Finite-element analyses of thrust tectonics: Computer simulation of detachment phase and development of thrust faults

Abstract

The mechanics of thrust fault formation in rectangular and wedge-shaped, homogeneous thrust blocks are examined using finite-element techniques incorporating elasto-plastic material behaviour. Compression is simulated by applying an incremental end displacement parallel to the top of the horizontal or inclined basement. Shear zones (or faults) can form where the material experiences plastic strain softening. These models simulate that thrusts initiate in sequences that are “pushed from behind”, by some tectonic force, over a decollement horizon. In all models, the formation of thrusts is preceded by the formation of a basal slip plane near the base of the block or in a weak substratum. The front of this slip plane does not control the location of thrusts formed subsequently. In the rectangular block, a thrust forms at the pushed end. In the tapered wedge, drained as well as undrained, a fault system composed of a thrust and a backthrust forms. It is argued that the geometry of the block is the main controlling feature for the formation of thrusts within the tapered wedge. The shape of reverse faults depends on the magnitude of shear stresses acting along the basal slip plane. The higher the shear stress, the more pronounced the curvature. In the drained tapered wedge, the intersection of the backthrust with the basal slip plane appears to control the location of the thrust. Under undrained conditions, the backthrust forms subsequent to the thrust and acts only as an accommodation feature.