Microstructural evolution during the deformation of polymineralic rocks

Mylonites are ubiquitous structural features of dynamic plate boundaries, and are widely assumed to represent the product of localized deformation at high pressure and temperature. There are two features of mylonites that distinguish them from typical host rocks: grain-sizes that may be reduced by orders of magnitude and mineral phases that generally well-mixed. Together, these microstructural characteristics are thought to promote rheological weakening over long geologic intervals, an essential feature of Earth-like plate tectonics. This poster will highlight recent progress towards understanding the processes of grain-size reduction and phase mixing. Deformation experiments to shear strains of > 50 have been conducted using the Large Volume Torsion Apparatus (LVT) at Washington University. Starting materials for these experiments include calcite and anhydrite, calcite and fluorite, and calcite and quartz. Phase mixing is determined to be the product of several independent mechanisms, the relative importance of which depends on pressure, stress, strain, composition, viscosity contrast, and the ratio of the initial grain-size to the recrystallized grain size.