  In laboratory experiments that impose shear deformation on partially molten
aggregates of initially uniform porosity, melt segregates into high-porosity
sheets (bands in cross-section). The bands emerge at 15-20 degrees to the shear
plane. A model of viscous anisotropy can explain these low angles whereas
previous, simpler models have failed to do so. The anisotropic model is
complex, however, and the reason that it produces low-angle bands has not been
understood. Here we show that there are two mechanisms: (i) suppression of the
well-known tensile instability, and (ii) creation of a new, shear-driven
instability. We elucidate these mechanisms using linearised stability analysis
in a coordinate system that is aligned with the perturbations. We consider the
general case of anisotropy that varies dynamically with deviatoric stress, but
approach it by first considering uniform anisotropy that is imposed a priori
and showing the difference between static and dynamic cases. We extend the
model of viscous anisotropy to include a strengthening in the direction of
maximum compressive stress. Our results support the hypothesis that viscous
anisotropy is the cause of low band-angles in experiments.
