Terminology
Flanking folds can occur wherever a host element (HE), a planar or linear marker structure such as a foliation, bedding, or a lineation, is transected by a disc shaped object or fault, known as the crosscutting element (CE). Flanking folds are deviations in the orientation of the HE close to the CE (Fig. 1). Several suggestions have been made for terminology to allow a quantitative description of flanking folds (Passchier, 2001; Grasemann and Stüwe, 2001; Grasemann et al., 2003; Exner et al., 2004; Grasemann et al., 2005; Wiesmayr and Grasemann, 2005; Coelho et al., 2005; Kocher and Mancktelow, 2006; Gomez et al. 2007), but no consensus presently exists. Here, we restrict ourselves to simple terminology to distinguish different geometries of flanking structures (Fig. 1). Wherever the HE bends into the CE to become more parallel, the geometry will be similar to that of shear bands and we refer to this shape as "shear band type" (Fig. 1); where the HE becomes less parallel when approaching the CE, the shape is that of a fish-hook, and we refer to it as "hook-type", after Hudleston (1989). Shear band type and hook-type flanking folds correspond to positive and negative lift of the HE above the intersection point of the HE and the CE as defined by Coelho et al. (2005). Both categories are separated by "neutral" lift, when no flanking folds exist (Fig. 1).
Besides the two possible geometries of flanking folds, the magnitude and sense of slip on the CE is important. Most of us are used to an intuitive concept where slip on faults will have a "drag" effect on the HE that will tend to decrease the amount of separation of the cut-off points as compared to a neutral situation, where the HE runs up to the CE without distortion (Fig. 2); we call this "normal slip". However, counterintuitive "reverse slip", where the amount of separation is increased (Fig. 2), is commonly observed in flanking folds. Reches and Eidelman (1995) recognized that drag occurs along a CE with reduced or non-existing friction, resulting in reverse drag at the CE centre and normal drag at the CE tips. The drag is a result of perturbations of the strain or flow field at the vicinity of the CE (Passchier et al., 2005) which causes the flanking structure to form.
