Aspects of shear strain in the East Rand Basin

Abstract

M.Sc. (Geology)Rocks of the Witwatersrand Super group in the northern portion of the East Rand Goldfield have been examined on surface and in underground exposures in an attempt to establish the chronology, movement vectors, amount of strain and displacement associated with bedding parallel faults. It was found that more than one age of movement occurred along most of the non-bedding parallel fault planes with normal, reverse and strike-slip senses of motion. The ductile bedding plane faults are manifested in all lithologies, but the shale units have, in particular, acted as a locus for shearing and it is suggested that they should be regarded as phyllonites. Bedding parallel faults are generally characterised by the presence of quartz veins and the development of phyllosilicates. They are categorised as mylonitic quartz schists. Syntectonic fault restricted quartz veins generally parallel the foliation, developed in the plane of flattening, and aided the movement of overlying strata during fault formation. Kinematic indicators imply a broad northerly up dip movement on the bedding parallel faults; implying they are thrust faults. Directionally specific kinematic indicators display at least two directions of thrust movement in the Central Rand Group, one to the NE and a second to the N to NW. The second group may represent two overlapping directions of movement. Only NE thrust movement are indicated in the West Rand Group. The NE thrust event occurred after emplacement of Ventersdorp dykes and before deposition of the rocks of the Black Reef Quartzite Formation. The N thrust fault event occurred after deposition of the Black Reef Quartzite Formation and before deposition of Karoo age rocks. A possible third event of Bushveld Igneous Intrusion age may be manifested as thrust faults towards the NW. The contact between the Central and West Rand Groups has acted as a major decollement during the N thrust event. The folds in this region may initially have been the result of thrust faulting towards the NE. Tilting of the fold axes during the postulated NW event is possible

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