Transposition structures associated with cleavage formation in the Otago schists

Abstract

Transposition occurs when a folded layer is disrupted in such a manner that the orientation of the individual segments no longer indicates the gross orientation of the parent layer. Transposition structures are described from an area of semischists near Dansey Pass, North Otago, where they appear to be most strongly developed near the hinge zones of macroscopic folds. Transposition of the form surface in the hinge zone of folds in higher grade rocks is thought to be one of the reasons for the difficulty in demonstrating large folds in the Otago schists by conventional mapping techniques. Although detailed petrofabric investigation of these structures has not been attempted, many of the field observations support the conclusions of earlier workers that the formation of slaty cleavage is associated with tectonic compaction and the expulsion of water from only partly consolidated sediments.