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University of Twente (2021)

Crustal and upper mantle imaging of Botswana using magnetotelluric method.

Akinremi, Stephen

Titre : Crustal and upper mantle imaging of Botswana using magnetotelluric method.

Auteur : Akinremi, Stephen

Etablissement de soutenance : University of Twente

Grade : Master of Science in Geo-Information Science and Earth Observation 2021

Résumé
There exist some unclear and debated hypotheses about important tectonic features and the geodynamics of Botswana. There are open debates on the existence of a buried Maltahohe microcraton in the southwest region, termination of the East African Rift System in northern Botswana (Okavango Rift Zone), the extension of the East African Rift System to central Botswana, and its influence on the 03 April 2017 earthquake. Further exploration of the crust and upper mantle beneath Botswana in these highlighted tectonic domains would fundamentally improve our understanding of the two main features of the south African lithosphere, the African superswell and the East African Rift System. Besides understanding these features, it would improve the understanding of the current tectonic settings of Botswana and the deformation history. This research presents a homogenous 3-D electrical model with an unprecedented spatial coverage, using a robust methodological scheme that requires no assumption about the directionality of the subsurface structure. The result of this study shows a resistive structure in southwest Botswana, which suggests the existence of the Maltahohe microcraton as a separate cratonic unit from the adjacent geologic terranes. In northern Botswana, the electrical conductivity model reveals a high conductivity structure around the Okavango Rift Zone, which connects with a deeper high conductivity structure that corresponds to the East African Rift System’s extension to northern Botswana. This gives a piece of evidence to the role of ascending hot fluids or melt, leading to weakening of the lithosphere and subsequent rifting in the Okavango Rift Zone. Finally, the result of the electrical model could not establish the link between the high conductivity structures due to the East African Rift System in northern Botswana and beneath central Botswana (location of the 03 April 2017 earthquake) because of poor constrain of the model due to sparse magnetotelluric site distribution in the area. The results of this study provides straightforward, connected, and precise geologic interpretations about different arguments raised in the literature on the tectonics and structure of the crust and upper mantle beneath Botswana. This work underscores the need to complement, confirm or refute findings from other geophysical data about the structure of the crust and upper mantle with electrical models derived from the magnetotelluric method.

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Page publiée le 17 décembre 2021