The brittle-to-viscous transition in polycrystalline quartz: An experimental study

Shear experiments on quartz gouge were performed at elevated confining pressures (predominantly 1.5 GPa) and temperatures (500 °C - 1000 °C) at shear strain rates of 3.5·10−6 s−1 to 2·10−3 s−1 to study the brittle-to-viscous transition. An unsystematic temperature dependence of strength at low temperatures changes towards a clear temperature weakening dependence above 650 °C. The transition from a pressure strengthening to a pressure weakening relationship takes place continuously between 650 °C and 800 °C. Strain rate stepping experiments reveal power-law breakdown at low temperatures (∼650 °C). Between 800 °C and 1000 °C, a stress exponent of n = 1.9 ± 0.6 and an activation energy of Q = 170 ± 72 kJ/mol indicate a combination of diffusion and dislocation creep. The Goetze criterion is confirmed as the upper stress limit for viscous deformation mechanisms. Localised deformation in the form of semibrittle shear bands with Riedel geometry at low temperatures changes to homogeneous deformation with a pervasive foliation accompanied by a continuous texture evolution between 700 °C and 1000 °C. Fracturing dominates at low temperatures accompanied by increasing amounts of dissolution and precipitation in fine-grained zones with increasing temperature. Above 650 °C, dislocation and diffusion creep are the dominating deformation processes, with dislocation creep being favoured in larger grains while dissolution-precipitation is active in the fine-grained fraction.