Numerical investigation of deformation mechanics in fold‐and‐thrust belts: Influence of rheology of single and multiple décollements

Abstract

Thin‐skinned fold‐and‐thrust belts related to convergence tectonics develop by scraping off a rock sequence along a weaker basal décollement 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 décollements. In addition, the influence of multiple weak layers in the stratigraphic column is studied. Model shale décollements 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 décollement simulations have been compared to an analytical solution concerning faulting versus folding. Results show that fold‐and‐thrust belts with a single frictional basal décollement generate thrust systems ramping from the décollement 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 décollement. Tectonic underplating and antiformal stacking occur if the within‐sequence décollement 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” décollement with a viscosity of 1018 Pa⋅s leads to isolated box folds (detachment folds). Multiple salt layers (1018 Pa⋅s) result in long‐wavelength folding. Our results for both frictional and viscous décollements are in bulk agreement with the Mohr‐Coulomb type, critical wedge theory.