Consequences of viscous anisotropy in a deforming, two-phase aggregate. Why is porosity-band angle lowered by viscous anisotropy?

TL;DR

This paper investigates how viscous anisotropy influences melt segregation in partially molten rocks, revealing two mechanisms that explain the formation of low-angle porosity bands observed in experiments.

Contribution

It introduces a comprehensive model of viscous anisotropy, elucidates the mechanisms behind low-angle band formation, and distinguishes effects of static versus dynamic anisotropy.

Findings

Viscous anisotropy suppresses tensile instability.

Viscous anisotropy creates a shear-driven instability.

Model explains low-angle porosity bands in experiments.

Abstract

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.