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 degrees C and 500 degrees C) and fast shear strain rates (similar to 1.8 x 10(-4) s(-1)) where the material deforms by semi-brittle flow. Samples deformed at 500 degrees C are systematically weaker than samples deformed at 300 degrees 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. (C) 2011 Elsevier Ltd. All rights reserved.