DIRECT-CURRENT RESISTIVITY AND LOTEM MODEL FOR THE DEEP-STRUCTURE OF THE NORTHERN EDGE OF THE KAAPVAAL CRATON, SOUTH-AFRICA

The transition from the high-grade metamorphic Southern Marginal Zone (SMZ) of the Limpopo belt to the low-grade Kaapvaal craton was studied by means of geoelectrical techniques. The d.c. data showed conclusively that the high-grade rocks were thrust over the low-grade rocks. This contradicts earlier geological ideas, but agrees with recent seismic reflection data. The area where high-grade rocks occur above low-grade strata was mapped out and the southern boundary of the SMZ was established. The conductor that occurs at lower crustal depths in the Kaapvaal craton was traced into the SMZ using the LOTEM method. The conductor shallows from approximately 30 to 22 km over 12 km on the surface, but does not reflect the difference in metamorphic depth of approximately 30 km seen at the surface. This indicates that the conductor is a transient feature of the crust. The conductor is not associated with a seismically reflective crust. The cause of the conductor remains enigmatic.