Interaction of flexural shear, S– C fabrics, and oblique shear during folding of micaceous quartzite

Abstract

This paper investigates the geometry, microstructure, and c-axis fabrics of an outcrop scale, micaceous quartzite fold produced under greenschist facies metamorphic conditions in the Moeda quartzite, Quadrilátero Ferrı́fero granite–greenstone terrain, southeastern Brazil. The fold limbs show development of opposed S– C fabrics and asymmetric quartz c-axis fabrics compatible with flexural slip along the fold surface. Towards the fold hinge, there is an increasing presence of oblique shear bands (here named S-bands) which gradually change to crenulations within the hinge zone. The oblique S-bands are interpreted to have formed through connection of several S-planes, increasing accommodation of antithetical shear along these S-planes and offset of the initial C-planes at intermediate stages of folding. This mechanism represents a kinematic inversion in the role played by the two sets of foliations in S– C structures. Our observations support flexural slip for early stages of folding. However, with progressive closure of the fold, the flexural slip mechanism involves increasing contributions from oblique shear on the S-bands, thus approximating an intermediate situation between flexural slip and passive folding (shear parallel to the axial plane).