The relationship between frictional and viscous deformation and the transition between these processes are poorly understood. We propose to perform gouge experiments with good mechanical reproducibility over a wide range of elevated pressures and higher temperatures (up to 1.5 GPa, up to 900°C) in order to gain insight of seismic and aseismic processes in the brittle-viscous transition regime. The project will be complemented by collaborative field studies of naturally deformed rocks associated with an exceptionally well-exposed normal fault system led by the University of Wyoming (PI: Prof. Barbara John). Two subprojects are proposed: Subproject A is concerned with the brittle-viscous behaviour of typical crustal rocks using polyphase gouges of granitoid and basaltic composition to represent the continental and oceanic crust. In subproject B, we want to re-focus on one of the most important key minerals of the Earth's crust - quartz - because quartz is a very well researched key mineral and can be used to infer stress and strain rate information based on existing flow laws and because insight gained from recent experiments shows the immense importance of brittle deformation processes for the general viscous deformation in quartz (especially during initial stages). The current submission is the ninth in a well-established research effort concerned with the deformational behaviour of rocks at various levels of the earth's crust. So far, all projects were supported entirely by the Swiss National Science Foundation and carried by the research group in Basel.