Anisotropic viscosity in geodynamical flow models – Rayleigh-Taylor instabilities as a test example

abstract: Rocks often develop fabric when subject to deformation, and this fabric causes anisotropy of physical properties such as viscosity and seismic velocities. While predicting anisotropy from mantle flow models is becoming increasingly popular, its effect on the flow has yet been investigated throughly. We employ analytical solutions and two-dimensional numerical flow models to investigate the effect of anisotropic viscosity on the development of Rayleigh-Taylor instabilities, a process strongly connected to lithospheric instabilities. Our results demonstrate a dramatic effect of anisotropic viscosity 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 anisotropic viscosity which favors horizontal shear. We also find that an interplay between regions with different initial fabric gives rise to striking irregularities in the downwellings. An inherent part of our investigation was the development and tracking of anisotropic fabric by the flow. We compare three methods for tracking fabric development - directors, finite strain ellipses and a kinematic crystallographic method, and find that directors provide a reasonably accurate and efficient prediction tool, appropriate for incorporation into self-consistent anisotropic mantle flow models. 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 anisotropic viscosity should be included. ----------------------------------------------------------------------------- Atelier de Dynamique des Systèmes Géologiques -----------------------------------------------------------------------------