Microstructural evolution of micaceous mylonites

Abstract

In crustal shear zones, the grain size and geometric arrangement of mica is observed to evolve significantly with strain. The transition from tabular, interconnected mica layers to a well-mixed fine-grained ultramylonitic microstructure is inferred to reduce rock viscosity. This evolution is predicted to play an important role in the strength of the crust, particularly at plate boundaries. However, the mechanisms of mica layer disaggregation and phase mixing at high strains have not been widely explored. In this study, we conducted deformation experiments (up to shear strains of $\gamma >100$) on synthetic composites of mica (fluorphlogopite) and either calcite or fluorite in the Large Volume Torsion Apparatuses (LVTs) at Washington University in St. Louis. Experiments were performed at T = 765 °C, P = 1.5 GPa, and strain rates of ∼1x10⁻⁴ s⁻¹. Deformation microstructures were analyzed using optical and electron microscopy and electron backscatter diffraction (EBSD). Microstructural evolution is sensitive to shear strain (γ) but largely insensitive to the composited material. Mica grain size is reduced through brittle mechanisms and mixing may be enhanced by phase boundary sliding. Interconnected layers of mica disaggregate and are fully mixed with the composited material at $\gamma >50$. This sequence of deformation processes is also inferred from the complementary analysis of specimens from the Kuckaus Mylonite Zone in the Namaqua Metamorphic Complex in southwestern Namibia. This process of mica grain size reduction and phase mixing may contribute to the transition from mylonite to ultramylonite in high temperature crustal shear zones.