Superposition of folding during either progressive displacement or different phases of deformation result in three-dimensional refold structures that are exposed on two-dimensional sections as interference patterns (Ramsay, 1962, 1967). In order to understand the complex geometry of these structures several models have been presented: Earlier models used card decks, which were cut into the shape of the first fold and than sheared parallel to the second fold axial plane (e.g. Carey, 1962, O'Driscoll, 1962, Brown, 1967). Scaled physical models showed the influence of layer buckling, competence contrast between layers and the influence of the initial fold geometry on the formation of refold structures (e.g. Reynolds and Holmes, 1954, Gosh and Ramberg, 1968, Watkinson, 1981, Odonne and Vialon, 1987, Grujic, 1993, Johns and Mosher, 1995). Kinematic forward modelling computer programs have been successfully applied to simulate three-dimensional refolding and to study two-dimensional interference patterns on arbitrary oriented sections through the modelled structures (e.g. Thiessen, 1986, Perrin et al. 1988, Jessell and Valenta, 1996; Vacas Peña, 2000, Ramsay and Lisle, 2000; Moore and Johnson, 2001). Although most of these programs have the same limitations as the card deck models, that folds are assumed to be (cylindrical) similar shear folds with passive initial layering, these studies have significantly contributed to the understanding of the great variety of interference structures and the classification of refolds.
Computer animations of the development of geological structures during progressive deformation are a powerful tool both in the advancement of understanding of processes and geological education. With the help of a computer program for modelling three-dimensional refold structures and their two-dimensional interference patterns, this contribution provides computer animations, which effectively improve the understanding of natural fold superposition and are therefore ideally suited for teaching in electronic classrooms or for use in online courses on the Internet. Furthermore the animations suggest that the Type 0 refold, which actually produces no interference patterns, has to be divided in three different classes, which theoretically exist but which are probably difficult to distinguish in the field. Because the presented study is based on the kinematic forward modelling software Noddy (Jessell and Valenta, 1996) the mechanical influence of contrasting rheologies is ignored and discussed elsewhere (e.g. Johns and Mosher, 1995 and references cited therein). Despite these limitations the presented results have many geometric similarities with natural refold structures justifying the use of kinematic modelling in exploring the complex shapes and interference patterns of fold superposition (Ramsay and Lisle, 2000).