The association of folds and veins in shear zones

Paired hook-shaped asymmetric folds, with shared limbs cut by axial planar veins, dikes or fractures are common in glaciers and rocks. The sense of offset across the vein or fracture is characteristically opposite to that expected from the sense of fold asymmetry. The kinematics of development of the folds and the relationship of the folds to the fractures or veins can be understood by considering the nature of glacial flow and by utilizing simple clay experiments. In zones of simple shear, dilational fractures may form at about 45° to the shear zone boundaries. The perturbation of strain around such a fracture during its development and subsequent rotation and closure under continued deformation causes layers or pre-existing foliation cut by the fracture to become folded. Thus folding is a consequence of the fracturing, and not vice versa. The precise shape of the fold depends on the nature of the perturbation, and particularly whether or not the fracture remains for a time open or fluid filled and thus has walls unable to sustain a shear stress during vein rotation. Paired hook folds provide evidence for non-coaxial strain and are good sense-of-shear indicators. However, not all folds with veins parallel to their axial surfaces form in the way described in this paper. In some cases vein emplacement occurs late during fold development or post-dates folding; in these cases offsets along the veins are consistent with fold asymmetry.