THE VREDEFORT IMPACT 'GLASSES': STELLENBOSCH RESEARCHER TAKES A FRESH LOOK AT ROCKS FIRST STUDIED HERE, A CENTURY AGO

            The venerable Professor S. J. Shand was one of the founders of Stellenbosch University and is considered the real founder of the Department of Geology (now Earth Sciences). Almost precisely 100 years ago (Shand, 1916), he coined the term 'pseudotachylyte' to describe widespread, dark, flinty veins in granitic and metamorphic rocks within the Vredefort dome of South Africa (Figs 1A & C). Following in his footsteps, with a colleague from the Geological Survey of Denmark and Greenland, Dr Martin Klausen (from the same Earth Science Department in Stellenbosch) has introduced the concept of seismic shaking as a key process in the formation of these large volumes of pseudotachylyte (Garde and Klausen, 2016) within what is now known to be a meteorite impact structure that formed a little over 2 billion years ago.

            The process of seismic shaking is well established on the Moon (e.g. Fig. 1B) and asteroids, from terrestrial earthquakes and rock slides, as well as from nuclear tests. However, this important mechanism has been largely overlooked in terrestrial cratering. Previous struggles to convincingly explain Vredefort's large pseudotachylyte volumes through either frictional heating or instantaneous 'shock' melting, are now elegantly resolved. The pseudotachylyte was formed by repeated breaking up of blocks, initially loosened within a branching system of fractures that formed during the early high-amplitude stage of seismic shaking (Fig. 1A). Continued high-frequency shaking led to size reduction and rounding (Fig. 1C), and eventually even frictional melting of certain minerals in the ground-up mass. Thus, contrary to previous ideas, most of the pseudotachylyte was not injected from anywhere else but rather formed in place. Indeed, analysis of the microscopic features in the Vredefort rocks shows that most of the pseudotachylyte is actually finely ground crystalline material rather material that was formerly molten.

Figure 1: (A) Thin pseudotachylyte veins in a branching fracture system, formed by the initial shock wave, as well as along a more regular, and presumed pre-existing, fracture or fault that cuts diagonally across the photo. (B) An approximately 1.5 hour seismogram of a Lunar meteorite impact that occurred on 13 May 1972 (modified from Nakamura et al., 1982). (C) A thicker pseudotachylyte pod with characteristic rounded rock fragments, typically formed, in place, through seismic shaking and repeated grinding of the corners of loose blocks. Both field photos are from the Esperanza quarry and show approximately the same field of view, with a R5 coin for scale in (A).

Garde, A.A. and Klausen, M.B. 2016. A centennial reappraisal of the Vredefort pseudotachylytes: shaken, not stirred by meteorite impact. Journal of the Geological Society (London), in press, doi: 10.1144/jgs2015-147.

Melosh, H.J. 1979. Acoustic fluidization: a new geologic process? Journal of Geophysical Research 84: 7513-7520.

Nakamura, Y., Latham, G.V. and Dorman, J. 1982. Apollo lunar seismic experiments - final summary. Journal of Geophysical Research 87: A117-A123.

Shand, S.J. 1916. The pseudotachylyte of Parijs (Orange Free State) and its relation to 'trap-shotten gneiss' and 'flinty crush rock'. Quarterly Journal of the Geological Society of London 72: 198-217.