Large deviations from ''cubic-law'' dependence of laminar fluid flow through fractures on the apparent mechanical aperture of a fracture can be explained by assuming: 1) cubic-law dependence of flow on the actual local aperture at the microscopic level; 2) conservation of rock volume when deforming the fracture; and 3) macroscopic flow properties are dominated by the critical neck (the smallest aperture along the path of highest aperture through the fracture). ¿ American Geophysical Union 1988