Articles | Volume 7, issue 5
Review article
21 Oct 2016
Review article |  | 21 Oct 2016

Folding and necking across the scales: a review of theoretical and experimental results and their applications

Stefan Markus Schmalholz and Neil Sydney Mancktelow

Abstract. The shortening and extension of mechanically layered ductile rock generates folds and pinch-and-swell structures (also referred to as necks or continuous boudins), which result from mechanical instabilities termed folding and necking, respectively. Folding and necking are the preferred deformation modes in layered rock because the corresponding mechanical work involved is less than that associated with a homogeneous deformation. The effective viscosity of a layered rock decreases during folding and necking, even when all material parameters remain constant. This mechanical softening due to viscosity decrease is solely the result of fold and pinch-and-swell structure development and is hence termed structural softening (or geometric weakening). Folding and necking occur over the whole range of geological scales, from microscopic up to the size of lithospheric plates. Lithospheric folding and necking are evidence for significant deformation of continental plates, which contradicts the rigid-plate paradigm of plate tectonics. We review here some theoretical and experimental results on folding and necking, including the lithospheric scale, together with a short historical overview of research on folding and necking. We focus on theoretical studies and analytical solutions that provide the best insight into the fundamental parameters controlling folding and necking, although they invariably involve simplifications. To first order, the two essential parameters to quantify folding and necking are the dominant wavelength and the corresponding maximal amplification rate. This review also includes a short overview of experimental studies, a discussion of recent developments involving mainly numerical models, a presentation of some practical applications of theoretical results, and a summary of similarities and differences between folding and necking.

Short summary
About 200 years ago in 1815 Sir James Hall made his famous analogue experiments, which showed probably for the first time that natural folds in ductile rock are the result of a horizontal compression. If such rocks are extended, then the rock layers can thin only locally, which is a process termed necking, and the resulting structure is often termed pinch-and-swell. We review here theoretical and experimental results on folding and necking on all geological scales.