Progressive fracture, subsequent cataclasis, and syntectonic alteration of feldspar to phyllosilicates transformed a massive granite into a quartz-mica phyllonite during early Tertiary deformation in a 20-30-m-wide shear zone in southeastern Arizona. The ductile deformation proceeded at temperatures and pressures approximating, middle to upper crustal levels. Most of the deformation was taken up by the feldspar and its alteration products. Quartz deformed by both brittle fracture and crystal plastic mechanisms. These results show that under certain temperature-pressure-strain rate conditions feldspars are weaker than quartz. The observed deformation mechanisms indicate that the brittle to ductile transition in fault zones is not simply a function of the onset of temperature-activated plastic deformation in quartz nor of the frictional behavior of rocks, as inferred in the familiar strength vs. depth curves. It is more likely that the transition is complex and depends on a variety of parameters. One such factor delineated in this study is the fracture and fracture-facilitated syntectonic alteration of feldspar under hydrated conditions.