Pseudotachylitic breccia and microfracture networks in Archean gneiss of the central uplift of the Vredefort Impact Structure, South Africa

Abstract

Since 1916 the Vredefort Dome, the erosional remnant of the central uplift of the Vredefort Impact Structure, has been known as the type locality for what has been known as “pseudotachylite”, which can be investigated in this structure at various scales and in a range of different host lithologies. Pseudotachylite sensu stricto is generally regarded as the product of friction melting. The formation of such (or similar ) melt breccias in impact structures – known as “pseudotachylitic breccias” - is highly controversial and has been ascribed alternatively to impact (shock compression) melting, friction melting, decompression melting, or various combinations of these processes. In order to contribute to the understanding of the processes that affect the target rock and result in the formation of central uplifts, in general, it is necessary to characterize the impact deformed rocks of the target as well as the impact-generated lithologies. The structures apparent in the rocks of the Vredefort Dome constitute relative time indicators for different stages of deformation during the impact event. Structural analysis of pseudotachylitic breccias and microfractures on a polished 3 × 1.5 m granite slab from a dimension stone quarry in the core of the Vredefort Dome, as well as orientation statistics for microfractures and pseudotachylitic breccias in a quarry in the northern part of the Vredefort Dome, form the basis for this investigation. Two microfracture systems are apparent in the granite slab. Fractures of an older system either cross-cut pseudotachylitic breccia or terminate against it, seemingly depending on different cooling times of melt veins of different thickness. This first-formed fracture system is found mostly in the wall rocks to the breccia veins and in their fragments, whereas a younger fracture system cross-cuts the pseudotachylitic breccias and the host rock. Unravelling the development of individual structural deformation features indicates that the various cross-cutting relationships between pseudotachylitic breccias and two generations of microfractures resulted from a sequence of four processes involving both deformation of the target rock and melt emplacement into the deformed host rock. The first microfracture generation was formed either pre- or syn-impact, pseudotachylitic breccia is syn-impact, and the second microfracture generation is either late-impact or post-impact in age.