Introduction
Flanking folds, first extensively described by Gayer et al. (1978) and Hudleston (1989), are fold trains in a primary planar or linear structure, such as a foliation, layering or lineation, which are "flanking" isolated secondary planar structures, such as faults, fractures or veins, which lie oblique to the primary one. Some of them are traditionally referred to as "drag folds", but we prefer to use the term "flanking folds" (Passchier 2001) because of its non-genetic connotation and the option to use it for all fold and deflection structures close to secondary elements in rocks, including shear bands. Flanking folds have been the subject of extensive experimental and field research over the last years, including the effects of flow conditions, strain, initial geometry and orientation on the final geometry (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). These studies have shown that it is theoretically possible to determine strain, flow conditions such as the kinematic vorticity number (Means et al. 1980), and some other parameters from flanking folds. In practice, the ideal circumstances needed to carry out such quantitative analyses will be rarely found in field examples. However, even if detailed quantitative analysis is not possible or needed, flanking folds can be very useful for two purposes; 1) they can be used as shear sense indicators; 2) they can indicate the presence of open voids and active brittle faults in otherwise ductilely flowing rocks, even if the faults and voids have only been present transiently and have been subsequently sealed and overprinted. This is obviously useful, since open voids or faults can be indicative of enhanced fluid pressure or excessive strain rates.
In most cases, flanking folds are isolated asymmetric structures in strongly deformed terrains and geologists will be tempted to use them as indicators for the sense of shear. This is possible, but not in all cases, and before one attempts to use these structures as shear sense indicators, it is important to consider why and how they form. We have carried out a number of simple analogue experiments, which we present here to illustrate the concept. In addition, the experimental results are used to interpret natural flanking folds in Namibia.