Controls of Grain Breakage on Shear Band Morphology and Porosity Evolution in Fault Gouges

Abstract

Gouge in fault zones generally undergoes grain breakage during shear slip events, resulting in changes in both shear mode and pore structure. We establish a discrete element model representing shearing of granular fault gouge for increasing normal stresses but constant shear velocity (v = 6 μm/s) to investigate the effects of grain breakage on shear band development and the evolution of fault friction and porosity. An increase in normal stress increases frictional strength by ∼20% accompanied by many small slip events triggered by grain breakage. The fragments generated by grain breakage reduce mean grain size and shift the grain size. Dilation and an absence of comminution under low normal stress increase porosity countered by high normal stress developing rapid compaction and grain breakage and decreasing porosity. We propose a concept of porosity evolution linked to volumetric strain. An increase in normal stress results in the principal breakage mechanism evolving from low efficiency abrasion to high efficiency splitting with grain size distribution converging to fractal distributions observed in nature. Heterogeneous grain breakage drives local reduction in porosity, the redistribution of contact stresses and realignment of force-chains, changing the slip pattern and microstructural characteristics through shear band development. At low normal stress, the grain deformation is mainly accommodated by slipping and rolling and the shear bands are dominated by Y shears. With the increase in normal stress, grain breakage promotes the development of the more highly inclined R shears.