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International Institute for Geo-Information Science and Earth Observation (ITC) 2015

Understanding the earth structure underneath Botswana : the tectonic model and its relationship to the basement and crustal thickness

Chisenga, Chikondi

Titre : Understanding the earth structure underneath Botswana : the tectonic model and its relationship to the basement and crustal thickness

Auteur : Chisenga, Chikondi

Etablissement de soutenance : University of Twente International Institute for Geo-Information Science and Earth Observation (ITC)

Grade : Master of Science in Geo-Information Science and Earth Observation 2015

Résumé
The geophysical studies in southern Africa in general and Botswana in particular have focuses on the selected areas within Botswana especially in the south and south- eastern part of the country . Such studies have contributed to the science world but have not tried to understand the tectonic evolution and the geodynamic processes that resulted in the formation and evolution of Botswana crust. The relationship of the crust thickness and the tectonic terranes to the geodynamic activities are still unclear. Furthermore, Botswana does not have a high resolution resolved discontinuity between the earth crust and upper mantle. These limitations can cause problems in the upper mantle tectonic studies. Upper mantle studies, including seismic tomography and 3D subsurface structure of crust and upper mantle cannot be effectively done if the tectonic boundaries and major discontinuities are not well resolved. In this research, the tectonic boundaries and terranes were delineated and improved using automatic lineaments extraction method . The method, which is mostly used on Digital elevation models (DEM) and satellite images (e.g. ASTER and LANDSAT) , was applied in the extraction of tectonic lineaments on geophysical data (gravity and magnetic) . The major mafic complexes in Botswana were mapped from magnetic data and added to the tectonic terranes to produce the Precambrian basement geology of Botswana. Furthermore, using the geology of the basement of Botswana , new method of mapping the geology in a covered environment was introduced . The method, called apparent physical mapping, combines the magnetic susceptibility and density distribution calculated from the magnetic and gravity data to predict the geology of the covered environment using colour scheme . The apparent physical mapping was used to improve the tectonic terrane boundaries based on the physical parameters of different tectonic terrane. Finally, high resolution Moho discontinuity topography was resolved using the iterative inversion method . The variations of crustal thickness in relation to the tectonic terrane were also discussed in Botswana. Fin ally, the geodynamic evolution of Botswana based on the evidence from geophysical data (gravity and magnetic data) was presented . The geodynamic evidence and crust movement were produced by combining information obtained from the tectonic terranes , the mafic complexes, geology , earthquake epicentre distribution map and the crustal thickness of Botswana. The combined information from these products have proved that the axis of suturing between the Kalahari and Congo craton in the Damara belt is not a straight line running through the centre of Botswana from north east to southwest as shown by Begg et al., ( 2009 )  ; Hutchins & Reeves, ( 1980 )  ; and Reeves & Canada, ( 1982) . No evidence from the new geophysical data supports such a theory. This study has found that the extension within Botswana is due to the sinking microcontinent in the western part, the recent reactivation of deep fault in the eastern part and the continuing r ifting movement in the north western and north eastern part of Botswana.

Mots clés : Tectonic Model, Basement Geology, Crustal Thickness Map, Lineament Extraction, Derivatives, Apparent Physical Mapping, Geodynamic Processes, Southern Africa Tectonic

Version intégrale (ITC)

Page publiée le 31 janvier 2018