A maximum distortional strain energy density criterion is proposed for shear fracture propagation under mixed mode loading conditions. It is shown that an isolated mode-II shear fracture would propagate in its own plane. However, the growth paths of two en e¿chelon shear fractures deviate from straight lines due to crack interaction. Using a displacement discontinuity boundary element method, we show that the shear fracture propagation paths of two en e¿chelon mode-II faults always converge toward each other regardless of the sense of shear and fault step. In contrast, opening (mode-I) fracture growth paths of two en e¿chelon faults depend on the sense of stepping and shearing. The predicted propagation paths of en e¿chelon shear fractures agree well with those observed in the field. ¿ American Geophysical Union 1993