The Palaeomagnetism of the Great Dyke of Southern Rhodesia

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Oriented cores have been secured from fourteen sites in the Great Rhodesian Dyke, by means of a portable sampling drill. The natural remanent magnetizations showed high dispersion at all sites except one. After demagnetization in alternating magnetic fields, nine sites gave well grouped directions of primary magnetization. These sites include five rock types distributed among three Complexes of the Great Dyke and two satellite dykes, over 200 miles of the length of the Dyke and through several thousand feet in depth as the rocks were originally intruded. The nine site mean directions of primary magnetization are closely grouped and are believed to represent directions of thermo-remanent magnetization at the date of intrusion of the Great Dyke. It is suggested that the dates of magnetization at the sites must cover a sufficient time interval to give a mean pole position close to the axial geocentric dipole freed from secular variation. On the assumption of a geocentric dipole field, the position of the mean South magnetic pole is 211/2 °N, 611/2 °E, with radius of 95 per cent confidence 9°. This pole position is close to positions of North magnetic poles given by studies of the palaeo-magnetism of the Pilansberg Dykes and Bushveld gabbro.