Experiments on folding in spreading-gliding nappes

Abstract

A small scale model of spreading-gliding nappe was used to study the development of folds which are controlled by the geometry of external boundaries. Throughout in the model, the deformation combines pure shear and simple shear. Recumbent folds are produced above irregularities of the base. The cylindricity of folds is directly dependant on the cylindricity of basal irregularities. Models with homogeneous rheological properties produce only recumbent folds whereas models of contrasting rheological properties produce recumbent folds, and “en échelon” upright folds in the zones of lateral shear. The experiments demonstrate that: 1. (a) large scale recumbent folds in nappes can result from passive amplification processes in unsteady shear flow 2. (b) upright folds and coaxial refolding result from mechanical instability due to contrasting rheological properties between layers and develop during a unique progressive deformation. These laws can be directly applied to a spreading-gliding nappe but also on a larger scale to all nappes in which the shear component dominates the deformation.