The effective viscosity of rocksalt: implementation of steady-state creep laws in numerical models of salt diapirism

A steady-state creep law for rocksalt, describing the two parallel mechanisms of dislocation creep and fluid-enhanced grain-boundary diffusion creep, has been used in numerical models of salt diapirism, to study the effective viscosity of rocksalt. Typical models included a 3-km-thick sedimentary layer on top of 1 km of rocksalt. The grain size of the salt has been varied between 0.5 and 3 cm and the geothermal gradient between 25 and 35 K/km. For strain rates of 10-‘2-10-‘5 s-‘, typical of salt diapirism driven by buoyancy alone, the diffusion creep mechanism dominates at the fine grain sizes, with dislocation creep becoming important in coarsely grained salt. The effective viscosity ranges from 10” Pa s for small grain size and high-temperature salt to lo*’ Pa s for large grain size and low-temperature salt. The viscosity is strongly dependent on grain size and moderately dependent on temperature. For the larger grain sizes, the dislocation creep mechanism is most effective during the diapiric stage, but the non-Newtonian effects in the salt are not important in determining the growth rate and geometry of the diapirs. The estimates for the Newtonian viscosity of salt that have traditionally been used in modelling of salt dynamics are at the lower end of the range that we find from these numerical experiments.