Microstructural observations of naturally faulted granitic gneisses show that feldspar grains are weaker than quartz grains at temperatures below 325°C and at depths of less than 8–10 km. Feldspar grains sustained most of the deformation by grain-scale faulting, slip along cleavage-controlled fractures and cataclasis. Fracture of feldspar grains within fault zones promoted their alteration to kaolinite. Quartz grains also deformed by fracturing, and often healed to form quartz porphyroclasts and mosaics in a comminuted matrix of feldspar and kaolinite. Syntectonic alteration of the feldspar grains may have weakened the fault zones over time and resulted in foliated textures within the fault zones. This study of naturally deformed rocks confirms published experimental results on the behavior of granitic rocks at low temperatures and pressures and, taken together, these data show that the theology of the upper 10 km of the crust is greatly influenced by cataclastic processes in feldspar.