3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Magnetotelluric studies across the Damara Orogen and Southern Congo craton(2016-05-10) Khoza, Tshepo DavidArchean cratons, and the Proterozoic orogenic belts on their flanks, form an integral part of the Southern Africa tectonic landscape. Of these, virtually nothing is known of the position and thickness of the southern boundary of the composite Congo craton and the Neoproterozoic Pan African orogenic belt due to thick sedimentary cover. In this work I present the first lithospheric-scale geophysical study of that cryptic boundary and define its geometry at depth. The results are derived from two-dimensional (2D) and three-dimensional (3D) inversion of magnetotelluric data acquired along four semi-parallel profiles crossing the Kalahari craton across the Damara-Ghanzi-Chobe belts (DGC) and extending into the Congo craton. Two dimensional and three-dimensional electrical resistivity models show significant lateral variation in the crust and upper mantle across strike from the younger DGC orogen to the older adjacent cratons. The Damara belt lithosphere is found to be more conductive and significantly thinner than that of the adjacent Congo craton. The Congo craton is characterized by very thick (to depths of 250 km) and resistive (i.e. cold) lithosphere. Resistive upper crustal features are interpreted as caused by igneous intrusions emplaced during Pan-African magmatism. Graphite-bearing calcite marbles and sulfides are widespread in the Damara belt and account for the high crustal conductivity in the Central Zone. The resistivity models provide new constraints on the southern extent of the greater Congo craton, and suggest that the current boundary drawn on geological maps needs revision and that the craton should be extended further south. The storage possibilities for the Karoo Basins were found to be poor because of the very low porosity and permeability of the sandstones, the presence of extensive dolerite sills and dykes. The obvious limitation of the above study is the large spacings between the MT stations (> 10km). This is particularly more limiting in resolving the horizontal layers in the Karoo basin. However the 1D models provide layered Earth models that are consistent with the known geology. The resistivity values from the 1D models allowed porosity of the Ecca and Beaufort group lithologies to be calculated. It is inferred that the porosities values are in the range 5-15 % in the region below the profile. This value is considered too low for CO2 storage as the average porosity of rock used for CO2 is generally more than 10 to 12 percent of the total rock unit volume.Item Cross-border correlation of the Damara Belt in Namibia and equivalent lithologies in northwestern Botswana from potential field and magnetotelluric interpretations(2015) Rankin, WilliamNorthwest Botswana holds a key position for the correlation of the Pan-African mobile belts of southern Africa (i.e. the Damara-Zambezi-Lufilian Orogeny). Phanerozoic cover (Kalahari Group) precludes direct correlation between Proterozoic lithologies of the Damara Belt and thick metasedimentary sequences of northwest Botswana. A combination of new geological and geophysical field observations, interpretation of 50 m resolution aeromagnetic data, and 2.2 km resolution gravity data of Namibia and Botswana, have led to the development of a new sub- Kalahari geological map of the Damara Belt and northwest Botswana. The interpretation of potential field and magnetotelluric (MT) data complemented with both new and published geological data, has improved the identification of the northern and southern margins of the Damara Belt and northwest Botswana, and tectonostratigraphic zones within them. In addition, these correlations have established that the northern margin of the Kalahari Craton on geological maps extends further north than previously noted. The northeast trending Damara Belt is confidently traced into northwest Botswana (Ngamiland) to ~19.5°S, 22.0°E. At this location, in map view, aeromagnetically interpreted structures follow a radial distribution from northwest-striking in the west to northeast-striking in the east. The lithostratigraphic units to the north of this location cannot be confidently correlated with lithostratigraphic units of the Damara Belt. Instead, these units are better correlated with lithostratigraphic units in southern Angola and/or Zambia. The southeastern margin of the Damara Belt is in tectonic contact with the northern margin of the Ghanzi-Chobe Belt as identified in the aeromagnetic images. The Ghanzi-Chobe Belt is correlated with the Sinclair Supergroup in the Rehoboth Subprovince in Namibia. The basal Kgwebe volcanics are correlated with the Oorlogsende Porphyry Member and Langberg Formation and the unconformably overlying metasediments of the Ghanzi Group are correlated with the metasediments of the Tsumis Group. The correlations are based on similar aeromagnetic signatures, lithologies, mineralisation and age dates constrained by carbon isotope chemostratigraphy. Physical property measurements were collected on Meso- to Neoproterozoic lithologies of the Damara Belt, northwest Botswana and Zambia. The measurements included hand held magnetic susceptibility measurements on 303 samples and density measurements on 174 samples. The measurements provide one of the largest physical property databases for Namibia, Botswana and Zambia. In general, the sedimentary units have the lowest magnetic susceptibility values of ~0.207 x 10-3 SI units, respectively. The exceptions are the iron formation and diamictite of the Chuos Formation and conglomerate of the Naauwpoort Formation of 15.2 x 10-3 SI units. The iron iii formation ranges in magnetic susceptibility from 3.34 x 10-3 SI units to 92.0 x 10-3 SI units and the diamictite has a magnetic susceptibility of 7.68 x 10-3 SI units. The igneous lithologies have a density and magnetic susceptibility range from 2.58 g.cm-3 to 3.26 g.cm-3 and 0.001 x 10-3 SI units to 11.6 x 10-3 SI units, respectively. The lower values are associated with pegmatites and rhyolites and the higher values are associated with mafic lithologies and magnetite bearing granites (Omangambo, Salem, Sorris-Sorris and Red Granites). The metamorphic lithologies have the widest range of density and magnetic susceptibility values, between 2.61 g.cm-3 and 3.37 g.cm-3, and -0.299 x 10-3 SI units and 49.5 x 10-3 SI units, respectively. The lower values are associated with low grade metamorphic facies of sedimentary origin, and the higher values are associated with high-grade metamorphic facies of an igneous origin. The first upper crustal-scale interpretation of the Southern African MagnetoTelluric EXperiment (SAMTEX) was developed. The results were derived from 1D Occam inversion models, at depth intervals of 1 – 5 km, 1 – 15 km and 1 – 35 km. The MT data were acquired across the semiparallel, north-south striking DMB, NEN and OKA-CAM profiles in the vicinity of the Namibia – Botswana border between 2006 and 2009. Beneath the MT profiles are two zones of enhanced conductivity, a northern and southern zone. The enhanced conductivity of the northern zone (> 100 Ωm) is associated with individual geological bodies. The southern zone forms an elongated belt of enhanced conductivity (> 300 Ωm) at a depth of less than 5 km. This zone of enhanced conductivity is associated with Proterozoic plate boundaries and subduction zones. Three ~350 km long, north-south trending magnetic profiles were 2D forward modelled to investigate the proposed northward subduction of oceanic crust and subsequently a portion of the Kalahari Plate beneath the Congo Craton. Additionally, the folding pattern of the Ghanzi- Chobe Belt was developed. The interpretation of the magnetic models suggests a northward subduction is a possible cause for the evolution of the Damara Orogen with the regionally eastwest striking negative aeromagnetic anomaly, in northern Namibia, being caused by a thick package (~12 km to 20 km) of metasediments with a modelled magnetic susceptibility of 0. 829 x 10-3 SI units. The Damara Orogen has passed through the subduction-collisional transition but did not evolve into a large-hot orogen. Evidence suggests that the Damara Orogen has gone through the transition of subduction of oceanic crust to terrane accretion (speculated to be represented by the Deep-Level Southern Zone and Chihabadum Complex) and continental collision. However, the doubly vergent wedges did not evolve into an orogenic plateau completing the transition from a small-cold orogen to a large-hot orogen. This is similarly observed in the Alps Orogeny.Item Tectonothermal evolution of the Southwestern central zone, Damara Belt, Namibia(2013-01-31) Longridge, LukeThis is an integrated study of the stratigraphy, deformation, magmatism, and metamorphism in the vicinity of the Ida and Palmenhorst Domes, an area in the southwestern Central Zone of the Damara Orogen, Namibia. The principal aim is to understand the timing of tectonic events through high-precision U-Pb dating of structurally constrained intrusions and anatectic rocks, and link these tectonic events across the Damara Orogen and Pan-African Orogeny. A secondary aim is to compare the Central Zone and Damara Orogen to other collisional orogens. The stratigraphy of the study area is similar to that noted elsewhere in the Central Zone, but the mapped distribution of lithologies differs slightly from previous work. Specifically, Damara Supergroup rocks have been found infolded with the Abbabis Complex, and the stratigraphic positions of certain units in have been locally reclassified. The mapped distribution of lithologies suggests a Type-2 fold interference pattern across the study area. This Type-2 fold interference is confirmed by structural analysis. A D2 deformation event formed strongly S- to SE-verging km-scale recumbent to shallow NW-dipping folds with smaller-scale parasitic folds. The long limbs of these folds are extended, and a number of shear zones are found on these extending limbs, as well as near the contact between the Abbabis Complex and the Damara Supergroup. NE-SW extension is associated with the late stages of D2, and forms a conjugate set of shear bands and a shallow NE-plunging mineral stretching lineation. This D2 event was overprinted by upright to steeply WNW-dipping km-scale D3 folds to form the domes in the study area. Mesoscale fold interference structures are rare, but D2 structures are shown to be consistently reoriented by D3 structures. D3 deformation does not have a strong vergence, and mesoscale D3 folds are rare. D2 and D3 were preceded by a D1 fabric forming event locally observed as rootless isoclinal intrafolial folds, and followed by brittle deformation. The Ida Dome is a fairly simple domal structure formed by the km-scale interference between a shallow NNW-dipping D2 anticline and an upright to steeply WNW-dipping D3 anticline. East of the Ida Dome, NE-trending D3 structures predominate, but are seen to overprint earlier D2 structures. The Palmenhorst Dome is a larger area where Damara Supergroup rocks have been infolded into the Abbabis Complex during D2 deformation. These isoclinal, N- to NW-dipping D2 folds have been refolded by upright D3 folds to form a Type-2 fold interference pattern. D2 structures along the southern margin of the Palmenhorst Dome dip steeply towards the south, in contrast to D2 structures elsewhere. This is interpreted to be the result of a lower-intensity km-scale D2 fold. The orogen-parallel extension and orogen-perpendicular recumbent folding that took place during D2 cannot be explained by previous structural models for the Central Zone and a new model is suggested where these structures form as the result of coeval irrotational NE-SW extension and S- to SE-verging simple shear during extensional collapse of the orogen. A number of intrusive rock types are found in the study area and have been dated using SHRIMP U-Pb. Amphibolite dykes have a chemical affinity to mafic rocks of the Goas Suite, and are suggested to be either pre-Damaran or early Damaran intrusives as they cut the gneisses of the Abbabis Complex, and are affected by D2. They have been dated at 2026.9 ± 2.3 Ma (zircon) or 557.2 ± 7.4 Ma (zircon) with metamorphic overgrowths in this sample giving 520 ± 6.9 Ma. Red, potassic granites emplaced near the contact with the Abbabis Complex and Damara Supergroup contain a D2 gneissic fabric and give ages of 536 ± 7.2 Ma (monazite), and zircons have lower intercept ages of 539 ± 17 Ma and upper intercept ages of 1013 ± 21 Ma. Grey granites are abundant in the study area, and form a continuum from dark grey granites (which are tonalitic to dioritic in composition and contain hornblende and abundant biotite) to light grey granites (which are leucogranitic and contain abundant K-feldspar and minor biotite). These grey granites show a fractionation trend from dark to light varieties, and cross-cutting relationships indicate that the lighter variety is younger than the darker variety. The grey granites show syn-D2 structural relationships and contain a fabric subparallel to the S2 fabric, and which is more pronounced in the darker varieties. They show similarities with granites described by earlier workers, and two samples have been dated at 519.1 ± 4.2 Ma and 520.4 ± 4.2 Ma (zircon). A variety of sheeted granites are found – quartz-feldspar-magnetite pegmatitic granites are associated with grey granites, occur axial-planar to F2 folds, and have metamict zircons which are dated at 530-525 Ma. Garnet (± cordierite) granites are leucocratic, have garnet poikiloblasts, are emplaced axial planar to F2 folds and are also folded and boudinaged by D2. They are associated with pelitic units in the Damara Supergroup and are dated at 520.3 ± 4.6 Ma (zircon) and 514.1 ± 3.1 Ma (monazite). Uraniferous leucogranites found are similar to those widely described in the Central Zone, but metamict zircons give imprecise ages of between 515 and 506 Ma. Pink pegmatitic leucogranites comprise pink perthitic feldspar and milky quartz, are emplaced into more brittle structures and gives an age of 434.4 ± 2 Ma (zircon). Almost all granites analysed appear to be crustal-melt granitoids, with the exception of the darker grey granites, which show a calc-alkaline affinity. No Salem-type granites are found in the study area. In addition, SHRIMP U-Pb analyses of zircons from three Abbabis Complex gneisses give ages of 2056 +11/-10 Ma, 2044 +32/-27 Ma and 2044 +17/-14 Ma, and titanites from an amphibolite sample give ages of 493.4 ± 6.4 Ma. Two anatectic leucosomes from D2 shear zones and shear bands give zircon ages of 511 ± 18 Ma and 508.4 ± 8.7 Ma in spite of high-U zircons. Lu-Hf data on zircons from an Abbabis Complex gneiss gives model ages of ca. 3 Ga, whilst similar data for a grey granite gives a model age of ca. 2 Ga. Zircons from the Abbabis Complex gneiss have variable O-isotopic values, whilst the grey granite gives O-isotopic values of ca. 7‰. These geochonological and isotopic data show that the Abbabis Complex is part of the Congo Craton, and that some amphibolites are pre-Damaran, whilst others may be related to the Goas Intrusive Suite, and represent a phase of early Damaran magmatism. In contrast to the chronology previously presented for the Central Zone, M1 in the study area appears to have occurred at 535-540 Ma, with M2 coeval with D2 deformation at 510-520 Ma. Elsewhere in the Central Zone, NW-verging D2 deformation is dated at 540-560 Ma, and the Central Zone appears to have a diachronous tectonometamorphic evolution along strike. It is suggested here that this represents the preservation of two separate tectonic events in the Central Zone at different crustal levels, one at 540-560 Ma and the other at 520-510 Ma. D3 deformation is suggested to have taken place at 508 Ma, immediately after D2 extension. The Central Zone began to cool following D2, and the 495 Ma titanite age reflects this cooling. Isotopic evidence from this and other studies shows that Damaran granitoids (with 1.5-2.2 Ga model ages) cannot be derived from the Abbabis Complex (with 3 Ga model ages) but must come from an alternative source, suggested here to be Kalahari Craton material subducted below the Congo Craton. Textural studies of a number of pelitic samples indicate syn-D2 low-pressure, high-temperature metamorphism. Differences in observed assemblages between various sample types are due to compositional differences, and samples appear to have reached similar conditions across the study area. Mineral compositional profiles show no prograde zoning, indicating mineral re-equilibration. Orthopyroxene is locally observed, suggesting lower-granulite conditions. This is confirmed by pseudosection modelling of a number of samples, which gives peak conditions of 750-850 °C and 4.5-5 kbar. This modelling shows lower-granulite facies conditions with higher temperatures than previous estimates based on mineral compositional geothermometers, which are affected by re-equilibration. These conditions are sufficiently high for fluid-absent biotite breakdown to form the voluminous anatectic leucosomes and granitoids in the southwestern Central Zone. Pseudosection modelling and phase relationships indicates a low-pressure (ca. 4 kbar) clockwise heating path, with slight decompression at the thermal peak. All metamorphism noted is 520-510 Ma M2 metamorphism, and no petrographic evidence exists for earlier 540-535 M1 metamorphism. This cryptic M1 is suggested to be related to the emplacement of the Goas Intrusive Suite and Salem-type granites early in the orogenic history, whilst M2 may be related to thermal relaxation following crustal thickening early in the orogenic history, but requires an additional heat source. The difference in ages for deformation and metamorphism between the study area and elsewhere in the lower grade portions of the Central Zone is suggested to be related to the preservation of different portions of the orogenic history in different areas. The results of this study together with previous work details a multi-stage evolution for the Central Zone involving subduction, continent-continent collision, crustal thickening, slab breakoff, magmatism, granulite-facies metamorphism and exhumation of the mid-crust. This multistage evolution explains the multiple ages for deformation and metamorphism in the Central Zone. NW-folding and thrusting documented in the Karibib area at 560-540 Ma is related to an early phase of crustal thickening owing to continent-continent collision following a brief period of subduction. Slab breakoff led to asthenospheric upwelling and heating of the lower crust, and produced the Goas Intrusive Suite and Salem-type granites, as well as providing heat for 540-535 Ma M1 metamorphism and the melting of the crust to produce anatectic red granites. SE-verging deformation, extension and granulite facies metamorphism recorded in this study is related to orogenic collapse following crustal thickening, and the heat source for low-P, high-T metamorphism may be highly radiogenic crust that was thickened , which is suggested to be either burial of crust enriched in heat-producing elements, or asthenospheric upwelling owing to delamination of the Congo Craton lithospheric mantle or asthenospheric upwelling owing to the position of the southwestern Central Zone on a major orocline. The events recorded for the Central Zone have been correlated across the entire Damara Orogen, and the timing of events can be correlated along strike into the Zambezi Belt. Events in the Kaoko Belt appear to predate those in the Damara Belt, which appears to also show a similar collisional timing to the Gariep Belt. It is therefore proposed that the Gariep and Damara Belts formed part of a younger orogenic episode to that which formed the Kaoko and Dom Feliciano orogenic belts. The Damara Belt shows similarities to both Alpine-style and Himalayan-style orogens. An evaluation is provided of a channel flow model for the Central Zone, but there are currently insufficient data for the Damara Belt to confirm or repudiate this model. Nonetheless, this study has identified a more complex tectonic history for the Central Zone than previously, with chronological and lithogeochemical evidence for two episodes of deformation and metamorphism that have been linked to the collisional history of the entire Damara Belt and have been correlated with events in other Pan-African belts.