Critical state finite element models of contractional fault-related folding: Part 1. Structural analysis

Abstract

Fourteen Lagrangian finite element models with a critical state mechanics constitutive model illustrate some of the primary controls on the formation of fault propagation, fault bend, and diffuse folds. The models demonstrate how variable mechanical stratigraphy, initial fault dip and inter-layer detachments affect the way faults propagate and thus exert a significant control on resultant fold layer geometry. For example, 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. Detachments accommodate shortening by bed-parallel slip, resulting in fault-bend fold kinematics and poorly expressed fault propagation across layers. Structural analysis of the numerical model results reveals that contractional deformation is a composite of lateral compaction, pure shear shortening, fault propagation along narrowly localized zones of reverse shear, and flexure of layers. The relative proportions of these shortening components vary in time and with mechanical properties (shale vs. sandstone). Depth-to-detachment calculations performed on selected numerical models suggest that reasonably accurate predictions can be made for detachment fold-thrust belts and toe-of-slope contractional systems. However, our study suggests that applications to mid-crustal level detachments within the basement beneath a sedimentary cover may be inaccurate.