Magnetotelluric studies across the Damara Orogen and Southern Congo craton
Date
2016-05-10
Authors
Khoza, Tshepo David
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Abstract
Archean 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.
Description
A thesis submitted to the Faculty of Science, University of the Witwatersrand,
in fulfilment of the requirements for the degree of
Doctor of Philosophy
University of the Witwatersrand
School of Geosciences
and
Dublin Institute for Advanced Studies
School of Cosmic Physics
Geophysics Section
February 2016