Numerical modelling of thrust structures in unconsolidated sediments: implications for glaciotectonic deformation

The mechanics of thrust faulting induced by a differential vertical load has been studied by application of a dynamic finite element model. The modelled scenario is analogous to the glaciostatic load imposed onto an unconsolidated sedimentary substrate by a stationary ice sheet. The objective is to evaluate whether large-scale glaciotectonic deformation may be induced by gravity spreading in front of a stationary glacier or if a push from the rear is required. The model is capable of handling large strains and rotations combined with visco-elastic-plastic rheologies. Model parameters are derived from a Pleistocene large-scale glaciotectonic thrust complex in the eastern Danish North Sea. The plastic behaviour of the unconsolidated sediments is described by a time and pressure dependent Drücker-Prager yield criterion combined with a non-associated flow law. The modelling results show that gravity spreading in front of a stationary ice-sheet can form strain localisations and possibly cause thrust faults to nucleate in unconsolidated sediments several kilometres beyond the snout of the ice-sheet, provided a décollement surface is situated at a suitable depth relative to the effective load of the ice-sheet. Hence it is concluded that large-scale glaciotectonic thrusting may be formed by gravity spreading alone, and a push from the rear is not generally required to explain the observed deformation.