The paper presents the results of 3D numerical modeling of fault formation due to strike-slip displacement of basement blocks on the example of model rock massifs differing in thickness. It is shown that the material thickness and initial stress state substantially influence the form of fracture structures arising in strike-slip faulting. The formation of a vertical fault with feathering Riedel fractures is characteristic of high-strength material and small thickness. If the material thickness is large, there arises a complex spatial flower structure with inclined fractures oriented in the horizontal plane at an angle of 40 to the shear axis. The inclined fractures represent strain localization bands or so-called shear bands developing as pairs of "flakes" that resemble opened oyster valves