| Summary: | ABSTRACT
While most impact craters are characterised by negative magnetic anomalies over their
central regions, aeromagnetic surveys over the Vredefort meteorite impact crater reveal
multiple concentric magnetic patterns with no significant anomaly at its centre. In the
rim, the patterns reflect the different sedimentary strata of the Witwatersrand Basin and
the intense anomalies in the rim are clearly related to iron rich shales. About halfway into
the basement there is a prominent negative magnetic anomaly that extends in a broad
semicircular belt around most of the basement core. The anomaly in the basement is more
pronounced in the northwest part of the basement.
A ground geomagnetic survey was conducted across a portion of the negative magnetic
anomaly identified from the aeromagnetic data, which coincides with the amphibolitegranulite
facies transition zone. The reasons for choosing the area was two fold, firstly to
understand the relationship between the magnetic anomalies and the geology and
secondly to compare the ground survey with aeromagnetic data. In addition to the main
survey, a more detailed geomagnetic survey was conducted over a small area (9 m x 9 m,
this being the total area) of intense magnetic field variation to help constrain
magnetization over shorter wavelengths. The latter part also included a palaeomagnetic
study and analysis of these and pre-existing data from the crater.
The data were analysed using two geophysical filters (upward-continuation and automatic
gain control) which were successful in comparing data from this study with existing
aeromagnetic data and in enhancing subtle features for comparison with the geological
map. Inverse modelling was conducted on the main magnetic study area as well as on the
9 m x 9 m grid, which was characterized by very variable magnetic field, in an attempt to
constrain the magnetization and depth of source bodies.
Magnetic anomalies defined by the data are most often negative and occur over a wide
range of wavelengths. The longest wavelength negative anomaly coincides well with the
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aeromagnetic data. This feature is centered over the amphibolite to granulite
metamorphic facies transition exposed in the basement. The upward continued map
coincides very well with the aeromagnetic data in that the amplitude and shape of the
long wavelength anomaly obtained in this study is similar to that seen in the
aeromagnetic data.
On the basis of the modelling conducted in this study it is concluded that the long
wavelength negative anomalies in the basement are due to Archaean basement rocks with
coherent vectors, that have been remagnetised as a result of temperature, pressure and
phase transitions at amphibolite-granulite transition at the time of the 2.0 Ga impact
event. Petrographic evidence shows that there is a marked increase in the intensity of the
impact related thermal and shock metamorphism (including the formation of single
domain magnetite) across the transition. The author suggests that this and the magnetic
anomaly are explained by focusing and defocusing of shock waves at a rheologic
interface.
On the other hand, negative anomalies occurring over smaller (20 to 100 m) wavelengths
often do not coincide with the surface geology. These features require a body below the
surface with very high magnetization intensities and thus cannot be modelled using the
same criteria as that for the long wavelength anomaly. Further, the magnetizations
determined from inversion over the smaller anomalies are not compatible with
conventional thermoremanent magnetism.
The scattered pattern displayed by the natural remanent magnetism data strongly suggests
that lightning strikes are the cause. The observed patterns displayed by the anisotropy of
magnetic susceptibility data could not have survived the plasma fields, and this is a
strong negation that the plasma fields were responsible for the random orientations of
natural remanent magnetism as postulated by others.
In this study the principal directions of the anisotropy of magnetic susceptibility were
found to coincide with the observed metamorphic fabric which suggests that at least some
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of the rocks were not heated that high to attain melting at the time of the impact event.
This is also in agreement with Verwey transition measurements in the basement rocks
that suggest that the basement rocks were not wholly heated above the Curie temperature
during or since the time of impact
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