REGIONAL-SCALE FOLIATION REACTIVATION AND REUSE DURING FORMATION OF A MACROSCOPIC FOLD IN THE ROBERTSON RIVER METAMORPHICS, NORTH QUEENSLAND, AUSTRALIA

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.