Fault gouge evolution and its dependence on normal stress and rock strength—Results of discrete element simulations: Gouge zone properties

Abstract

In order to study the process of gouge zone evolution, and its dependence on normal stress, σn, and uniaxial compressive strength, σucs, we simulate the breakdown of fault blocks and growth of fault gouge zones using the distinct element method (DEM) in two dimensions. Breakable elastic bonds were added between adjacent, closely packed particles to generate the fault blocks with a given σucs ranging from 100 to 260 MPa. DEM experiments were conducted by shearing the fault blocks along an initially flat surface for a range of σn from 10 to 100 MPa. The simulated fault gouge zones experience two distinct stages of evolution, i.e., fast growth and slow growth, distinguished by a switch in deformation mechanism from dominantly wear of the fault blocks to dominantly shearing of existing fault gouge. The rate of the gouge zone thickening decreases exponentially during the fast growth stage (to about 20% shear strain, i.e., 7.4 mm shear displacement) and then reaches a relatively constant value, marking the beginning of the slow growth stage. The thickening rate increases with increasing σn and decreasing σucs during the fast growth stage, but the dependency reverses during the slow growth stage. The mean grain size of the gouge shows a first‐order dependence on shear displacement, and varies less significantly with σn and σucs. In our simulated shear zones that undergo both surface wear and grain comminution, gouge grains develop power law size distributions characterized by a two‐dimensional fractal dimension D ranging from 0.6 to 2.4.