Microstructural development and strain partitioning of experimentally sheared granitic rocks at the brittle ductile transition

The formation of interconnected weak minerals is one of the processes which causes strain weakening and shear zone development around brittle-ductile transition zone of crustal rocks. Toward the understanding of mechanical properties around this zone, many experimental studies have been conducted. However, there are not so many studies using poly-phase mineral aggregates (such as granitic rocks). Previous experimental studies on polyphase aggregates have been performed at the conditions of brittle to semi-brittle (e.g., Bos and Spiers, 2002; Pec et al., 2012) and fully ductile zone (Holyoke and Tullis, 2006), and their rheological behaviors and microstructural development of minerals are described in detail. However, the detailed microstructural development and strain partitioning at the brittle-ductile condition is not clear yet. To resolve these problems, we conduct deformation experiments with granitic rocks. Our experiments were conducted with Griggs-type solid medium deformation apparatus. Experimental sample is Gneiss Minuti, which was used in Holyoke and Tullis (2006), and mainly consists of 20 30% quartz, 40 50% plagioclase, and 20 40% biotite. Samples with approximately 1 mm thickness are cut into halves normal to shear direction, and Ni strain marker is inserted. At temperature of 800°C, strain rate of 2 x 10^-5 s^-1 and confining pressure of 1.5 GPa, peak strength is 700 800 MPa at γ of 1.0. Then, the strength monotonously decreased towards 200 MPa with further increases in strain but did not attain the steady state flow even at γ of 3.7. With SEM-EDS analysis, sample deformed to γ of 3.7 show the development of several narrow shear zones which are defined by elongated biotite grains. The shear zones are oriented oblique (20 30°) to clockwise against the shear direction. The strain at the outside of shear zone estimated from the strain markers is approximately 1, while the rest of the strain is almost accommodated by slip along the shear zones (γ 2.7). Strain partitioning associated with weakening by the development of the interconnected biotite network causes mechanical transition from pervasive ductile flow to localized slip.