We examine the effect of anisotropic viscosity on the thermal structure of  subduction zone mantle wedges. Abundant observations of seismic anisotropy in subduction zones suggest that the material in the mantle wedge has a strong fabric and may be mechanically anisotropic. Using two-dimensional finite-element kinematic models we find that anisotropic viscosity causes three  substantial changes:
(1) a hotter slab-wedge interface
(2) a smaller partially molten region
(3) time variability of the melt production rate and excess temperatures

A hotter slab-wedge interface can change the depth extent of the seismogenic zone, limit the depth to which hydrous minerals can carry water, and influence flux melting. Time-variability, a result of heterogeneity in material alignment,
can explain temporal changes in subduction zone magmatism without invoking a change in the wedge geometry, slab age or composition. We therefore recom mend that anisotropic viscosity, as well as time-dependence, be considered in future models of wedge thermal structure.

The degree of anisotropic viscosity and the grain size of upper mantle minerals are two important rheological parameters that are generally poorly constrained.
We use numerical models of asthenospheric flow to determine the grain
size and anisotropic viscosity required to explain the observed confinement of seismic anisotropy to a layer at the top of the convecting upper mantle. We find that a grain size larger than 10mm gives the best fit to the observations. The ratio of shear viscosity to normal viscosity is 0.3 or higher (less anisotropic), depending on the grain size.