Semi-brittle deformation of granitoid gouges in shear experiments at elevated pressures and temperatures

The physical and chemical processes acting in the ‘brittle-to-plastic’ transition are of great interest for a better understanding of fault rheology. We performed a series of experiments on granitoid gouge material under high confining pressures (Pc = 500–1500 MPa), temperatures (T = 300 °C and 500 °C) and fast shear strain rates (∼1.8 × 10−4 s−1) where the material deforms by semi-brittle flow. Samples deformed at 500 °C are systematically weaker than samples deformed at 300 °C over the whole examined confining pressure range indicating a non-frictional component of the deformation. All samples develop an S–C′ fabrics and deformation localizes in slip zones containing ‘amorphous’ feldspar material with an intermediate composition (Na, Ca and K-rich). Further, we observe changes in composition of feldspars (enrichment in the albite component) in the highly fragmented – but crystalline – regions with increasing finite shear strain. Our results indicate that mass-transfer processes keep pace with frictional deformation even at high strain rates and together with viscous flow of the ‘amorphous’ material are responsible for the observed strength difference.