Factors that control the development of fault-bend versus fault-propagation folds: Insights from mechanical models based on the discrete element method (DEM)

We investigate the role and relative importance of a range of geometric and mechanical factors in the development of contractional fault-related folds, with an emphasis on defining the factors that promote the development of fault-bend and fault-propagation folds. We construct a series of discrete-element mechanical models in order to test the effects of fault dip, bulk material strength, mechanical layer anisotropy and spacing, sedimentation rate, and boundary conditions on the style of fault-related fold that develops. We find that fault-bend folding is most favored at low fault ramp dips, with thinly-spaced mechanical layers, and strong layer strength contrasts. In contrast, conditions that inhibit slip on a potential upper detachment surface, such as increased friction and a fixed foreland boundary, encourage the development of fault-propagation folds. Additionally, steeper fault dips, more widely-spaced mechanical layers, and decreased layer strength contrast favor the increased localization of shear during the growth of structures. This leads to structures that deform by a mixture of fault-bend and fault-propagation folding styles. Observations of the distortional strains that develop in the model provide insight into the relationship between the different deformation mechanisms, such as flexural slip and localized shear, which accommodate structural growth and ultimately determine fault-related folding style. Thus, these models provide a context for understanding how rock and fault properties influence whether structures evolve as fault-bend or fault-propagation folds, or as combinations of these end members. We apply these insights to interpret two natural examples from the offshore Niger Delta outer fold-and-thrust belt that exhibit changes in structural style through time as a result of changes in fault properties and syntectonic sedimentation.