THE EFFECTIVE VISCOSITY OF ROCK-SALT - IMPLEMENTATION OF STEADY-STATECREEP LAWS IN NUMERICAL-MODELS OF SALT DIAPIRISM

A steady-state creep law for rocksalt, describing the two parallel mec hanisms of dislocation creep and fluid-enhanced grain-boundary diffusi on creep, has been used in numerical models of salt diapirism, to stud y 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 gradi ent between 25 and 35 K/km. For strain rates of 10(-12)-10-(15) s(-1), typical of salt diapirism driven by buoyancy alone, the diffusion cre ep mechanism dominates at the fine grain sizes, with dislocation creep becoming important in coarsely grained salt. The effective viscosity ranges from 10(17) Pa s for small grain size and high-temperature salt to 10(20) Pa s for large grain size and low-temperature salt. The vis cosity is strongly dependent on grain size and moderately dependent on temperature. For the larger grain sizes, the dislocation creep mechan ism 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 dynami cs are at the lower end of the range that we find from these numerical experiments.