Development of anisotropic contiguity in deforming partially molten aggregates: 2. Implications for the lithosphere-asthenosphere boundary

Abstract

In this article, 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. During both modes of deformation, the trace of the contiguity tensor remains nearly unchanged. In the companion article [labeled DHM], we outline the numerical methods and present the synthetic micrographs from our numerical deformation experiments. 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 3% anisotropy between shear waves vibrating perpendicular and parallel to the planes of melt films.