Sense of fold asymmetry in single-phase and superposed folding

Abstract

The sense of asymmetry of folds depends upon the sense of shear strain parallel to the enveloping surface. In bulk pure shear, S folds develop by a layer-parallel shortening combined with a sinistral shear strain along the enveloping surface when this surface rotates clockwise in progressive deformation. In layers which shorten but rotate anticlockwise, Z folds form under a dextral shear strain. In simple shear the sense of layer-parallel shear strain is the same as that for bulk simple shear, provided the layering is at a low angle to the direction of shearing. If this angle is large (> 45°) the sense of layer-parallel instantaneous shear strain is opposite to that of bulk simple shear. The sense of asymmetry of the folds can therefore change in the course of progressive simple shear. During refolding of early asymmetric folds (F1) the pattern of S- and Z-shaped F1 folds on two limbs of F2 will depend upon several factors, e.g. the initial sense of asymmetry, the initial angle between the F1 and F2 axes, and the mechanism of F2 folding. The different types of fold asymmetry in type 1, type 2 and type 3 fold interference have been considered. In ductile shear zones where the F1 hinge lines are deformed to hairpin bends over F2 folds, the sense of asymmetry of F1 parasitic folds on adjoining limbs of F2 may be either the same or opposite. In the latter case the pattern of fold asymmetry may be similar or opposite to that of congruous parasitic folds. There may be an association of congruous and incongruous patterns when parasitic folds of different generations are present. This association may be of different types but usually two generations of parasitic folds can be distinguished from their asymmetry and other geometrical characters.