INITIATION OF DUCTILE SHEAR ZONES

Ductile shear zones occur on all scales in the crust, but microstructural evidence of the processes for initiation is obliterated by subsequent strain. To determine the processes which lead to strain localization, shear experiments have been performed using a fine-grained (d~50-100 mm) aggregate consisting of 13% biotite, 32% plagioclase, and 55% quartz. The biotite grains are aligned but not interconnected. Samples with foliation parallel to the shear plane were deformed at 800^oC, P=1500 MPa and a shear strain rate of 2x10^-5/s, with no water added, to g of 1.8, 1.9 and 6.3. In a sample held hydrostatically for 93 hrs at these conditions <1% of biotite reacts (bt + qtz + plag=gt + pyx + Kspar + H₂O).

The high strain sample shows pronounced strain weakening and localization; the strength decreased from 1200 to 380 MPa between g=1.5 and 6.3. Yielding begins by semi-brittle deformation of quartz and feldspar grains in zones of high stress concentrations between weak biotite grains, which undergo easy (001) slip. At slightly higher strain (g=1.9) these processes lead to interconnection of biotite grains along several distinct strands which coalesce into a single very narrow (10-30 mm) shear zone. Deformed biotite grains in the shear zone react extensively to form micron sized garnet + pyx + Kspar grains, causing further weakening.

Strain is highly partitioned into the shear zone. Although the bulk strain rate remains constant at 2x10^-5/s, the strain rate inside the shear zone increases to >10^-4/s and that outside the shear zone decreases to <10^-6/s. These changes in strain rate cause changes in the deformation mechanisms, such that the initial semi-brittle microstructures are overprinted. Inside the shear zone, strain occurs by grain boundary sliding and diffusion creep of the reaction products, regime 1 dislocation creep of quartz and kinking/slip of remnant biotite. Outside of the shear zone, quartz deforms by regime 2 dislocation creep. These experimental results demonstrate that (1) there is a mutual enhancement of deformation and reaction, (2) the mechanisms which initiate shear zones may not be the ones which dominate after deformation is localized, and (3) dehydration reactions during prograde metamorphism can enhance the formation of localized shear zones, even though the product phases are stronger.