studies of the crust and uppermost mantle of South Africa.
Date
2014-03-05
Authors
Kgaswane, Eldridge Maungwe
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Abstract
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.
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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.