Mechanical controls on the rate of phase transformations in deforming rocks: a numerical study applied to quartz-coesite transformation

Interactions between deformation and metamorphic transformations influence plate motions, earthquake nucleation and fluid transport and remain a key issue in geodynamics. Here we explore how mechanical parameters (viscosity and strain rate) influence the spatio-temporal evolution of phase transformations in deforming rocks by using numerical models of matrix-clast systems applied to the quartz-coesite transformation example. The amount of transformed material depends on the dynamic pressure magnitude, while in time the transformation shows an exponential evolution toward a steady-state. The magnitude of dynamic pressure depends on the applied deviatoric stress and scales with the Deborah number. However, our parametric study reveals that the transformation (densification) rate is not controlled simply by stress magnitudes, but by the ratio of the deviatoric stress and the characteristic Maxwell time. This means that for a given viscosity distribution, the strain rate controls the densification rate. Our results represent a further step towards predicting the spatio-temporal evolution of the stress field, deformation mechanisms, and mineral phase distributions in natural shear zones or laboratory experiments.