Folding in high-grade rocks due to back-rotation between shear zones

Abstract

Folds with a sense of asymmetry opposite to the bulk shear sense may form in high-grade rocks due to back-rotation between ductile shear zones. Layers back-rotated into the shortening field can undergo additional buckle folding. With progressive deformation, shear zones and domains of back-rotated foliation between shears rotate towards the X– Y (flattening) plane of the finite strain ellipsoid. In high-grade gneiss, the migration of melt into shear zones may facilitate displacement along them and accentuate back-rotation of material between shear zones. Folding during back-rotation occurs when the spacing between shear zones does not increase sufficiently to accommodate the length of back-rotated layers or between non-parallel shear zones. Folds formed by back-rotation between shear zones commonly have pegmatites sub-parallel to their axial planes, thickened overturned limbs and occur in localised packages. Associated shear zones have the same sense of shear on both attenuated limbs of folds formed by back-rotation. Examples are given for folds between transcurrent and normal ductile shear zones in the Albany Mobile Belt and Leeuwin Complex (Western Australia) and the Dharwar Craton (India).