Combined control of décollement layer thickness and cover rock cohesion on structural styles and evolution of fold belts: A discrete element modelling study

A series of numerical experiments based on the discrete element method were performed to simulate the formation of fold belts, with varied décollement layer thickness and cover rock cohesion, to investigate their controls on structural styles of fold belts. Each model consists of a weak, unbonded basal décollement layer with a relatively low to high thickness, and a bonded cover with a relatively low to high cohesive strength. Horizontal shortening of the particle assemblage was achieved by horizontal motion of a vertical boundary wall, resulting in deformations in the system. The results show that shortening was mainly accommodated by detachment folds (sinusoidal and box folds), fault-related folds and opening-mode fractures in the models. The combination of a lower cohesion and a thinner décollement resulted in more distributed strain and a larger number of folds, whilst the combination of a higher cohesion and a thicker décollement led to more pronounced strain localisations in fewer folds. Surface uplift and fold amplitude (diapir height) are mainly positively affected by the décollement thickness, i.e. the thicker the décollement is, the greater the surface uplift and fold amplitude are. The propagation rate of deformation is predominately controlled by the cover rock cohesion. A lower cohesion led to a higher propagation rate of deformation. The model with a relatively low cohesion and décollement thickness produced regularly-spaced box folds, which are comparable to those in the southern Zagros Fold-and-Thrust Belt. The models presented demonstrate that the combined effect of décollement layer thickness and cover rock cohesion can play a critical role in the structural styles and kinematic evolution of fold belts.