Abstract
D.Phil. (Geology)
This study examines the nature, distribution and origin of a
distinctive chert-like fault rock in the West Rand Goldfield of the
Witwatersrand Basin in South Africa. These fault rocks, termed
pseudotachylites, are characterized by an aphanitic groundmass
enclosing subangular to rounded clasts of the host rocks. No glass
has been observed in the matrix but features such as spherulites,
coronas and altered margins to the host rocks as well as
geochemical evidence, suggest that the pseudotachylite formed as a
result of melting of the host rocks due to the heat generated by
friction on faults. The colour of the pseudotachylite is a function
of its chemical composition and parentage.
The pseudotachylite has abrupt contacts with the host rocks which
comprise a lower Proterozoic to Archaean succession of rocks
belonging to the predominantly sedimentary Transvaal Sequence, the
predominantly volcanic Ventersdorp Supergroup and the predominantly
. sedimentary Witwatersrand Supergroup. The orientation of many of
the pseudotachylite fault veins parallels a pre-existing set of
mylonitic faults. These pseudotachylite fault veins most commonly
occur in sub parallel southward dipping pairs and are accompanied by
injection veins. If treated on a statistical basis, the vergence
concept can be extended to injection veins to give the approximate
movement direction of the fault system. The pseudotachylite is
thought to be genetically related to brittle or semi-brittle
extensional faulting of post-Transvaal age.