A threshold stress for pressure solution creep in rock salt: Model predictions vs. observations

Abstract

At low deviatoric stress, flow of rock salt is expected to be controlled by pressure solution creep. This requires a fluid phase to be present in grain boundaries in the form of adsorbed grain boundary films or a dynamically wetting island-channel structure. However, whether such structures can persist at low stress or will be eliminated by healing phenomena is open to question. In this study, a model is derived describing a threshold stress for pressure solution, below which healing occurs, and creep is stopped. The results predict that pressure solution creep in rock salt, at depths up to 3 km, is inhibited below a deviatoric stress of 0.07 to 0.7 or at most 0.90 MPa. When deviatoric stresses in rock salt drop below this threshold stress, a decrease in strain rate from ~10−13 to 10−18 s−1 is expected, due to a switch to dislocation creep. The model needs to be tested against CT experiments or optical observations on grain boundary behavior under stress, and against confined creep tests at very low deviatoric stress, as far as technically feasible. If confirmed, the effect of this threshold stress needs to be considered in future numerical studies of flow in rock salt.