The thrust directions in foreland thrust belts have not been explained on theory. Based on Coulomb fracture criterion and deformation asymmetry in foreland thrust belt, the origins of fore-thrust and back-thrust faults are analyzed in this paper. Two potential conjugate fractures would occur in initial deformation stage, and the fracture requiring less applied forces might develop into thrust fault under the quasi-static equilibrium caused by deformation asymmetry in foreland thrust belts. The applied forces needed to create fault movements include the frictions along both the detachment surface and the fault surface. The fore-thrust faults will occur in most deformations in foreland thrust belts. However, where either the principal stress axes tilt toward foreland or the intersections point of the two conjugate fractures are on the detachment surface, the back-thrust faults will be preferable to occur. The applied force, the frictions along the detachment surface and the geometries of the slipping terranes will determine the principal stress axes. The findings will help to explain the selectivity of the fault dips in both contractional and extensional deformation areas.
