Polyphase folding in Precambrian low-grade metamorphic rocks, Middle Gordon river, S.E. Tasmania

Abstract

New exposures on the southern side of the proposed damsite on the middle stretches of the Gordon River reveal unequivocal examples of polyphase folding in the Precambrian, low-grade metamorphic rocks of southwestern Tasmania. Three groups of folds are deformed by kink bands. The first group comprises very tight, almost isoclinal,and recumbent folds with north-south trending hinges, and was formed by considerable subhorizontal ovierriding of the western block towards the east. These early, concentric folds are folded by more open, similar folds. The hinges of the second group of folds pitch shallowly north-northeastward on steeply dipping axial surfaces parallel to which there is a prominent transposition foliation developed in the phyllites. These folds were produced by a steeply inclined thrusting of the eastern block towards the west during low greenschist-facies conditions of metamorphism. Small-scale, similar-style puckers pitching steeply in the transposition foliation comprise the third group of folds, and were produced by subsequent subhorizontal movement in the phyllites of the eastern block southwards.