Interaction of folding and thrusting during fold-and-thrust-belt evolution: Insights from numerical simulations and application to the Swiss Jura and the Canadian Foothills

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We use two-dimensional numerical simulations to investigate the interaction of folding (buckling) and thrusting (localized shearing) during the structural evolution of fold-and-thrust belts (FTB). We model layer-parallel shortening of mechanically strong layers resting above a weaker layer, referred to as detachment. We consider a viscoplastic rheology with linear viscosity and frictional-plastic Drucker-Prager yield criterion. The detachment thickness is larger in one model half and only this geometrical thickness perturbation triggers folding and thrusting in the simulations. We conduct a systematic analysis to quantify the impact of model parameters, such as magnitude of the thickness perturbation or detachment rheology (frictional or viscous), on the FTB evolution. The magnitude of the perturbation determines whether, or not, folding is favoured over thrusting and, hence, controls the progressive interaction of folding and thrusting during FTB evolution. The asymmetry of modelled anticlines is proportional to the relative height of the initial perturbation. We also adapt the model configuration to specific mechanical stratigraphies of two regions in natural FTB, namely the Haute-Chaîne of the Jura Mountains (Switzerland) and the Foothills of Alberta (Canada). The modelled fold and thrust geometries agree with different types of observed anticlinal structures, such as symmetric detachments folds and asymmetric thrust-folds. Our results suggest that an initial stage of low-amplitude buckling likely occurred prior to the main folding and thrusting phase. We argue that folding and thrusting are equally important and interacting processes during the structural evolution of many natural FTB.