Development of anisotropic contiguity during deformation and implications for the Lithosphere-Asthenosphere Boundary

Abstract:

The microstructure of partially molten rocks strongly influences the macroscopic physical properties. Contiguity, a geometric parameter, is a tensorial quantity that describes the area fraction of intergranular contact in a partially molten aggregate. It is also a key parameter that controls the effective elastic strength of the grain network. As the shape of the grains evolve during deformation, so does the contiguity of each grain. We present the first set of numerical simulations of evolution of grain-scale contiguity of an aggregate during matrix deformation using a Fast Multipole Boundary Elements Method (FMBEM) based model. Using the results of contiguity, we calculate the seismic anisotropy resulting from melt redistribution during pure and simple shear deformation. Deformation strongly modifies the geometry of melts initially occupying three grain junctions. The initially isotropic fractional area of intergranular contact, contiguity, becomes anisotropic due to deformation. Consequently, the component of contiguity evaluated on the plane parallel to axis of maximum compressive stress decreases. In pure shear deformation, the principal contiguity directions remain stationary while they rotate during simple shear. The ratio between the principal components of the contiguity tensor decrease from 1 in an undeformed aggregate to 0.1 after 45% shortening in pure shear and to 0.3 after a shear strain of 0.75 in simple shear. In both pure and simple shear experiments anisotropy in the shear wave velocity increases with the strain in a strongly nonlinear fashion. In pure shear deformation, the steady-state microstructure produces nearly 4% anisotropy between shear waves vibrating perpendicular and parallel to the planes of melt films.