Influence of layer interface geometry on single-layer folding

Geometrical heterogeneities along layer interfaces play a key role to determine the geometries of folds developed during shortening of competent layers. We present a series of numerical simulations to investigate the influence of initial sinusoidal perturbations on folding of single layers. Models consist of a competent viscous single layer embedded in a softer matrix, with the layer oriented parallel to the shortening direction. We first generalise the wide spectrum of sinusoidal perturbations accounting for asymmetries along and across a competent single-layer, using two parameters: transversal asymmetry (A’) and longitudinal asymmetry (φ). These two parameters allow studying a transition between classical fold shape and pinch-and-swell geometries. The parameter A’ describes the development of fold hinges with different geometries between upper and lower layer interfaces and abnormal curvatures between outer and inner arcs of fold hinges. The parameter φ induces a strong polarity on folds, with systematic preferred orientation of the pinch and swell regions of the layer, even if there is no shear component parallel to the layer. Our results demonstrate the importance of structural inheritance on the resulting fold geometries and suggest that caution must be taken when using certain types of asymmetric folds as strain markers and kinematic indicators.