REGIONAL-SCALE FOLIATION REACTIVATION AND REUSE DURING FORMATION OF AMACROSCOPIC 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 responsi ble for formation of the macroscale geometry of the fold. Redistributi on of progressive shearing strain, due to strain partitioning during p rogressive D-4 deformation, resulted in the accommodation of D-4 shear ing strain along S-2 differentiated crenulation cleavages that had bee n synchronously rotated into favourable orientations. On the antiforma l 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 wa s the same as that operating on the bulk scale of the fold (i.e. synth etic), D-4 shearing strain was accommodated dominantly by the favourab ly oriented, approximately axial-planar S-1 fissility. In zones where the progressive shearing component of D-4 deformation was relatively m ore intense, the S-2 cleavage was also rotated into parallelism with S -4 and was also re-used, as opposed to reactivated. Detailed microstru ctural analysis, particularly of porphyroblast-matrix relationships, c ombined with field observations have resolved the processes operating during folding and reveal that, despite the intensity of D-4 deformati on, a separate cross-cutting S-4 cleavage has rarely been produced at either the meso- or microscale. Similar processes have probably operat ed at all scales in other orogenic belts.