A dissertation submitted to the Faculty of Science, University of Witwatersrand in the fulfilment of the requirements for the degree of Master of Science. Johannesburg, 2015. / Northwest 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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/18545 |
| Date | January 2015 |
| Creators | Rankin, William |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
Page generated in 0.0028 seconds