A thermodynamic formulation for pack ice is presented that includes a means of relating stresses in floes to thermally induced strain rates. The formulation combines some basic thermodynamics as detailed in previous research. The mechanics component includes viscous creep and mechanically induced strain rates due to typical floes being too large to bend or twist in response to spatially varying strain rates. Moreover, the rheology quantifies the impact of existing cracks in the floe on the overall strain rate in the floe and the resulting stress state. A paradigm is proposed which specifies tensile fracturing and the resulting stress relief as proportional to the amount of existing cracks in the floe and some measure of the tensile strength of the ice. This allows the thermomechanical equations to be used for both first-year and multiyear floes by differentiating the two by the extent of existing cracks. The thermomechanical equations are applied in an inverse fashion to solve for flaw structure for a first-year floe as well as a multiyear floe. This is done by matching stresses predicted by the thermomechanical equations to observed stresses in the floes. The results indicate that the top two thirds of the ice in the first-year floe was relatively free of cracks while the bottom third of the floe had cracks covering ~50% of the ice. In the multiyear floe, cracks covered 70% to 90% of the ice in the top two-thirds of the floe while the bottom third of the floe had cracks covering ~30% of the ice. ¿ 1998 American Geophysical Union