Discrete element modelling of contractional fault-propagation folding above rigid basement fault blocks

Many studies have shown that discrete (blind) faults at depth are commonly linked to more distributed deformation, in particular folding, at higher levels. One category of fault-related folds, forced folds, is common where there is a distinct mechanical contrast between faulted basement and sedimentary cover. Outcrop, numerical and analogue modelling studies indicate that such folds form as upward widening zones of distributed deformation (monoclines) above discrete faults at depth. With increasing displacement the folds are often cut by faults as they propagate upwards into the cover. While the trishear kinematic model of fault-propagation folding appears to approximately represent the geometric development of such structures, comparatively little is known of the mechanical controls on their development. Here we present a 2D discrete element model of sedimentary cover deformation above a contractional fault in rigid basement. The elements consist of a series of soft spheres that obey Newton's equations of motion and initially interact with elastic forces under the influence of gravity. Particles are bonded until the separation between them exceeds a defined breaking strain at which time the bond breaks, simulated by the transition from repulsive-attractive forces to solely repulsive forces. The model is used to investigate the influence of basement fault dip and sedimentary cover strength on the geometry of the folds developed and the rate of fault propagation. In all cases an upward widening monocline occurs above the basement fault. We find that shallow basement fault dips produce homogenous thickening of the monocline limb while steeper dips produce contemporaneous thinning and thickening within the monocline. Thinning and thickening within the monocline are accommodated by a combination of small-scale faulting and folding. With decreasing cover strength, the zone of deformation becomes wider, localization does not occur on a single fault and fold geometries resemble trishear fold profiles with low propagation to slip ratios (p/s similar to 1). In contrast, a stronger cover produces a narrower zone of deformation, localization on a single fault and more rapid fault propagation (similar to trishear fold profiles where p/s similar to 2-3). The fault propagates into the cover at approximately the same angle as the basement fault. The model reproduces well many of the features observed in analogue modelling and reported from outcrop and seismic studies. (C) 2002 Elsevier Science Ltd. All rights reserved.