dominated by a northerly to westeriy overthrusting. The fault rocks probably did not form during a single event but as a result of a minimum of two periods of activation of the fault zone separated in time by at least the period required to deposit in excess of 4 km of sediments belonging to the lower part of the Transvaal Sequence.Equivocal evidence suggests that the pseudotachylites, which probably formed as a result of frictional fusion on fault planes, may have formed at similar depths to the mylonites. Consequently the related parameters of pressure and temperature are not considered to be as important in determining whether brittle or ductile deformation occurs, as are pore fluid pressure and strain rate. The age, direction of tectonic transport and a characteristic enrichment in lead all suggest that there may be a genetic Iink between the pseudotachylites in this fault zone and the type pseudotachylites at Vredefort, 60 km to the south.
R~sum~Cette note donne une description d'une zone failleuse prot~rozoi'que parall~le ~. la stratification dans le bassin du Witwatersrand (Afrique du Sud). Cette zone failleuse comporte, de mani~re pr~dominante, des pseudotachylites qui y constituent les tectonites les plus jeunes; on y rencontre aussi des veines de quartz et des cataclasites, plus jeunes que les mylonites qui leur sont spatialement associ~es. I1 s'agit essentMlement d'un charriage de direction nord ~i ouest, qui provoque la lois des suppressions et des redoublements tectoniques. Les roches tectonis6es de la zone failleuse ne se sont pas formfes en une fois, mais t6moignent d'au moins deux p6riodes d'activit6, s@ar6es au moins par le temps n6cessaire au d6p6t de 4 km de s6diments appartenant ~ la partie inf6rieure de la S6rie du Transvaal.Les observations sugg~rent, de mani~re un peu 6quivoque, que les pseudotachylites, form6es vraisemblablement par fusion le long des plans de faille, peuvent avoir 6t6 engendr6es ~l la m4me profondeur que les mylonites. Par cons6quent, dans les conditions d'apparition d'une d6formation soit cassante, soit ductile, les param6tres pression et temp6rature doivent jouer un r61e moins dfterminant que la pression des fluides et la vitesse de la d6formation. L'~.ge de I'accident, la direc-
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