Rheological behaviour of synthetic rocksalt: the interplay between water, dynamic recrystallization and deformation mechanisms

The ductile deformation of rocks in nature can be greatly enhanced by the presence of water. Part of the water-induced weakening of rocks at depth may come from fluid-assisted deformation or recrystallization mechanisms that are absent in dry rocks. In this study, we investigate the effect of water on the rheological behaviour of rocksalt. We focus on quantification of the contribution of individual deformation and recrystallization mechanisms to deformation. We also aim to calibrate a flow law that incorporates the effect of all the relevant microphysical processes and hence more accurately describes the flow of rocksalt in nature. For this purpose, the mechanical behaviour and microstructural evolution of synthetic rocksalt samples that are similar, except for differences in water content (determined using FTIR analysis), are investigated. The samples are deformed to natural strains of 0.07-0.46 at 50 MPa confining pressure, strain rates of 5×10−7-1×10−4 s−1 and temperatures of 75-240 °C, resulting in flow stresses of 7-22 MPa. The flow stress of samples with a water content below ∼5 ppm ('dry') is higher than that of samples with a water content of ∼9-46 ppm ('wet') at all strains under the investigated conditions. The difference in flow stress can be explained as due to the operation of only work hardening dislocation creep without dynamic recrystallization in the dry material versus combined dislocation and solution-precipitation creep plus fluid-assisted grain boundary migration in the wet material. The results allow us to calibrate a flow law for wet rocksalt that incorporates the effects of solution-precipitation creep and fluid-assisted grain boundary migration. The results also suggest that strain localization in natural rocksalt is more likely to be localized due to fluid infiltration and associated rheological weakening, than due to progressive removal of strain hardening substructure by grain boundary migration.