The rheology of fault rocks (brittle faults and ductile shear zones) is a key problem in structural geology and tectonics. Rheological information is important to model localization of deformation, to study and predict the occurrence of earthquakes, and to understand orogeny in general. The proposed research focusses on the identification of deformation processes and their rheological consequences in low temperature fault rocks. The potential transition in rheology due to a switch in deformation mechanism during the deformation history is the main topic of investigation. The approach is based on a microstructural comparison of experimentally and naturally deformed rocks. The transition will be studied as a consequence of fluid-rock interaction after initial fracturing.