THE MECHANICS OF THRUST FAULT INITIATION AND FOLD AND THRUST BELT EVOLUTION: INSIGHTS FROM DISCRETE ELEMENT SIMULATIONS

Discrete element method simulations provide unique views into the kinematics and mechanics of contractional systems, yielding constraints on deformational processes and failure conditions within fold and thrust belts. Numerical simulations were carried out on assemblages composed of ~50,000 particles, settled under gravity and bonded, defining cohesive 2D domains 120 km long and 6 km high. Horizontal compression is introduced by moving one wall inward above a weak, non-cohesive decollement surface. Wedge deformation is strongly influenced by cohesive strength: low-cohesion materials exhibit distributed deformation and broad folding. With increasing bond strength, deformation becomes progressively more localized, ultimately producing large-offset spaced forethrusts. The stress field shows significant spatial and temporal variations, influenced by cohesion, decollement strength, and pre-existing faults. In general, maximum compressive stresses rotate from subvertical outboard of the wedge to shallowly inclined inboard of the deformation front, consistent with frictional slip along the decollement. Differential vs. mean stress values sampled throughout the wedge define unique shear strength criteria for each configuration, which are used to map the failure potential across the wedge (i.e., proximity of the material to failure). All cohesive systems exhibit triangular regions of high failure potential outboard of the deformation front, widest and thickest for the highest cohesion. The formation of each new fault causes a local drop in failure potential, and a forward transfer in stress and high failure potential, predicting the location of the next fault. Lower decollement strengths induce high failure potential in the immediate footwalls of frontal thrusts, favoring the formation of backthrusts and pop-up structures at the toe of the wedge, whereas failure potential is highest some distance from the frontal thrust above strong decollements, favoring new forethrusts.