Palaeomagnetic studies on some South African rocks

Doctoral Thesis


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University of Cape Town

A brief review of the subject of palaeomagnetism as it affects the study of the behaviour of the earth's magnetic field and the problems of Continental Drift and Polar wander is presented, giving the reasons why a systematic palaeomagnetic study of the Cape and Karroo Systems of South Africa would be of outstanding significance. This task was vigorously tackled by sampling the Karroo System at vertical intervals of approximately 50 ft. in two separate areas, using the techniques that were then available. The results, although negative, provide material for a discussion of the possible reasons for the scattered directions of magnetization of the samples. A palaeomagnetic study of the Karroo dolerites was undertaken in an attempt to (i) determine 1 the position of the geomagnetic pole at the time of the intrusions, and (ii) possibly assess the importance of the remagnetization of Karroo sediments by the thermal effects of the younger intrusions. Samples from surface exposures in the eastern half of South Africa, from the shafts of a gold mine and from a railway tunnel were collected and studied, giving a reliable mean direction of magnetization of the Karroo dolerites of Declination= 341°, Inclination= -60°. At the commencement of the Jurassic period the geomagnetic pole relative to Southern Africa had the present day co-ordinates of Longitude 74½ 0 E, Latitude 70° S. Both normally and reversely magnetized dolerites were found and evidence in favour of a true reversal of the earthts magnetic field is advanced. It is also suggested that in the area studied, the dolerite was intruded in two distinct phases. Because the direction of magnetization of the Karroo dolerites is very close to that of the present magnetic field, it is difficult to separate samples remagnetized at the time of the intrusions from those remagnetized in the present field. However, some light is thrown on the problem of the scattered directions of magnetization of the Karroo sediments by the fact that dolerite samples collected from surface exposures are much less consistently magnetized than those from underground workings.