Discrete-element modeling (PFC3D) of polyaxial servo-control experiments: Testing the orthorhombic fault model

Plane-strain deformation of the upper crust commonly results in the formation of two (conjugate) sets of faults. It has been proposed that nonplane-strain deformation can result in the formation of three or more sets of faults. This hypothesis is based on the results of rock-deformation experiments (e.g., Reches and Dieterich, 1983) and field observations (Reches 1978; Krantz 1988) of shallowly deformed rocks. The four set of faults are thought to be arranged in orthorhombic symmetry to the principal axes of bulk finite strain. This model has been extended to explain the formation of multiple sets of en echelon vein arrays and kink bands during bulk nonplane-strain deformation (Kirschner and Teyssier 1994; Kirschner and Teixell, 1996). We have used discrete-element modeling to test the orthorhombic hypothesis for faults. Using the DEM software PFC3D sold by Itasca Consulting, Inc., we have tried to qualitatively reproduce the experimental results of Reches and Dieterich (1983). It is possible to make the boundaries of the sphere-packed cube to be frictionless, which was a very important consideration in the rock deformation experiments. Results have been obtained for plane-strain and nonplane-strain contraction, and extension "experiments".