A study of the palaeomagnetism of the bushveld gabbrot

Abstract

Data are presented on the natural remanent magnetization at five sites of the Main Gabbro zone of the Bushveld Igneous Complex. It is found that the directions of N.R.M. at the five sites would agree more closely if the rock at each site were rotated about the strike of the pseudo-stratification so as to make its dip zero. This indicates that the gabbro became magnetized before the crust subsided to form the basin in which the Complex now lies. The same results show that the N.R.M. of the gabbro has been stable since before the crustal subsidence. Stability of the N.R.M. is further demonstrated by the highly consistent directions of magnetization making a large angle with the present field. After correction for dip, and on the assumption that the gabbro was magnetized in the field of a geocentric dipole, the mean north magnetic pole from the five sites lies at 23° N. 36° E. The age of the Complex is close to 2·0 × 10⁹ years. The palaeomagnetic results show that the crust supported the load due to the gabbro intrusion without subsidence for at least the time taken by this intrusion to cool to the Curie temperature of its ferrimagnetic constituents. A lower limit to this time is estimated, on reasonable assumptions, to be of the order of 10⁵ years, so that the South African crust 2 × 10⁹ years ago appears to have had very considerable strength.