THE INTERACTION BETWEEN BRITTLE- AND DISLOCATION CREEP IN QUARTZ

Understanding the impact of rheology on fault behavior is of fundamental importance for plate tectonics and earthquake generation. Lithosphere scale fault zones operate over a broad range of pressure - temperature conditions and therefore a variety of deformation mechanisms contribute to the integrated strength of the lithosphere. Deformation is achieved by a complex spatial and temporal interplay between “viscous” and “brittle” processes. Conceptual understanding and experimental results show that brittle and viscous deformation can occur simultaneously in a single mineral phase. The interaction becomes most pronounced around the so called “brittle - viscous transition” where viscous flow cannot accommodate all the imposed displacement and abundant pervasive fracturing occurs and where rocks deform by “semi-brittle” flow.

Here we combine a brittle creep flow law with a wet dislocation power law flow law to investigate the conditions under which we would expect the co-existence of brittle and viscous behavior in one mineral phase. We attempt to quantify the contribution of the brittle and viscous deformation mechanisms to the bulk deformation. Due to the limited experimental data available we concentrate on the brittle to ductile transition in quartz. We compare our calculations with findings from rock physics experiments. Both flow laws show a good agreement with measured data points, however, they both show a high dependence on the input parameters. Theoretically both of the two flow laws can be dominant in the explored parameter space depending on the choice of input parameters. With the currently available flow laws fitted to experimental data that have been published for quartz or quartz rich rocks we are not able to find a parameter space where both flow laws contribute significantly to the overall strain. We discuss the uncertainties in extrapolation of the flow laws and encouraging new studies in rock-physics as well as theoretical considerations.