Permeability enhancement via shear slip is commonly accepted as the main stimulation mechanism. However, the mechanism of permeability increase appears to have been understood to be limited to shear dilation and perceived to exclude the propagation in tensile and shear mode of the natural fractures that experience slip. This has led to the claims of the discovery of a new stimulation mechanism, namely, stimulation via wing crack propagation. The root cause of the misconceptions is likely the inability to model natural fracture propagation and coalescence. However, natural fracture propagation in general and wing cracks in particular are to be viewed as an integral part of the shear slip stimulation mechanism because shear slip increases the stress-intensity at the fracture tips, potentially leading to fracture propagation. In an effort to better illustrate the underlying mechanisms in the geothermal reservoir stimulation process, a displacement discontinuity (DD) model is developed and used to simulate secondary crack propagation associated with natural fracture slip. The model uses Mohr-Coulomb joint (contact) elements and rigorously accounts for fracture propagation. The model is applied to explore the conditions conducive to shear and tensile mode fracture propagation. When natural fractures undergo shear slip due to direct and indirect water injection, out-of-plane wing (tensile) cracks form and propagate at injection pressures below the minimum in-situ stress level and turn toward the maximum in-situ stress direction as they grow longer. It was found in our results that the injection pressure is stabilized at approximately the minimum in-situ stress level. The secondary cracks form as wing cracks and/or shear cracks. The predominance of wing cracks and their lengths and propagation paths were found to be controlled by the relative value of differential and mean stresses, natural fracture length, as well as rock and natural fracture frictional properties. In deeper geothermal reservoir with relatively low differential stress conditions and/or high mean stress levels, the shear crack propagation could play a major role in fracture network formation and permeability enhancement.