Experimental Investigation on the Brittle-Ductile Transition of Natural Mafic Granulite

Semi-brittle and plastic deformation behaviors of mafic granulite are significant for evaluating characteristics of ductile zones in the lower crust region and the rheological strength of the lower crust. Axial compression experiments were carried out in this study with natural mafic granulite collected from the North China Craton, using a gas medium apparatus at 950–1,150°C and 300 MPa with strain up to 17%. The samples are composed of 57 vol.% Plagioclase, 19 vol.% Clinopyroxene, 20 vol.% Orthopyroxene, and 4 vol.% magnetite and ilmentite. The mean grain size is 300–700 μm. The bulk structural water content is 891 ± 399 wt ppm. At 950–1,000°C, the samples were brittly broken by scattered cracks and localized fault zones. At 1,050–1,075°C, the samples were deformed by ductile shear zones that broadened with increasing temperature, the deformation behavior is characterized by a steady-state semi-brittle creep; mechanic data yield a flow law of $\dot{\epsilon }={10}^{6.0\pm 0.3}{\text{MPa}}^{-5.8}{s^{-1}\sigma }^{5.8\pm 0.1}\exp \left(-\frac{651\pm 68\text{kJ}/\text{mol}}{RT}\right)$. At 1,100–1,150°C, the samples plastically deformed with dislocation creep, and the deformation strength is reduced by recrystallization and partial melting; mechanical data yield a flow law of $\dot{\epsilon }={10}^{2.7\pm 0.8}{\text{MPa}}^{-4.1}{s^{-1}\sigma }^{4.1\pm 0.2}\exp \left(-\frac{442\pm 13\text{kJ}/\text{mol}}{RT}\right)$. The strength profile based on our data implies that North China Craton has a wet and cold continental lower crust. Recrystallization and cataclastic flow involve grain size reduction that can lead to steady-state ductile behaviors of fault zones.