40AR-39AR DATING OF PSEUDOTACHYLITE FROM THE VREDEFORT DOME, SOUTH-AFRICA - A PROGRESS REPORT

To date, six pseudotachylite samples from the Vredefort dome have been analysed by the stepheating 40Ar39Ar technique. The resulting ages (Ga) (2.25 ± 0.04 (KAr), 2.01 ± 0.06, 1.39 ± 0.08, 1.44 ± 0.08, 1.33 ± 0.04 and 1.09 ± 0.10) may be understood as suggesting several pseudotachylite-forming events at Vredefort. This, and other chronological and field evidence, may be interpreted as supporting the possibility that Vredefort pseudotachylite is not derived from a single catastrophic, central shock event, but was generated in several local, high-strain (possibly seismic-shock) events. Diffusion calculations have shown that it is possible that several thermal spikes could have effectively degassed pseudotachylite at relatively low temperatures and during short periods of time (e.g. at 340 ° C in 10000 yrs). There is, however, no geological or isotopic evidence directly supporting the existence of these proposed temperature spikes at ∼ 2.0, 1.3-1.4 and 1.1 Ga in the region of the Vredefort structure. On the contrary, field and laboratory observations can be invoked that seem to favour the formation of multiple generations of pseudotachylite in the Vredefort dome and in the surrounding Witwatersrand Basin. However, it must be noted that likewise no direct geological evidence for local tectonic activity in the region of the Witwatersrand Basin at those times can be presented, with the exception of emplacement of Pilanesberg age dykes. Further tests on the validity of the 40Ar39Ar ages on Vredefort pseudotachylite are in progress and are discussed. © 1990.