A Thesis submitted to the Faculty of Science, University of the Witwatersrand,
Johannesburg, in fulfillment of the requirements for the degree of Doctor of Philosophy
Johannesburg, 2018 / Two dimensional gravity models and a few magnetic models limited to short profiles in
the eastern sections of the BC have been used to propose conflicting geometries for the
Rustenburg Layered Suite (RLS), especially between the western and eastern lobes; one
school argued for completely separate intrusions, while the other proposed that the
outcropping lobes are connected at depth. These competing models suggest different
emplacement models. They provide a valuable starting point, but, 2.5D potential field
modelling is not well suited to modelling complex three dimensional geology. Also, in
previous work the magnetics or only the gravity data were modelled independently. Here I
present the first full three dimensional potential field forward modelling of the central and
southern Bushveld Complex (BC) which has been used to test the geometry of the
Bushveld Complex in areas obscured by younger geological cover. Both gravity and
magnetic data have been used to develop a geological model that is consistent with both
data sets. Joint 3D modelling of regional gravity and magnetic data combined with
published crustal thickness models derived from broadband seismic tomography studies
were used to create a 3D model of the central and southeastern sections of the BC, as
well as the southern part of the northern lobe. Eight downhole logs with more than
730 000 new density and magnetic susceptibility measurements from the RLS were
combined with existing data to determine values for Bushveld Complex lithologies that
were used in the model. The 3D model has a complex geometry with thick continuous
RLS in most of the western lobe, thinner RLS in the south-eastern lobe, but a disrupted
RLS in the eastern lobe. Large domes of Transvaal dolomite or thicker granites and
granophyre in the eastern lobe interrupt the continuity of the RLS. The western and
eastern lobes are therefore broadly connected, with major disruptions largely in the
eastern lobe. However, they are not separate intrusions, but represent a single magma
chamber affected by pre-existing and syn-magmatic updoming. The model includes the
whole thickness of the crust and requires dense material at the crust mantle boundary.
Thus, for the first time, I have imaged the remnants of magma staging chambers. The
chambers, constrained by the extent of the diffuse crust-mantle transition as determined
from teleseismic data, underlie the whole Bushveld Complex, but is thickest under the
northern, south-eastern and western parts of the complex. I propose feeders that
correlate with the chambers in the northern lobe near the large gravity high near
Polokwane and in the south-eastern lobe. / MT 2019
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/27082 |
| Date | January 2018 |
| Creators | Cole, Janine |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | Online resource (294 leaves), application/pdf |
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