Ductile shear zones from brittle precursors in feldspathic rocks: The role of dynamic recrystallization

Intact samples of aplite, albite rock, and quartzite deformed at 900-1100°C, 10^-6/sec, and 1500 MPa all show homogeneous strain by dislocation creep. Pre-faulted samples of aplite and albite rock subsequently deformed at these conditions develop localized ductile shear zones along the former faults, whereas pre-faulted quartzite samples deform homogeneously. The difference in behavior of the feldspathic and quartzite samples is due to the different accommodation mechanisms for dislocation creep in these minerals at these conditions. Feldspars undergo recrystallization-accommodated dislocation creep; recrystallized grains form by grain boundary migration, and are initially strain-free and weaker than the work hardened original grains. In contrast quartz undergoes climb-accommodated dislocation creep; recrystallized grains form primarily by subgrain rotation, and have a similar dislocation density and thus a similar strength as the original grains. In all three materials the early-formed fault gouge sinters at the higher temperature, and forms a planar zone of fully recrystallized material, but only in the feldspathic rocks does this constitute a weaker zone. The same behavior is observed for samples with added water. At natural strain rates, feldspars undergo recrystallization-accommodated dislocation creep at greenschist to amphibolite grade, and for feldspathic rocks deformed at these conditions, pre-existing faults on all scales should preferentially recrystallize and become ductile shear zones. In contrast, pre-existing faults in feldspathic rocks deformed at granulile grade, or in quartzites deformed at greenschist through granulite grade, where the dominant deformation mechanism is climb-accommodated dislocation creep, should not become ductile shear zones.