Dislocation-accommodated deformation in blueschists from high-pressure experiments

Subduction zone dynamics depend on the rheological behavior of shear zones defining the plate interface, which can accommodate a spectrum of slip from earthquakes to creep. In cold subduction zones, high-pressure, low-temperature metamorphism of the oceanic crust forms blueschist-facies rocks within the interface. Geologic observations demonstrate that deformation at the plate interface is often localized into blueschists with fabrics dominated by glaucophane, a sodic amphibole. Microstructural observations of exhumed blueschists suggest that glaucophane typically deforms by dislocation-accommodated mechanisms. Recent experiments have made progress in developing new flow laws for diffusion creep and dislocation creep. However, the rheological behavior of blueschists remains under-constrained due to challenges arising from phase stability that limits experimental temperatures. Therefore, we conducted experiments at very high pressures to suppress brittle deformation and favor dislocation-accommodated mechanisms to investigate low-temperature plasticity in glaucophane. We conducted cyclical loading experiments in the deformation-DIA on blueschist cores, both parallel and perpendicular to foliation, and cold-pressed glaucophane powders. Experimental conditions covered confining pressures of 6–8 GPa and temperatures of 20–800 °C at strain rates of ~10^–4 s^–1. We observe temperature-dependent yield and flow stresses and nearly temperature-independent backstresses. These mechanical results as well as the microstructures are characteristic of deformation dominated by dislocation glide. These experiments provide the foundation for a constitutive law describing low-temperature plasticity in glaucophane, which will serve as an input for geodynamic models that aim to understand the physics of strain localization, quantify steady-state interface strength, and explain the mechanics of slip transients.