We consider the problem of defining the fracture permeability tensor for each grid block in a rock mass from maps of natural fractures. For this purpose we implement a statistical model of cracked rock developed by M. Oda, where the permeability tensor is related to the crack geometry via a volume average of the contribution from each crack in the population. In this model, tectonic stress is implicitly coupled to fluid flow through an assumed relationship between crack aperture and normal stress across the crack. We have included three enhancements to the basic model. (1) A realistic model of crack closure under stress has been added along with the provision to apply tectonic stresses to the fracture system in any orientation. The application of compressive stress results in fracture closure, and consequently, a reduction in permeability. (2) The fracture permeability can be linearly superimposed onto an arbitrary anisotropic matrix permeability. (3) The fracture surfaces are allowed to slide under the application of shear stress, causing fractures to dilate and result in a permeability increase. Through two examples we demonstrate that significant changes in permeability magnitudes and orientations are possible when tectonic stress is applied to fracture systems. ¿ 1998 American Geophysical Union