Browsing by Author "De Wit, Maarten"

Now showing 1 - 12 of 12
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    African river basins : their present geometry and recent past as a framework for their evolution
    (2004) Stankiewicz, Jacek; De Wit, Maarten
    Fractals and scaling laws abound in nature, and it is said that geometry of river networks and basins is an epitome of this. This study investigates how on the tectonically unique African continent, scaling parameters, and in particular deviations from 'perfect fractal patterns' relate to parameters like underlying geology, climate, and vegetation through which the river flows. Stream and basin patterns are also used to reconstruct the past network geometry of rivers, and to shed some light on the drainage evolution of major African rivers.
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    Denudation rates and geomorphic evolution of the Cape Mountains, determined by the analysis of the in situ-produced cosmogenic 10BE
    (2012) Scharf, Taryn E; De Wit, Maarten; Codilean, Alexandru T
    Southern Africa is host to a unique mountain system, the Cape Mountains, which includes the coastal Cape Fold Belt (CFB) and an inland Escarpment. Apatite fission track analysis has shown that this mountain system is an erosion feature, exhumed from beneath 2-7 km of overburden by large-scale denudation processes affecting the subcontinent during Gondwana break-up (ca. 140 – 65 Ma). Despite its antiquity and location on a passive continental margin, the ruggedness of the present-day topography of the Cape Mountains compares to that of the world’s active orogens. The coastal Cape Mountains are traversed by deeply-incised, meandering rivers that cut canyons through the most resistant quartzite ridges of these mountains, perpendicular to their structural grain inherited from the CFB. The evolution of this landscape is poorly understood, because little quantitative data exists on the denudation history of the Cape Mountains. This study presents the first in situ-produced cosmogenic 10Be inventories determined for quartz from catchment sediments and bedrock surfaces within the coastal Cape Mountains, with which to quantify denudation rates, exposure ages and the recent geomorphic evolution of these Cape Mountains. River sediments sampled from catchments within the Langeberg and Swartberg Ranges of the Western Cape, as well as bedrock from the Tradouw River traversing the Langeberg Range, were analysed. In addition, charcoal from alluvial material was collected for radiocarbon dating. Catchment-averaged denudation rates reported from these mountains range between 2.1 ± 0.3 and 6.9 ± 1.9 m·Myr-1. These are amongst the lowest reported rates globally, despite the rugged terrain of the mountain system. The spatial consistency between the low denudation rates suggests a landscape approaching geomorphic steady-state. This finding is best attributed to lithological control on denudation rates in a tectonically quiescent environment, and a relatively dry climate.
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    Geographic information system (GIS) integration of geological, geochemical and geophysical data from the Aggeneys base metal province, South Africa
    (1994) Naicker, Isayvani; De Wit, Maarten
    Geographic Information System (GIS) technology aids in storage, manipulation, processing, analysis and presentation of spatial data sets. GIS can effectively interrogate large multidisciplinary exploration data sets in the search for new mineral exploitation targets. A spatial database, the AGGeneys Exploration Database (AGGED), has been created, comprising exploration data gathered during two decades of exploration for base-metals in the Aggeneys area, Bushmanland, South Africa. AGGED includes data extracted from analog maps, as well as digital remotely sensed sources, stored in vector and raster data structures, respectively. Vector data includes field based observations such as the extent of outcropping geological units, litho- and chrono-stratigraphic data; structural data; laboratory data based on regional geochemical stream sediment and traverse sampling; cadastral data and known mineral occurrences. Raster data includes Landsat satellite TM imagery and airborne magnetic data. Spatial variation within single data maps are examined. Spatial correlation between three different data maps are facilitated using colour analysis of hue, saturation and value components in a perceptual colour model. Simultaneously combining lead and zinc data with Landsat TM and geophysical magnetic data spatially delineates four new "geoscience" anomalies in the area under investigation. Two distinctive anomalies occur on the farms Aroams and Aggeneys. The Aroams anomaly (GSAl) has not been previously recognised, whereas the Aggeneys anomaly (GSA2) has been located before. The two other "geoscience" anomalies, on the farm Haramoep (GSA3 and GSA4 ), are slightly less distinct. Overlaying fold axial trace patterns and anomalies on the farm Haramoep, indicate that F2 and F3 fold structures are closely associated with these two anomalies. The location of the Aroams anomaly occurs along the same east-west trend of the four known major ore-deposits viz. Big Syncline, Broken Hill, Black Mountain and Gamsberg. Extrapolating F2 and F3 fold patterns using magnetic data locates this Aroams anomaly along the F3 axial trace extending from Big Syncline through to Gamsberg. The elevated Pb-Zn geochemical anomaly and structural data associated with the Aroams anomaly makes it a promising future exploitation target. The AGGED database can be expanded both in geographic extent to include surrounding areas, and to allow for inclusion of future surveys. Analytical processing of data in AGGED can also be continued and expanded. GIS is a burgeoning field and developments in GIS technology will impact on the explorationist. Developments in object-oriented and knowledge-based database technologies, visualisation techniques and artificial intelligence, incorporated in future GIS need to be closely monitored and evaluated by geoscience explorationists.
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    Landscape evolution and equilibrium in Southern Africa : insights from cosmogenic noble gases in Karoo dolerites and geospatial analyses
    (2010) Decker, John E; De Wit, Maarten; Niedermann, Samuel
    Southern Africa’s landscape is characterised by a high plateau (elevation > 1,000 m) bound by escarpments and extensional continental margins, and the region has been influential in the development of geomorphological theory. However, the evolution of this landscape in response to tectonic and climatic forcing remains poorly understood, and is investigated here though an analysis of the topography and surface exposure history of early Jurassic (~183 Ma) Karoo dolerites.
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    Mesozoic alkaline volcanism and mantle evolution of the southwestern São Francisco craton, Brazil
    (1996) Bizzi, Luiz Augusto; De Wit, Maarten; Smith, Craig
    This thesis explores the nature of the subcontinental lithosphere underlying the southwestern margin of the São Francisco craton and the relation of variations in the petrochemistry of kimberlites and related alkali igneous rocks to variations in age, thickness and thermodynamic history of their continental lithospheric hosts. The São Francisco craton is a mid- to late-Archean basement granite-greenstone terrain flanked to the west by the Proterozoic Tocantins Province (Almeida, 1977, Almeida et al., 1981). New Rb-Sr and Sm-Nd data are presented for both onand off-craton crustal rock sequences. The ultramafic greenstone association of the Rio das Velhas Supergroup yields 3.2 Ga Rb-Sr and Sm-Nd ages, in agreement with widespread 3.2 Ga old zircons from the area. Granitic gneiss and juvenile granitoids associated with the greenstones in the Congonhas area give a Transamazonian 2128 Ma Rb-Sr age, which is in agreement with a published 2124 Ma zircon age. Funher west, syntectonic granitoids and metabasalts from the Araxa Group define a 711 Ma Rb-Sr isochron. This latter age is interpreted as a Sr-isotope rehomogenization related to the development of the Brasilia orogenic and foreland thrust belt. A 823 Ma Sm-Nd errorchron indicate that these rocks may be coeval to the felsic volcanism of the Araxa Group which was recently dated at 794 Ma by zircon work (Pimentel et al., 1991). Funher to the west still, combined samples from the Nique!andia mafic-ultramafic igneous complex and associated granitic basement rocks yield a 1.26 Ga Rb-Sr isochron, which is best interpreted as a metamorphic age. Crystallization ages of the crustal sequences decrease and ENd values increase with increasing distance westward from the Archean craton margin. The isotopic characteristics are consistent with a model which requires that large volumes of crust, derived in the Proterozoic from mantle reservoirs similar to the sources for modern oceanic basalts, were accreted onto the pre-existing Archean nucleus during the Brasiliano orogenic event.
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    Quantifying spatial association between mineral deposits and geology across three African crustal segments of different age, with implication for secular change in mineralization during earth history
    (2006) Mabidi, Tshifhiwa; De Wit, Maarten; Thiart, Christien
    Variations in enrichment of mineralization, expressed in ore deposits, in the continental crust may be one way to test for secular changes in crustal genesis. This study collates and analyses fundamental information about mineral deposits with which to 'fingerprint' the metal endowment of African crust of different age. Three areas of juvenile African crust (e.g. mantle derived over similar lengths of time of ~500 million years, and excluding recycled older crust) of different ages with similar geology are compared. The areas range in age from 0.5 to 3.0 Ga, [e.g. the Zimbabwe Craton (2.5-3.0 Ga), the Birimian Shield (1.8-2.3 Ga), and the Arabian-Nubian Shield (0.5-1.0 Ga)]. The three areas have a total of 2671 mineral deposits, which are divided into six groups according to their geochemical affinities. Using these known deposits, mineral potential maps are created through a data driven approach, using weights of evidence (WotE). The layers/themes used in Woffi are (1) lithology, (2) structures (faults and shear zones), and (3) lithological contacts. The analysis shows that there is strong lithology control on mineralization in all three areas. Archean crust has high predictive values compared to the younger crust. A measure of spatial association (spatial coefficient), based on the WotE approach, is also used to 'fingerprint' the met I endowment in the three selected regions of African crust. The patterns of the mineral deposits distribution within all regions shows that each region has a unique metal endowment, and that there is a greater concentration of mineral deposits in the crust of the Archean Zimbabwe Craton relative to the younger crust of the Birimian and Arabian-Nubian Shields. The analysis of this study therefore quantitatively corroborates studies that suggest older crust is more mineral diverse and more enriched in mineral deposits than younger crust. Thus, secular changes in mineralization or rates of tectonic processes, or both, are implicated, and mineral endowment in the African crust has undergone major evolutionary changes from Archean to Neoproterozoic time.
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    Receiver function analysis of crustal and upper mantle structure beneath Southern Africa
    (2001) Stankiewicz, Jacek Maciej; De Wit, Maarten
    Bibliography: leaves 101-111.
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    Seismic interpretation, distribution, and basin modelling of natural gas leakage in block 2 of the Orange Basin, offshore South Africa
    (2010) Boyd, Donna Louise; De Wit, Maarten; Anka, Zahie; Di Primio, Rolando
    The aims of this study are to: (1) characterize different natural gas leakage features present throughout the basin, and (2) understand the relationship of natural gas leakage with structural and stratigraphic elements, and (3) quantify liquid/gas hydrocarbon generation, migration and seepage dynamics through the post-rift history of the basin.
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    Stratigraphic and structural interpretation of seismic reflection data across selected sections of the Kaapvaal Craton
    (2001) Tinker, Justine; De Wit, Maarten
    The Kaapvaal Craton is one of the best preserved of all Archean cratons. It is partially covered by the supracrustal sequences of the Witwatersrand, Ventersdorp and Transvaal Basin (and correlated Griqualand West Basin), which span almost a billion years (~3.1 to 2.2 Ga). This thesis describes and interprets eight newly available seismic reflection profiles, acquired by the vibroseis method to 6 seconds TWT, and totaling ~720 km in length. New stratigraphic and structural features are identified across three main regions: the Kaapvaal Craton's western margin, the northern margin or Bushveld lines (flanking the Thabazimbi-Murchison Lineament and across the western extremity of the Bushveld Igneous Complex) and the Kaapvaal Craton interior. The seismic data was interpreted using Charisma seismic interpretation software, Geoframe version 3.6 (developed by Geoquest, Schlumberger) on a UNIX, SUN workstation.
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    Structural, geophysical and geochemical characterisation of a Mesoarchean paleosuture zone, Barberton Greenstone Belt, South Africa
    (2012) MacLennan, Scott Angus; De Wit, Maarten; Weckmann, Ute; Spangenberg, Erik; Ritter, Oliver
    Images of the electrical conductivity of the subsurface below the Barberton Greenstone Belt (BGB) reveal two electrically conductive zones that extend to a depth of at least 5km. These high conductivity zones correlate well with regional shear zones geologically mapped at surface. The serpentinised Stolzburg Layered Ultramatic Body (SLUB) occurs along one of these shear zones, the Saddleback-Inyoka Shear Zone (SISZ). In an attempt to identify the conductive rocks or phases responsible for the observed electrical conductivity anomalies, impedance spectroscopy measurements on samples collected from regional rock types and shear zone related lithologies were undertaken, and show that the rocks from the BGB are generally very resistive. Sulphide rich greenschists and greywackes collected at depth from the northern extension of the SISZ have elevated conductivities, but are not comparable to the anomalies observed in inversions of the magnetotelluric data. Magnetite veins from the SLUB were the only materials that had electrical resistivities low enough to be comparable with the conductivity anomalies. In order to better understand the structural evolution of the western extent of the greenstone belt, a field investigation was undertaken in the SLUB.
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    U-Pb geochronology and structural geology across major shear zones of the Southern granulite terrain of India and organic carbon isotope stratigraphy of the Gondwana coal basins of India : their implications for Gondwana studies
    (1998) Ghosh, Joy Gopal; De Wit, Maarten
    This thesis is divided into 2 parts. The first part (Chapters 1-5) concentrates on unravelling the . Precambrian geological history of the high-grade poly-metamorphic and poly-deformed basement of southernmost India, using field geology and U-Pb geochronology. Because Peninsular India occupied a central position in Gondwana, this work bears on reconstruction models of central Gondwana between circa 600 Ma and 200 :f1,a. The second part of the thesis (Chapter 6) tests the use of organic carbon isotopes as a chemostratigraphic tool to correlate between the lower part of terrestrial Gondwana (Upper Carboniferous to Triassic) sequences in India. This work contributes to (i) intra-basinal and inter-basinal stratigraphic correlation and carbon isotope variations during the Pennian-Triassic extinction event of Gondwana; and (ii) refines the inter-continental correlations of the Gondwana type sequences in India with those in South Africa and Madagascar. This century much attention has been focused on the reconstruction of Gondwana, and as a result, the broad relative positions of different Gondwana fragments and their later dispersal histories is now well understood. Finer-scale reconstructions, based on modern geochronological and isotope studies integrated with field studies of continental rocks, are still needed to resolve the detailed kinematic histories of continental break-up and the evolution of continental lithosphere in general. Shear zones that cross the boundaries of continental fragments provide 'piercing points' in fitting the fragments back together. To realize this, the kinematics and the timing of major shear zones need to be determined. The first part of this thesis, therefore, primarily focuses on understanding the kinematics and geochronology of a number of subvertical continental scale shear zones in the Southern Granulite Terrain of India (SGT).