The progressive development of a macroscopic upright fold pair during five near-orthogonal foliation-producing events: complex microstructures versus a simple macrostructure

Abstract

A macroscopic anticline in the Adelaide Fold Belt, with a profile similar to first deformation folds with an axial-plane slaty cleavage elsewhere in the orogen, folds bedding, schistosity, and a differentiated crenulation cleavage. All these structures are readily visible in outcrop and a young crenulation cleavage has locally developed parallel to the axial plane of the fold. Yet foliations preserved as inclusion trails, in andalusite, garnet and staurolite porphyroblasts, reveal a history of five foliations that formed successively in steep, shallow, steep, shallow and finally steep orientations without producing a refold geometry in bedding at the macroscopic scale. The predominantly dextral (looking north) asymmetry of curvature of overprinting foliations on both limbs of this fold tightly constrains any model for its development. The anticline formed as one hinge of an open fold couplet with a sub-vertical axial plane slaty cleavage during the first deformation. It was progressively tightened during each subsequent event, without producing a refold geometry in bedding. Steep axial-plane foliations were overprinted by shallow differentiated crenulated cleavages that transected the fold couplet. These shallow foliations were then folded during the next deformation, producing another steeply dipping foliation, as the fold couplet tightened. Thus simple macroscopic folds, in orogens with a multideformational history, may appear to have formed as late structures because they fold differentiated crenulation cleavages and a bedding-parallel schistosity, but actually formed very early during orogenesis and were then tightened by a succession of near-orthogonal foliation-producing deformations. Such deformation histories provide a simple explanation for the lack of large-scale refold geometries in many fold belts with a complex history of deformation. Small-scale structures are not always a direct reflection of larger-scale geometries in fold belts.