FINITE ELEMENT MODELS OF CONTRACTIONAL FAULT-RELATED FOLDING

In this study we investigate the kinematic and mechanical evolution of contractional fault-related folding phenomenon using forward models. A series of numerical simulations were run to show the effects of material properties, initial fault dip, and the presence of weak inter-layer detachment horizons. We employ a Lagrangian finite element method with adaptive remeshing and a constitutive model that is based on critical state mechanics. This approach allows for large deformation, volumetric changes, and captures the evolution of the failure envelope during progressive deformation. We demonstrate that material properties affect the way faults propagate and thus exert a significant control on resultant fold layer geometry. Models of uniform sandstone properties exhibit efficient strain localization and clear patterns of fault tip propagation. Uniform shale properties tend to inhibit fault propagation due to distributed plastic deformation. Models with mixed inter-layered sandstone and shale deform in a disharmonic manner, resembling lobate-cuspate arrangements that are common to many folds observed in outcrop. Shale-layer detachments accommodate shortening by bed-parallel slip, resulting in fault-bend fold kinematics and a general absence of fault propagation across layers. Constant area based restoration of the deformed models recovers the first-order contractional deformation, for a more accurate inverse model of fault-related folding, however, residual non-flexural-slip strains need to be applied. These results suggest that appreciable discrepancies exist between simplistic flexural-slip based kinematic restoration techniques and more complicated strain paths for lithologically diverse fault-related folds in nature. We track stress paths through time, investigate the complex stress field developed throughout different fault-related folds and explain in a critical state mechanics framework under what circumstances failure results in localized shear band formation versus mechanical compaction. In general, the results of the numerical simulations reveal that the stress state developed in the forelimb is more prone to geomechanical damage than in the backlimb.