Quartz Flow Law Revisited: The Significance of Pressure Dependence of the Activation Enthalpy

An accurate flow law for dislocation creep of quartz aggregates is critical for the understanding of continental rheology, numerical modeling of lithospheric processes, and the interpretation of microstructures of natural quartz-bearing mylonites. Despite many decades of research, considerable discrepancies still exist among quartz flow laws determined from different experiments. We demonstrate that the key to reconcile these discrepancies is to consider the pressure dependence of the activation enthalpy. From existing high-quality creep experiments on quartz aggregates, we critically identified test runs having microstructures and stress-strain curves indicating steady state regimes 2 and 3 dislocation creep and used the data to obtain a set of flow law parameters most consistent among existing experiments. Because estimates of strain rate and stress from natural mylonites are bound with large uncertainties, they cannot be used to construct a flow law better than one determined from well-controlled creep experiments. A large number of geological studies and modern GPS observations suggest that crustal scale ductile shear zones likely flow at a strain rate range between 10(-13) and 10(-11)s(-1). In this strain rate range, our flow law predicts flow stresses broadly consistent with stress estimates from many natural mylonites based on well-calibrated piezometers for dynamically recrystallized quartz grains.