Historical Overview

Shand (1916) introduced the term "pseudotachylyte" (modern spelling "pseudotachylite") to describe the voluminous breccias (Figs 2a, b) occurring in the granitoid core of the Vredefort Dome in South Africa. He compared these breccias with "trap-shotten gneiss" (a term coined initially for magmatic breccias from India, where clasts of country rock were noted to occur in a matrix then thought to be of igneous origin) and with so-called "flinty crush-rock" from Scotland, occurring as dark-matrix breccia veins that, at the time,

Fig. 2. Prominent exposures of pseudotachylitic breccia in the Vredefort Dome: Fig. 2a (top) Part of the massive exposure in the main quarry at Leeukop Hill. Face shown is ca. 10 m wide. Fig. 2b (bottom) An exploratory cutting on the eastern side of the Leeukop Hill. When comparing these two photographs, note the distinct difference in shape of large clasts -generally rounded in Fig. 2a which shows part of a several m thick, at least 50 m wide, occurrence, and mostly angular blocks in Fig. 2b that shows a large block broken off the massive granite on the right side of this view. However, narrow cracks in this broken and only locally brecciated material have been invaded by melt, providing some support for the suggestion that melt may be produced at generation planes and then intrudes into open spaces.

Fig. 2. Prominent exposures of pseudotachylitic breccia in the Vredefort Dome: Fig. 2a (top) Part of the massive exposure in the main quarry at Leeukop Hill. Face shown is ca. 10 m wide. Fig. 2b (bottom) An exploratory cutting on the eastern side of the Leeukop Hill. When comparing these two photographs, note the distinct difference in shape of large clasts -generally rounded in Fig. 2a which shows part of a several m thick, at least 50 m wide, occurrence, and mostly angular blocks in Fig. 2b that shows a large block broken off the massive granite on the right side of this view. However, narrow cracks in this broken and only locally brecciated material have been invaded by melt, providing some support for the suggestion that melt may be produced at generation planes and then intrudes into open spaces.

Fig. 2c. Strongly brecciated and intricately fractured hinge zone of the large (hundreds of meters) fold structure just south of Parsons Rest farm in the northwestern sector of the collar. The entire hinge zone is invaded with up to decimeter thick veins of pseudotachylitic breccia. The numerous quartzite clasts are generally angular. Not much evidence for movement beyond the centimeter to decimeter scale has been observed. Person in upper half of image for scale.

Fig. 2c. Strongly brecciated and intricately fractured hinge zone of the large (hundreds of meters) fold structure just south of Parsons Rest farm in the northwestern sector of the collar. The entire hinge zone is invaded with up to decimeter thick veins of pseudotachylitic breccia. The numerous quartzite clasts are generally angular. Not much evidence for movement beyond the centimeter to decimeter scale has been observed. Person in upper half of image for scale.

were also believed to be of igneous origin. He concluded that a catastrophic process (Shand actually used the term "shock") was responsible for the abundant brecciation observed in the dome. Other early workers, such as Hall and Molengraaff (1925), emphasized the remarkable abundance of this breccia type in the Vredefort Dome, but, like their contemporaries, could only speculate on possible endogenic processes responsible for this unique brecciation phenomenon.

Whilst the term "pseudotachylite" has been used to describe breccias from other mid- to large-sized impact structures (for example, the Ries Crater [Germany, 24 km diameter], Rochechouart [France, 23 km diameter], or Manicouagan [Canada, 100 km diameter] - for a comprehensive review, cf. Dressler and Reimold 2004), there is only one other known terrestrial impact structure where such material occurs in similar abundance and volume - the Sudbury Structure in Ontario, Canada (Dressler and Reimold 2004, and references therein).

By the 1960s, structural geologists working in brittle fault and mylonitic shear zones discovered dark-matrix breccias (e.g., Park 1961; Philpotts 1964). Early debate centered on whether the matrices of these breccias were cataclastic or melt-derived. At the time, neither Vredefort nor Sudbury was seriously considered to be of impact origin and the term pseudotachylite was applied to these breccias in fault and shear zones that resembled the material initially described by Shand (1916). Given the relative scarcity of terrestrial impact structures, and the limited number of confirmed impact structures listed around that time, it is unsurprising that the usage of the term in the geological literature rapidly assumed tectonic connotations. It was only in the 1970s that impact workers started to try to claim back the term. However, they were hampered not only by the lingering uncertainty concerning the origin of the breccias in the Vredefort type locality, but also by the fact that the Vredefort breccias dwarfed all other occurrences except Sudbury, besides which the origin of Vredefort by impact or internal processes was still highly controversial.

Was this article helpful?

0 0

Post a comment