Experimental study of sheath fold development around a weak inclusion in a mechanically layered matrix

Abstract

We present quantitative laboratory models investigating the mechanics of sheath fold formation around a weak inclusion in simple shear. Sheath folds are intriguing, highly non-cylindrical structures that stimulated extensive field and experimental studies, leading to an ongoing debate concerning their formation and evolution. Through a parametric study, we test the influence of a mechanically layered matrix on the development of sheath folds using silicone models. Our models show how (1) the viscosity ratio between the layers in the matrix and (2) the layer thickness control the shapes of the resulting folds and their visibility. All experiments with a weak inclusion resulted in strong deformation of the layers. An increase in viscosity ratio, however, leads to less evolved sheath folds, which are shorter and show a larger opening and dip angle. In contrast to former studies, we show that a mechanically layered matrix does not hinder the formation of sheath folds. The visibility of the sheath folds in our models strongly depends on the aspect ratio between the inclusion height and the layer thickness: we observed sheath folds for a ratio larger than 7.5. The experiments reproduced the first-order features of natural sheath folds. Our results challenge previously published studies where sheath folds were considered as purely passive structures. We demonstrate that sheath folds readily develop around slip surfaces, suggesting that this might be a relevant formation mechanism in nature.