Abstract
The processes that initiate and maintain prograde ductile shear zones are not well understood. We have performed shear experiments (γ = 1 to 3) on a fine-grained (100– 150 μm) gneiss (59% quartz, 28% plagioclase, 13% aligned but not interconnected biotite) to determine the evolution of deformation mechanisms that produce dramatic strain weakening and localization, at conditions (1.5 GPa, 800 °C,