How does viscosity contrast influence phase mixing and strain localization?

Ultramylonites with well-mixed mineral phases are thought to be an essential feature of Earth-like plate tectonics, because coupled phase mixing and grain boundary pinning enable rocks to deform by grain-size-sensitive, self-softening creep mechanisms over long geologic timescales. In isoviscous two-phase composites (and in the absence of chemical exchange or reaction), bulk “geometric” phase mixing occurs via the sequential formation and disaggregation of compositional layering. However, the effects of viscosity contrast on the mechanism(s) and timescale(s) for geometric mixing are poorly understood. Here, we describe a series of high-strain experiments on non-isoviscous calcite-fluorite composites (viscosity contrast, η_ca/η_fl ≈ 200). Under our experimental conditions (500°C, 0.75 GPa confining pressure, 10-6–10-4 s-1 shear strain rate), calcite and fluorite deform by dislocation and diffusion creep, respectively. At low to intermediate shear strains (γ ≤ 10), polycrystalline domains of the individual phases become sheared, and form compositional layering. As layering develops, strain progressively localizes into the weaker phase, fluorite. Strain partitioning impedes mixing by reducing the rate at which calcite layers deform, attenuate, and disaggregate. By employing two-phase flow laws that account for strain partitioning, we show that ultramylonites, shear zones, and plate boundaries form most efficiently when strain partitioning is minimized; that is, when viscosity contrasts are small.