Investigating the Mechanics of Fault-Bend Folding with the Discrete Element Method

Using the discrete element method (DEM), we examine the mechanical controls on fold development related to slip on an underlying thrust fault. The emergent mechanical behavior of these “numerical sandbox” models is directly governed by the physical interactions between the discrete elements, or particles. Our 2D models are composed of tens of thousands of bonded, frictional, elastic disks. These discrete particles are generated above a pre-defined fault surface and form the numerical stratigraphy through a process of gravitational compaction that mimics natural sedimentation processes. Once compacted, layers of elements are selected to receive bonding at particle contacts in order to match strength and elastic moduli parameters derived for real rocks.

After producing this mechanically layered hangingwall strata, we initiate slip on the underlying fault by driving an elastic, vertical backstop. This backstop produces shortening and contraction in the system that localizes as folding above a bend in the fault. For this study, we examine both anticlinal (ramp-to-flat) and synclinal (flat-to-ramp) fault geometries of various ramp dips. Additionally, new particles are generated and deposited as shortening of the model progresses, thereby producing sequential growth layers which record the history of folding. These resulting growth and pregrowth fold architectures are then analyzed in comparison to kinematic theories often used to relate fold characteristics to fault activity and slip.

These analyses show that the DEM models produce the first-order fold geometries predicted by the kinematic theory of fault-bend folding. We note, however, that the development of folding within the growth strata deviates from this theory as manifest by a steepening of the limb dips with depth. We believe this deviation from kinematic fault-bend folding to be a realistic feature of fold development resulting from an axial zone of finite width produced and controlled by the mechanical properties of the strata.