Growth and connection of preexisting fractures have been recognized as the driving mechanism of rock fracturing. This paper aims at throwing light on the geological implications of the fracturing behavior in pre-cracked rock observed in rock mechanics-oriented experiments, by numerical interpretation on the interaction of en echelon fractures. To properly simulate grain crushing involved in cataclastic deformation of nonporous rocks, a sophisticated numerical scheme incorporating rock morphology and crystallography into the grain-based model is proposed. In line with a previous experimental study, seven grain-based models with different configurations of en echelon flaws are numerically tested under uniaxial compression. The key results are (1) evaluation of the influences of preexisting flaws on mechanical properties and damage degree of the host rock, (2) visualization of the microfracture process zone into visible cracks, and (3) generalization of three different patterns of major fracture formation from en echelon fractures, during which the ratio of the number of intergranular to intragranular microcracks is used for evaluation of the cataclastic deformation process and the stress level. Based on the numerical results, we provide valuable interpretation on (1) the field occurrence of strike-slip fault system with typical fracturing features, focusing on the effects of the sense of left or right stepping and the degrees of separation/overlap of the master fractures, and (2) three different mechanisms of fault formation from preexisting fractures. As an initial attempt to relate rock fracturing observed in laboratory experiments with the field occurrence, capabilities and limitations of this study are also presented.