A compilation of buckle fold concepts and results of analogue experiments. (a) Single layer buckle folding, with layers of increasing competence (1–5), with the matrix competence equal to that in layer 1 (modified after Ramsay 1967). (b) The concept of contact strain, adjacent to a buckled single layer (modified after Ramsay 1967). (c) Numerical models of evolving buckled single layer (modified after Reber et al. 2010). (d) Result of an analogue multilayer model subjected to layer-parallel contraction that synchronously developed folds and faults (modified from a photograph by Price & Cosgrove 1990). (e) Evolution of stress–strain relationships during buckling, using the deformation history outlined by Casey & Butler (2004).

Complex fold-fault patterns. (a and b) illustrates idealised patterns of 'accommodation faulting' in antiform hinge zones, modified after (Price & Cosgrove 1990, fig. 12.23) while (c) shows a natural example developed in turbidite sandstones (modified from a photograph from Price & Cosgrove 1990, fig. 12.26). (d) shows the subsurface interpretation of the Ventura Avenue Anticline in California (modified after Mitra 2003, fig 10). (e) is a subsurface interpretation of the Anschutz Ranch East oil field in the Utah-Wyoming thrust belt (modified after Boyer 1986, fig. 16).

Plan views showing the linkage of folds by lateral hinge-line propagation. (a, b) Redrawn from an analogue experiment using a plasticene multilayer by Dubey & Cobbold (1977).

The development of fold trains in analogue (a, b) and numerical (c, d) experiments. a–b is the evolution of a single experiment using a plasticine and silicone putty multilayer with increasing contraction, reported by (Dixon & Liu 1992, fig. 3). Antiforms and layers are labelled for reference in text. The numerical experiments are after (von Tscharner et al. 2016, fig. 4) and show contraction of a rheological multilayer against a ‘basement step’ analogous to deformation of a thick basin fill (c) and a thin basin fill (d).

Are detachment folds and buckle folds really different? (a, b) Groshong's (2015) conceptualization of these styles, detachment folding and buckling respectively. (c) One source of confusion, arising from cropped and centred images recording experiments on analogue materials – exemplified here, retraced from photographs in Cobbold (1975; with additional annotations). The control layer (X) is encased in a lower-viscosity matrix upon which was printed a passive grid (red lines) that charts the contact strain zone. One of the grid lines is identified here and correlated between deformation states (Y). The low-strain state evolves into the high strain – shown here in Cobbold's framing (left side) and re-hung relative to the passive marker Y (right side). Note that the determining subsidence of synforms depends on the adopted reference frame – or ‘regional’. (d, e) Results of Simpson's (2009) numerical modelling of fold-trains developed above a very-low-viscosity décollement layer. The ‘regionals’ are determined using the undeformed section to the left side of each model.

The interpretation of the Incahuasi anticline in the Bolivian foothills, after Heidmann et al. (2017). (a) Simplified long cross-section through the thrust belt provided for context (simplified from fig. 8 of Heidmann et al. 2017). Total's evolving interpretation of the Incahuasi anticline with the acquisition of well data is shown in the remaining parts of the diagram (b–d); modified from fig. 15 of Heidmann et al. (2017). (b) The pre-drill interpretation; (c) shows a modified interpretation after the first well-bore; (d) shows a final interpretation that incorporates information from the first well-bore and its side-track.

A reinterpretation of the Incahuasi structure as a buckle-folded multilayer – with continuity of the axial surface to depth. Contrast with the pre-drill interpretation and its evolution (Fig. 7b–d).

Evolution of fold–thrust structures in the Central Peak anticline in the Livingstone Range, Canadian Rocky Mountains foothills; modified after Cooley et al. (2011, fig. 16).

Interpreted cross-section through the front of the Bornes sector of the Subalpine fold–thrust belt of the French Alps (modified after Butler et al. 2018, fig 17b).

The nucleation of anticlines on pre-existing heterogeneities. These two examples come from the western Alps and show the Urgonian limestone (Hauterivian–Barremian), which is generally assumed to form a competent formation within an alternating series of limestones and shales (control bed in the sense of Price & Cosgrove 1990). (a) Interpreted cross-section from the Col de la Bataille, Vercors, France. (b) Annotated photograph from the Col de Sanetsch, Switzerland (visible cliff-height c 700 m, to the summit of Spitzhorn).

Conceptual fold nucleation on pre-existing structures and the lateral propagation of fold hinge lines – shown here in plan view of the top of a control unit. (a) The initial distribution of minor normal faults that will serve to nucleate the initial fold clusters (b). (c) The lateral propagation of these hinge lines into previously unfaulted parts of the horizon. Considerations of cross-sections in these unfaulted areas would fail to identify the full causes of fold development.

Buxtorf's (1916) oft-reproduced cross-section through the Jura mountains of Switzerland, based on wells and railway tunnel.

A comparison of publication history of papers that cite the terms ‘fault-bend folding’, ‘detachment-folding’ and buckling/buckle folding. The sample was created by searching Scopus and filtering on papers classified as Earth and environmental science. Vertical scales refer to number of papers per three-year bin, coded to the type of fold.