PSEUDOTACHYLITE IN THE SOUTH BOUNDARY FAULT AT THE COOKE SHAFT, WITWATERSRAND BASIN, SOUTH AFRICA

Abstract

The steeply-dipping South Boundary Fault in the Cooke Section on the West Rand goldfield hosts a series of crosscutting pseudotachylite fault and injection veins, and pseudotachylitic breccias that formed progressively by abrasive wear in a brittle (elastico-plastic) regime. The crosscutting pseudotachylite veins and breccias are hosted by faulted supracrustal rocks of the Witwatersrand Basin that are exposed by underground workings and production-development borehole cores. Evidence of frictional melting is represented by quenched vein margins, corona texture, turbulent flow and glassy texture. Underground mapping and logging of boreholes has confirmed the existence of at least three generations of pseudotachylite, which were tectonically-induced. The relative age of tectonically-induced pseudotachylite formation is constrained to pre-2.58 Ga, or before deposition of the lower Transvaal Basin sequences, because 3D seismic data show that pseudotachylite-bearing faults do not penetrate the base of the Transvaal Basin. The conclusions made in previous research that the Hekpoort Andesite Formation (2224 ± 21 Ma) of the Transvaal Supergroup provides a maximum age for all pseudotachylite in the West Rand goldfield may therefore need revision.