Influence of multiple décollement and cover rock rheology on the structural evolution of thin-skinned fold-and-thrust belts: insights from discrete element modelling

A variety of thin-skinned fold-and-thrust belts are associated with multiple décollements, which have low frictional basal and intermediate décollements related to mechanical stratigraphy and or overpressure conditions. The present study considers six ternary series of numerical discrete element (DE) models to simulate and explore the effects of mechanical stratigraphy with varied décollement layers, the number of cover sequences and thickness in the structural style, evolution, and strain partitioning of thin-skinned fold-and-thrust belts. Horizontal layer-parallel shortening of particles was induced by horizontal motion of a vertical boundary wall. The modelling results show weak décollements promoted the decoupled deformation of the fold-and-thrust belt with continues shortening. The results indicate that shortening was mainly accommodating by thrust-related folds with significant differences in the structural style. The formation of décollement, box, fault-propagation, and fault-bend folds, as well as extensional faults represents the ultimate style of the fold-and-thrust belts developed in DE-models. DE-models with thin and fewer number of décollements demonstrate that box-shaped décollement folds with less secondary disharmonic folds developed in the limbs of main fold structures. Furthermore, DE-models with thickest upper décollements are characterized by structural decoupling, more flow of material into the core of anticlinal structures and formation of complicated structures. The results of this study show that the structural style and decoupling can be affected by the rheology, number, and thickness of décollements. The DE-modelling results compared with natural examples, as well as analogue and numerical models, show that our mechanical modelling can overall match thin-skinned fold-and-thrust belts with multiple décollements that present different structural style.