Numerical investigation of deformation mechanics in fold-and-thrust belts: Influence of rheology of single and multiple decollements

Thin-skinned fold-and-thrust belts related to convergence tectonics develop by scraping off a rock sequence along a weaker basal decollement often formed by water-saturated shale layers or low-viscosity salt horizons. A two-dimensional finite element model with a viscoelastoplastic rheology is used to investigate the structural evolution of fold-and-thrust belts overlying different types of decollements. In addition, the influence of multiple weak layers in the stratigraphic column is studied. Model shale decollements are frictional, with lower friction angles as the cover sequence. Model salt layers behave linear viscous, due to a lower viscosity as the cover sequence, or with a power law rheology. Single viscous decollement simulations have been compared to an analytical solution concerning faulting versus folding. Results show that fold-and-thrust belts with a single frictional basal decollement generate thrust systems ramping from the decollement to the surface. Spacing between thrust ramps depends on the thickness of the cover sequence. The structural evolution of simulations with an additional low-frictional layer depends on the strength relationship between the basal and the intersequential decollement. Tectonic underplating and antiformal stacking occur if the within-sequence decollement is weaker. In the frontal part of models, deformation is restricted to the upper part and imbrication occurs with a wavelength depending on the depth of the intermediate weak layer. Salt decollement with a viscosity of 10(18) Pa.s leads to isolated box folds (detachment folds). Multiple salt layers (10(18) Pa.s) result in long-wavelength folding. Our results for both frictional and viscous decollements are in bulk agreement with the Mohr-Coulomb type, critical wedge theory.