Strain Localization and Mechanical Coupling during Fault-Bend Folding: Insights from Discrete Element Simulations

Using the discrete element method (DEM), we study the influence of mechanical stratigraphy and strain localization on the evolution of fold development produced by slip on an underlying thrust fault. These simulations produce emergent behavior that is directly governed by the physical interactions between the discrete elements, or particles. Our 2D models are composed of tens of thousands of frictional, elastic disks that are generated above a pre-defined fault surface and allowed to gravitationally settle, compact, and bond to produce a mechanically stratified hangingwall section. Slip is then initiated on the underlying fault through the use of a horizontally driven, vertical backstop. As shortening of the model progresses, new particles are generated and deposited, thereby producing sequential growth layers. These growth layers record the evolution of deformation in the system, which localizes as folding above a bend in the fault. In this study we focus on the deformation field that develops during folding

We examine the partitioning of strain in these simulations using incremental displacements obtained by differencing particle positions. A nearest neighbor gridding algorithm is employed to obtain spatially averaged displacement vectors on a dense grid imposed over the model domain. Components of the 2-D displacement gradient tensor are then calculated for each grid node, and these in turn are used to calculate the incremental tensors for finite strain and deviatoric strain. These analyses show that the mechanical stratification built into our models encourages the development of what can be described as flexural slip surfaces. These zones of intense shear localize the strain during deformation and folding in a manner very similar to what might be expected in real rock that has developed penetrative mechanical layering. We document the partitioning of strain, as it accommodates folding, into interlayer slip, cross-layer fracture and compaction (dilation), and bed rotation.