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 Complex.
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.
