/\ dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg for the Degree of Master of Science. Johannesburg 1994. / This dissertation investigates the upper mantle discontinuities beneath South Africa by analysing seismic waves converted from p to Sv recorded at an array of broad band seismometers. Events, with desirable characteristics recorded at each station are transformed to a suitable coordinate system, normalised, and stacked to enhance the weak converted phases. Additional low pass filters are also applied to improve the signal to noise characteristics of the data. No data suitable to processing were found for stations outside the Kaapvaal craton Despite additional problems related to the application of sub-optimum numerical routines to a limited data base, by combining results with those from an independent study two models were derived for the '400 km' and '670 km' discontinuities beneath the Craton. Models for the '670 km' discontinuity accord with a simplified global earth model while models for the '400 km' discontinuity indicate the presence of a low shear velocity layer not supported in a global earth model. Addltlonal broad band seismic data must be acquired to investigate this phenomenon further. / AC2017

Two thermal convection problems of geophysical interest are examined, theoretically. First, convection in the earth's mantle is treated on the basis of a one-dimensional 'strip model'. This model results from further simplification of the well known 'Rayleigh model'. For homogeneous, Newtonian fluids, the strip model yields results similar to those obtained by the Rayleigh method. The strip model is used to determine the critical Rayleigh number for convection in an internally heated two-phase fluid. The critical number depends on the parameters of the phase transition, the physical properties of the fluid, and the depth of the fluid layer. Depending on these factors, a univariant phase transformation may either enhance or hinder convective instability. For the olivine-spinel and spinel-oxides transitions in (MgFe)₂SiO₄ which are thought to take place in the upper mantle, it is shown that the critical Rayleigh number is altered only slightly from the critical number for convection in a fluid with one phase. This result holds both for convection in the entire mantle or convection restricted to the upper mantle. Hence the phase changes are of minor importance regarding the existence of mantle convection in general. A method for estimating the order of magnitude of the displacement of the phase surface as a function of Rayleigh number is outlined for a fluid with only one phase transition. The strip model is also used to treat convection in non-Newtonian fluids obeying a power law rheological equation. If the mantle is governed by a flow law of this type, it appears that convection can take place. Lastly, the procedure for applying the strip model to fluids with variable viscosity and thermal conductivity is outlined. The second convection problem concerns some aspects of convection of fluids in thin vertical fractures in the crust. A steady state model is developed to estimate the magnitude of the mass flow as a function of fracture thickness. It is shown that fractures of the order of a millimeter thick or greater can carry a measurable convective flow. A time dependent model is used to estimate the rate of decay of the mass flow with time. The results indicate that in fractures of the order of a centimeter thick, a measurable decrease of the mass flow takes place after a period of the order of a day. This rapid decay rate suggests that the principal effect of sea water convection in extensive fracture systems which are expected on mid-ocean ridge crests is to cool a volume of crustal rock in the vicinity of the fractures. Circulation of sea water in vertical fractures in the upper crust may provide an explanation of 1) the relatively low conductive heat flow measured at some locations on ocean ridge axes and 2) the very 'noisy' data obtained in the axial zone. / Graduation date: 1972

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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.

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Brandt, Martin B.C. 2011. Imaging the African Superplume – Upper mantle, Tomography and Moment tensor. Ph.D. thesis, Faculty of Science, University of the Witwatersrand, Johannesburg, South Africa. The African Superplume, African Superswell and East African Rift System are amongst the most prominent geophysical features on Earth, but the structure, evolution and interaction between these features is controversial. In my thesis I conducted a range of investigations in an effort to better understand these issues. The thesis presents the investigations into the structure and expressions of these features. These include: (I) A study of the upper mantle shear velocity structure beneath southern Africa to investigate the source of the buoyancy that has powered the Superswell; (II) Statistical hypothesis testing of middle-mantle shear velocity tomographic models to evaluate evidence for links between the Superplume and low velocity features in/near the transition zone; and (III) Computation of three new regional moment tensors for South Africa to assess crustal stress in the Kalahari craton, and its link with mantle structure and dynamics. Waveform data were obtained for the study on the upper mantle shear velocity structure and the moment tensor inversions from the Southern African Seismic Experiment Kaapvaal craton array. For the statistical hypothesis testing on global tomography images, new travel-time data from both global and AfricaArray stations were added to Grand’s global shear velocity data set. The principal findings of this study are summarized below. I. The upper mantle shear velocity structure beneath the Kalahari craton is similar to that of other shields, except for slightly slower velocities from 110–220 km depth. The difference may be due to higher temperatures or a decrease in magnesium number (Mg#). If the slower velocities in the deep lithosphere are due solely to a temperature anomaly, then slightly less than half of the unusually high elevation of the Kalahari craton can be explained by shallow buoyancy from a depleted hot lithosphere. Decreasing the Mg# of the lower lithosphere would increase density and counteract higher temperatures. If an excess temperature of 90 K over a 110 km depth range and a corresponding decrease in Mg# of -2 between the Kalahari and the other cratons are assumed, this would match the seismic velocity difference but would result in essentially no buoyancy difference. We conclude that the high elevation of the Kalahari craton can only be partially supported by shallow mantle buoyancy and must have a deeper source. We determined a thickness of 250±30 km for the mantle transition zone below eastern southern Africa, which is similar to the global average, but the corresponding velocity gradient is less steep than in standard global models (PREM and IASP91). Velocity jumps of 0.16±0.1 km/s (eastern) and 0.21±0.1 km/s (central) across the 410 km discontinuity were found. Our results indicate a thermal or chemical anomaly in the mantle transition zone, but this cannot be quantified due to uncertainty. II. Statistical hypothesis testing on our global tomography images indicated that the African Superplume rises from the core-mantle boundary to at least 1150 km depth, and the upper mantle slow-velocity anomaly extends from the base of the lithosphere to below the mantle transition zone. The model that links the African Superplume with the slow-velocity anomaly in the upper mantle under eastern Africa has an equal probability to an alternative hypothesis with a thin slow-velocity “obstruction zone” at 850 to 1000 km depth. III. Finally, we calculated three regional moment tensors for South Africa and made progress towards resolving the discrepancy between the local and moment magnitudes we observe for the region. Moment tensors/focal mechanisms in southern Africa change from normal faulting (extension) in the northeast near the East African Rift to strike-slip faulting in the southwest. This confirms previous studies stating that not only eastern Africa, but also southern Africa is being actively uplifted by lithospheric modification at its base and/or the African Superplume.

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 iv 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.

xv, 151 p. : ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / To explain the origin of several distinct aspects of the Galápagos volcanic hotspot, such as the broad geographical extent of recent volcanism and the unusual pattern of geochemical anomalies, we conducted seismic tomography studies of the upper mantle and crust beneath the Galápagos Archipelago. The studies combine measurements of group and phase velocities of surface waves and delay times of body waves. We find that upper mantle seismic velocities are lower than those beneath other regions of comparable age in the Pacific and consistent with an excess temperature of 30 to 150°C and ∼0.5% melt. We attribute the excess temperature and presence of melt to an upwelling thermal mantle plume. Crustal seismic velocity is up to 25% lower than that of very young crust at the East Pacific Rise (EPR) and is comparable to that of Hawaii, which we attribute to heating by increased intrusive activity above the Galápagos plume and the construction of a highly porous volcanic platform. In addition, we find that the Galápagos hotspot is underlain by a high-velocity region whose thickness varies from 40 to 100 km. The tomographic images reveal that the upwelling mantle plume tilts northward (towards the nearby Galápagos Spreading Center) as it rises and then spreads laterally when it reaches the bottom the lid. The lid, which we attribute to residuum from melting, is thickest where it is farthest from the spreading center, suggesting that ridge processes may affect the generation and amount of thinning of the residuum layer. In addition, the thickness of the lid correlates well with the geographical pattern of geochemical anomalies of erupted lavas, suggesting that the lid may control the final depth of decompression melting. We conclude that many of the distinct characteristics of the Galápagos can be attributed to the interaction of the upwelling plume with the lid and the nearby ridge. We further suggest that the ridge affects the geometry of plume upwelling in the upper mantle and also the pattern of lateral spreading of the plume due to its effect on the thickness of the residuum layer. This dissertation includes previously published co-authored material. / Committee in charge: Dr. Douglas R. Toomey, Chairperson; Dr. Eugene Humphreys, Member; Dr. Emilie Hooft Toomey, Member; Dr. Paul Wallace, Member; Dr. John Conery, Outside Member

Ces travaux de thèse apportent de nouvelles contraintes sur la concentration et le comportement de l’H dans le manteau lithosphérique et reposent sur l’étude pétro-géochimique de trois séries de xénolites de péridotites à spinelle, associant les concentrations en élément majeurs et en traces y compris l’H dans les minéraux dits anhydres (les NAMs, ici, olivine, pyroxènes) et hydratés (amphibole). Les effets de la remontée des xénolites dans du magma hôte sur les concentrations en H des NAMs ont pu être étudiés sur une série de xénolites de péridotite du champ volcanique d’Eifel (Allemagne). Une variation intra cristalline de concentration en H peut être identifiée dans l’olivine et non dans les pyroxènes coexistant. Ces profils de concentration en H peuvent être utilisés pour estimer des vitesses de remontées des magmas. Dans le cas des volcans étudiés, ces vitesses sont estimées entre 3.5 et 12 m.s-1. Ces résultats suggèrent que les pyroxènes sont de meilleurs proxy que l’olivine pour quantifier la concentration mantellique de l’H. Les xénolites de Ray Pic (Massif Central, France) ont permis de discuter dans un contexte de point chaud, l’effet de la fusion partielle et du métasomatisme à grande échelle sur les concentrations en H des NAMs. Les concentrations en H des minéraux ne suggèrent pas de lien avec le métasomatisme subit, qu’il soit modal, cryptique, à rapport liquide/roche élevé ou faible. Cependant, en comparant les concentrations en H avec un marqueur de la fusion partielle (Yb du cpx), l’H semble avoir un comportement similaire à une MREE (e.g., Sm ; D(cpx/liquide)~0.29). Enfin, des xénolites composites associant une péridotite accolée à un agent métasomatique ont permis de cibler l’influence du métasomatisme de petite échelle (pluri-centimétrique). L’interaction magma roche identifiée sur une harzburgite en contact avec du basalte montre dans les cas des olivines, des variations chimiques couplé entre éléments majeurs en fonction de leur proximité au filon et les concentrations en H. Plus l’olivine se rééquilibre avec le liquide moins elle contient d’H. Parallèlement dans cette étude, trois échantillons présentent une lherzolite en contact avec une pyroxénite avec 14% d’amphibole, une clinopyroxénite avec 40 % d’amphibole et une amphibolite (98% d’amphibole). Cette contiguïté avec un filon métasomatique permet l’étude du comportement de l’H en contexte de percolation en bordure de filon. Chaque échantillon présente des concentrations en H homogènes pour chacune des phases minérales. Cependant plus les filons contiennent d’amphibole moins il y a d’H dans les NAMs. D’autre part, une nouvelle fois, la corrélation positive entre les concentrations en H des NAMs et le Sm(cpx) en tant que marqueur de métasomatisme suggère que l’H se comporte comme une MREE.En conclusion, les minéraux des spl-harzburgites contiennent en moyenne un peu plus d’H que ceux des spl-lherzolites. Les concentrations en H des olivines sont sensibles à la dévolatilisation lors de la remontée dans le système magmatique et le rééquilibrage avec le magma. Au contraire, les concentrations en H des pyroxènes, spécialement l’opx, sont très homogènes suggérant des concentrations mantelliques. Le comportement de l’H lors de la fusion partielle et du métasomatisme reste complexe ; nos données suggèrent que l’H suit les MREE tel que le Sm. / This thesis provides new constrains on H concentrations and H behaviour in the lithospheric mantle and is based on a petro-geochemical study on 3 spinel-peridotite xenoliths series with major and traces elements analyses, including H in nominally anhydrous minerals (NAMs, olivine and pyroxenes) as well as hydrous minerals (amphibole).Ascent effects through a magmatic system on H concentration of NAMs are studied for a xenoliths series from the Eifel volcanic field (Germany). Intracrystalline variation in H concentration are observe in olivine but not in the coexisting pyroxenes. Such H concentration profiles are used to calculate the rate of magma ascent. For the studied volcanoes, the calculated rate of magma ascent is between 3.5 and 12 m.s-1. Such H concentration variations imply a devolatilisation affecting only olivine, whereas the pyroxenes are homogeneous and then can be used as a better proxy for mantle H concentrations.Ray Pic xenoliths (French Massif Central) belongs to a mantle plume setting, implying the possibility to assess the effect of partial melting and large scale metasomatism on H concentration of NAMs. The H concentrations determined do not suggest a strong link with the suffered metasomatism whether modal, cryptic, at low or high melt rock ratio. However, using H concentrations and a marker of the partial melting (Yb in cpx), H seems to behave as a MREE (e.g., Sm, D(cpx/melt) ~ 0.29).Finally, the composite xenoliths with a peridotite adjacent to a metasomatic agent allow to target the influence of small scale metasomatism (pluri-centimetric). A magma-rock interaction between a harzburgite and a basaltic patch shows that, for olivines, chemical variations in major element as a function of olivine proximity to the vein, is coupled to H concentrations of NAMs. More the olivines are close to equilibrium with the basalt, more the H concentrations are low. Alongside in this study, three samples consist of a lherzolite adjacent a pyroxenite (14% amphibole), to a clinopyroxenite (40% amphibole) and to an amphibolite (98% amphibole) respectively. This special relationship with a metasomatic vein allows to study the behaviour of H during wall rock percolation. Each sample display homogeneous H concentration within each NAMs. However, H concentration is inversely correlated to modal content in amphiboles in the peridotite. Furthermore, a positive correlation between H in NAMs and Sm(cpx) here as a marker of metasomatism suggests, again, that H behaves as a MREE.To conclude, minerals from harzburgite contain in average a bit more H than the one in lherzolite. The H concentration in olivine are sensitive to degassing during magma ascent toward the surface and reequilibrium with magma. On the contrary, H concentration in pyroxenes, especially opx, are very homogeneous suggesting mantle concentration. The behaviour of H during partial melting and metasomatism is complex. However, our data suggest that H broadly follows MREE.