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,

= 2 × 10−5/s) where pure quartz aggregates deform homogeneously by dislocation creep. Initial yield occurs where stress concentrations at the tips of weak biotite grains produce semibrittle deformation in intervening quartz or plagioclase, allowing local biotite interconnection by slip on (001) and initiating strain weakening. After yield, the interconnected biotites kink and thus strengthen, but the highly strained parts of grains react to a fine-grained, mixed-phase assemblage which deforms by grain-size–sensitive creep, allowing further strain weakening and localization. At γ = 3.3, the strain and strain rate in the ∼100-μm-thick shear zone are 100 times that of the enclosing host rock, similar to the localization observed in natural shear zones. Thus, the processes that initiate strain localization are not necessarily the same as those that preserve the weak shear zone, and once formed, a shear zone may be permanently weakened.

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