Analytical modelling of viscous diapirism through a strongly non-Newtonianoverburden subject to horizontal forces

We study the early stages of diapirism and analyse the gravitational and bu ckling instabilities of a buoyant viscous layer overlain by a layer of stro ngly non-Newtonian power-law rheology (when a power-law exponent tends to i nfinity). This situation models rocksalt under a layer of a perfectly plast ic overburden. The growth rate of small perturbations on the interface betw een the two layers and the wavelength of the most unstable perturbations ar e found and compared with those of structures consisting of two Newtonian o r two strongly non-Newtonian viscous layers. Effects due to the effective v iscosity and thickness ratios between the two layers are assessed. Consider ing the effective viscosity of the overburden to be much greater than the v iscosity of the buoyant salt layer, we obtain the following results. In the case of simple gravitational instability and no-slip boundary conditions, the instability pattern is similar to that in a strongly non-Newtonian powe r-law material. An increase in the thickness of the overburden decreases th e dominant wavelength of the most unstable mode, while the dominant wavelen gth is lengthened in the case of Newtonian viscous layers. When the system of layers is subjected to either horizontal extension or shortening, and th e upper boundary of the system is stress-free, the buckling instability ove rwhelms the gravitational instability, and the dynamic growth rate of the i nstability depends linearly on the effective viscosity ratio. We conclude t hat the introduction of strongly non-Newtonian power-law rheology into diap ir overburdens greatly affects instability parameters such as growth rate a nd dominant wavelength of perturbations and hence, alters interdiapir spaci ngs. (C) 2001 Elsevier Science Ltd. All rights reserved.