Modeling the evolution of salt structures using nonlinear rocksalt flow laws

Abstract

We analyzed the evolution of a single wavelength (λ = 13.4 km) salt structure of initial amplitude Ao = 100 m by means of a 2D finite-volume numerical model. The model incorporated two laboratory-derived rheological laws for rocksalt: a low-stress, dislocation-creep power law (ε˙ασ3.4) and a fluid-assisted diffusion creep law (ε˙ασ/Td3), in whichε˙is strain rate, σ is the equivalent stress, T the absolute temperature and d the grain size. Our model also accounts for salt thermal conductivity dependence on temperature. The models comprised 3.6 km of sediments overlying a 1.2-km-thick salt layer with a density difference between sediments and salt of 100 kg/m3 and sediments viscosity μ2 = 3.0 × 1019Pa s. We assumed that deformation is driven solely by buoyancy forces. Results predict that salt structures evolve faster (2- to 3.5-fold) in fluid-assisted models than in dislocation creep models. Equivalent stresses and viscosities predicted by the fluid-assisted law are lower than those predicted by the power law, by factors of 3, and 1 to 2 orders of magnitude, respectively, depending on the degree of structural maturity. In contrast, strain rates predicted by both rheological laws are nearly the same, near 10−14 s−1 for similar degrees of structural maturity. Temperature distribution in the model domain was controlled by the thermal conductivity contrast between salt and sediments, the shape of the salt structure and thermal conditions imposed at model boundaries. At the early stages of evolution, a positive thermal anomaly of +30°C developed at the top of the salt structure. At more advanced stages (e.g., diapiric stage), the thermal anomaly decreased, maintaining values between +5°C and +20°C.