Regional-scale foliation reactivation and re-use during formation of a macroscopic fold in the Robertson River Metamorphics, north Queensland, Australia

Abstract

The rocks defining a macroscale antiform (25 km 2) in the Proterozoic Robertson River Metamorphics have been affected by four deformations of distinctive style, with the fourth deformation, D 4, being responsible for formation of the macroscale geometry of the fold. Redistribution of progressive shearing strain, due to strain partitioning during progressive D 4 deformation, resulted in the accommodation of D 4 shearing strain along S 2 differentiated crenulation cleavages that had been synchronously rotated into favourable orientations. On the antiformal limbs this commonly resulted in reactivation of S 2 because shearing during D 4 was in a sense that was antithetic relative to that on the bulk scale of the fold. Continued D 4 deformation caused unfolding of D 2 crenulations, resulting in straightening of sigmoidally folded S 1 and its rotation toward the axial plane of the synchronously forming macroscopic D 4 fold. In zones where the sense of shear during D 4 was the same as that operating on the bulk scale of the fold (i.e. synthetic), D 4 shearing strain was accommodated dominantly by the favourably oriented, approximately axial-planar S 1 fissility. In zones where the progressive shearing component of D 4 deformation was relatively more intense, the S 2 cleavage was also rotated into parallelism with S 4 and was also re-used, as opposed to reactivated. Detailed microstructural analysis, particularly of porphyroblast-matrix relationships, combined with field observations have resolved the processes operating during folding and reveal that, despite the intensity of D 4 deformation, a separate cross-cutting S 4 cleavage has rarely been produced at either the meso- or microscale. Similar processes have probably operated at all scales in other orogenic belts.