HR: 11:50h
AN: T12B-07    [Abstracts]
TI: Anisotropic viscosity in geodynamical flow models - Rayleigh-Taylor instabilities as a text example
AU: * Lev, E
EM: einatlev@mit.edu
AF: Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02140, United States
AU: Hager, B H
EM: brad@chandler.mit.edu
AF: Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02140, United States
AB: Rocks often develop fabric when subject to deformation, and this fabric causes anisotropy of physical properties including effective viscosity and seismic velocities. While predicting anisotropy from mantle flow models is becoming increasingly popular, the effect of viscous anisotropy on the flow has yet to be investigated thoroughly. We employ analytical solutions and two-dimensional numerical flow models to determine the effects of anisotropic viscosity on the development of Rayleigh-Taylor instabilities, a process strongly connected to lithospheric instabilities. Anisotropic viscosity has significant effects on the development of instabilities --- their timing, location, and, most notably, their wavelength, are strongly affected by the initial fabric. We find a significant increase in the wavelength of instability in the presence of anisotropic viscosity orientated to favor horizontal shear. We also find that interplay between regions with different initial fabrics gives rise to striking irregularities in the downwellings. An inherent part of our investigation is 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. We find the directors method to 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. Thus anisotropic viscosity should, and can fairly easily, be included.
DE: 0545 Modeling (4255)
DE: 1236 Rheology of the lithosphere and mantle (7218, 8160)
DE: 8020 Mechanics, theory, and modeling
SC: Tectonophysics [T]
MN: 2007 Fall Meeting