Regional slaty cleavage formation and fold axis rotation by re-use and reactivation of pre-existing foliations: the Fiery Creek Slate Belt, North Queensland

Abstract

A deformation history, comprised of six separate deformation events of differing intensity, has affected the rocks of the South Palmer River region of the the Hodgkinson Province, north Queensland. Within this region, a zone of pervasive slaty cleavage, herein termed the Fiery Creek Slate Belt, has developed as a result of the superposition of fabrics formed during several of these events. The most important processes in the formation of this composite cleavage were the re-use and reactivation of the favourably oriented, steep, N-S-trending S 2 foliation by the intense fourth deformation event, D 4. This produced micro-, meso- and macroscopic folds in an originally shallow S 3 foliation, produced during the intervening D 3 deformation, with an axial planar S 2–S 4 foliation. The D 4 stretching lineation, L 4 4, plunges subvertically to steeply north and indicates that shear during D 4 was oriented steeply north-south. In the Fiery Creek Slate Belt, D 2 fold axes are interpreted to have formed in much shallower orientations than their present moderately N-S-plunging to subvertical orientations. We consider this to be a result of D 4 shear, which caused variable degrees of rotation of D 2 fold axes toward the D 4 stretching lineation due to subparallelism of the bulk shortening directions of the D 2 and D 4 events. Near-total destruction of the pre-D 4 foliations during slaty cleavage formation has produced a misleading impression of a simple deformation history. There is no relationship between metamorphic grade and intensity of slaty cleavage development.