The geology of the Rustenburg Fault

Abstract

The N.N.W.-S.S.E. striking Rustenburg Fault zone, in the western Transvaal Basin, South Africa, has been mapped, in order to unravel its tectonic history. Thickness differences in the Daspoort Formation of the Pretoria Group on opposite sides of the Fault suggest that the Fault was active during Pretoria Group sedimentation, with normal faulting producing localised second-order basins on the down-thrown side of the Fault. In post-Pretoria Group times, but before the intrusion of the Bushveld Complex at ~ 2050 Ma, the area surrounding the Fault zone underwent two compressive events. The first was directed N .E. - S.W., producing S.E.-N.W. trending folds, and the second was directed N.W.-S.E., producing N.E.-S.W. trending folds. The second set of folds refolded the first set to form typical transitional Type 1-Type 2 interference folding, and this compression ultimately caused reactivation of the Rustenburg Fault, so that dextral strike-slip movement displaced the Pretoria Group sediments by up to 10.6 km. The subsequent intrusion of the Bushveld Complex into the adjacent strata intensely recrystallised, and often assimilated, the strata along the Fault zone. The fault rocks within the Fault zone were also recrystallised, destroying any pre-existing tectonic fabric. Locally, the Fault zone has been assimilated by the Bushveld Comnlex After the intrusion of the Bushveld Complex, little movement has occurred along the Fault, especially where the Fault passes under areas occupied by the Bushveld Complex. It is thought that the crystallisation of the Bushveld Complex has rheologically strengthened the neighbouring strata, preventing them from being refaulted. This model presented above is at variance with previous assumptions that continuous regional extension during Pretoria Group sedimentation culminated in the intrusion of the Bushveld Complex.