Bibliography: pages 191-202. / The surface heat balance of the Benguela upwelling area on the west coast of southern Africa is analysed. Measurements of the components of the heat balance were made in the St Helena Bay area from 14-21 October 1986. Additional long-term data was obtained from Alexander Bay and Cape Town. An average net heat gain of 227 W.m⁻² was received over the eight days of the field study. The presence of cold water determined that latent heat loss by the sea surface was small, while the sensible heat flux represented a small gain by the sea. These two turbulent heat fluxes are roughly equal and opposite and therefore approximately cancel each other. Use of a model, assuming idealised conditions, indicated that most turbulent heat exchange between the air and takes place in the nearshore region where air-sea contrasts are greatest. The net radiation was found to provide a good estimate of the total heat balance, thus the major contributing term to a high heat balance over the Benguela area is the input solar radiation. Minimal synoptic variation in the heat balance during the eight-day field programme was observed, but additional global radiation data analysed revealed that synoptic variations over the 3-6 day period are in fact more significant than the longer term seasonal variations. Both synoptic and seasonal variations in the heat balance are greater in the south than in the north. The high heat flux into the sea surface is capable of increasing the temperature of the upwelled water at a fairly rapid rate. During summer the heat exchange is capable of increasing the temperature of the upper 10 m mixed layer by as much as 0.65°C over one day. This input heat is used to realise the high biological potential of the upwelled waters.

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.

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. Johannesburg, 2016 / The Mesoproterozoic Pilanesberg Complex, South Africa, is the world’s largest alkaline intrusive complex. Mapped geological field relationships suggest the Complex has circular inward dipping layers. However, it is unclear how the dipping layers extend at depth. As a result, the 3D geometry of the Pilanesberg Complex is unknown. Modelling of the Pilanesberg Complex uses 2D forward models as well as 3D forward and inversion, gravity and magnetic data models, to set limits on the 3D geometry of the Pilanesberg Complex. The 2D Bouguer gravity models and geology maps indicate that some of the Bushveld Complex Main Zone shifted to the west of the Pilanesberg Complex during emplacement. This, and a highly faulted country rock, accounts for a portion of how the host rock was able to accommodate the Pilanesberg Complex intrusion. The geometry of the Complex is explored with test gravity models where the model of outward dipping and vertically dipping cylinders are unable to match the Bouguer gravity signal over the Complex, but the inward dipping model matched the data to provide a possible solution for the geometry of the Complex. The Pilanesberg Complex geometry is modelled with 3D magnetic inversion, 3D forward gravity models and 2.5D gravity test profiles that were all constrained by the surface geology. The different models correlate so that best data fit for the Complex is represented by an overall inward dipping structure. Surface geological measurements indicate that the northern edge of the Complex dip out to the north. The 3D forward modelling was able to produce a positive solution that matched the gravity data with a northward dipping northern edge. The dipping northern edge is also observed on the University of British Columbia, UBC, 3D gravity inversion and the Euler deconvolution gravity profile solutions. The depth of the Pilanesberg Complex from 3D forward gravity modelling is estimated to be between 5 and 6 km. The Complex is suggested to have undergone block movement where the northern block and southern block are separated by the 30 km long Vlakfontein fault, which bisects the Complex from the north-east to the south-west. The image processing contact depth, Euler deconvolution solutions and the 3D Voxi inversion model suggest that the fresh bedrock is closer to surface in the north, while the southern block appears to be approximately 1km deeper than the northern block. The northern dip and block movement are explained by complicated structural events that include trap door graben settling which hinged on the northern edge as well as faulting and external block movement during a regional lateral extensional event. The Pilanesberg Complex intruded during a larger system of alkaline intrusions, known as the Pilanesberg Alkaline Province. The intrusions are associated with the Province due to their ages and chemical affinity. This Province includes two dyke swarms that radiate to the north-west and south of the Pilanesberg Complex, as well as smaller circular clinopyroxenite intrusions throughout the Bushveld Complex. The Pilanesberg dyke system and the circular clinopyroxenite intrusions are reversely magnetised with IGRF corrected values ranging between -150 to -320 nT compared to the normally magnetised 166 to 330 nT values of the Pilanesberg Complex. This suggests that a magnetic reversal occurred between the emplacement of the Pilanesberg Complex and the dyke System. The age data of the Complex and dyke Swarm suggest a magnetic reversal could have occurred between the emplacement of the Pilanesberg Complex and the Pilanesberg dyke System. The Complex is dated at 1602 ± 38 Ma and 1583 ± 10 Ma, from two white foyaite samples from the southern edge (using 40Ar/39Ar amphibole spectrum analysis). These ages are vastly different from previously reported ages, which ranged between 1200 Ma and 1450 Ma (Harmer R., 1992; Hansen et al., 2006). The error analysis has improved considerably from the published dates making the proposed dates plausible for the intrusion of the Pilanesberg Complex as the first and main intrusion of the Pilanesberg Alkaline Province. The Pilanesberg dyke System intruded much later between 1219 ± 6 Ma to 1268 ± 10 Ma for the red syenite dyke samples (using 40Ar/39Ar on feldspars spectrum analysis) and 1139 ± 18 Ma obtained for the grey syenite dyke (using 40Ar/39Ar on amphiboles inverse isochronal analysis). The dyke Swarm dates are significantly younger than the previously published ages for the dykes, which were between 1290 Ma and 1330 Ma (Van Niekerk, 1962; Emerman, 1991). / LG2017

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