Rayleigh-Taylor instabilities with anisotropic lithospheric viscosity

Rocks often develop fabric when subject to deformation, and this fabric causes anisotropy of physical properties such as viscosity and seismic velocities. We employ 2-D analytical solutions and numerical flow models to investigate the effect of anisotropic viscosity (AV) on the development of Rayleigh–Taylor instabilities, a process strongly connected to lithospheric instabilities. Our results demonstrate a dramatic effect of AV on the development of instabilities—their timing, location, and, most notably, their wavelength are strongly affected by the initial fabric. Specifically, we find a significant increase in the wavelength of instability in the presence of AV which favours horizontal shear. We also find that an interplay between regions with different initial fabric gives rise to striking irregularities in the downwellings. Our study shows that for investigations of lithospheric instabilities, and likely of other mantle processes, the approximation of isotropic viscosity may not be adequate, and that AV should be included.