The brittle-to-viscous transition in experimentally deformed quartz gouge

This study investigates the transition from brittle to viscous deformation in polycrystalline quartz. It focuses on the semi-brittle field and the transition to dominating viscous processes. The transition is influenced by several parameters (e.g. temperature, confining pressure, strain rate, grain size) that affect operating deformation mechanisms.
The influence of temperature, confining pressure, shear-strain rate and shear strain is analysed with the help of deformation experiments in a Griggs apparatus. The starting material is a crushed quartz (grain size below 100 μm) originated from a natural single crystal. This powder simulates fine-grained crushed material within fault zones that can be deformed while the fault zone is reactivated. Some samples are hot pressed for 20 h at 1000 °C and 1.5-1.6 GPa confining pressure before deformation starts. Experiments are conducted at temperatures between 500 °C and 1000 °C and confining pressures of 0.5 GPa to 1.5 GPa. The shear-strain rate varies between 3·10−3 s−1 and 2·10−6 s−1. Experiments are performed at constant shear-strain rates except for some strain-rate-stepping experiments. Afterwards, the microstructural evolution and the textural evolution are analysed using light microscopy and scanning electron microscopy.
Coesite, a high-pressure polymorph of quartz, is formed in some samples. Therefore, the relationship between the stress state in the sample and the thermodynamic pressure, which determines the phase transformation of quartz to coesite, is analysed. The formation of coesite as well as the back transformation to quartz is observed in a single sample and related to increasing and decreasing differential stresses, respectively. But the PT range of the stability field of coesite is not reached by differential stress nor confining pressure or mean stress. Only the maximum principal stress is above the quartz-to-coesite phase trans- formation, which is in contrast to the common assumption of considering the mean stress as equivalent for the thermodynamic pressure.
At high confining pressure the temperature dependence of stress is ambiguous for temperatures below 650°C. For higher temperatures, differential stress (∆σ) decreases with increasing temperature. The largest decrease occurs between 650 °C and 700 °C at 1.5 GPa confining pressure and 2.5·10−5 s−1, which correlates with falling below the Goetze criterion (∆σ = Pc). Similar to stress, the friction coefficient (μ) slightly depends on temperature below 700 °C and it is approximately 0.45. A strong inverse temperature dependence is indicated between 700 °C and 1000 °C at 2.5·10−5 s−1 and 2.5·10−4 s−1. Lower shear-strain rates (4·10−6 s−1) are characterised by low friction coefficients (μ < 0.1) and small temperature dependence. At 650 °C a positive pressure dependence is observed between 1.0 GPa and 1.5 GPa whereas the pressure dependence is ambiguous at 700 °C. No pressure dependence occurs at 800 °C between 1.0 GPa and 1.5 GPa confining pressure but a decrease of Pc to 0.5 GPa appears to cause strengthening. Hence, brittle deformation mechanisms dominate at low temperatures and confining pressures. They mainly depend on confining pressure or rather normal stress. Temperature slightly affects these processes. Increasing temperature dependence and pressure independence indicate an increasing amount of crystal plastic processes.
Heterogeneous deformation dominates in low-grade samples and at low confining pressures. It is characterised by Riedel geometry but through-going fractures do not exist. Crack propagation is inhibited and cataclastic flow is predominant. With increasing temperature or confining pressure, the microstructure evolves into an S-C’ fabric. At 700 °C and high shear strain, a penetrative foliation is established by slightly elongated recrystallised grains. The S-C’ fabric persists at high shear strain for samples with differential stresses above the Goetze criterion. Samples with differential stresses below the Goetze criterion already establish an S-C’ fabric at peak stress. With increasing strain by predominantly dislocation creep, recrystallisation overprints the S-C’ fabric. The mode of recrystallised grain sizes increases with increasing temperature (1.8 μm at 700 °C; 10.1 μm at 1000 °C for 1.5 GPa and 2.5·10−5 s−1). Already at 600 °C, small, equiaxed grains form by dissolution-precipitation processes in fine-grained material in C’ orientation. These zones widen with increasing temperature and they are common in samples with S-C’ fabric.
A random crystallographic preferred orientation (CPO) exists in the undeformed material and persists at low temperatures or low confining pressure. C-axes develop a peripheral CPO with increasing crystal plasticity and the development of an S-C’ fabric. At 700 °C and 800 °C, a peripheral maximum occurs that is rotated anticlockwise with the sense of shear in S-C’ fabrics. The maximum rotates with the sense of shear with increasing shear strain and the development of a penetrative foliation. The a-axes form a girdle that is slightly inclined (∼10°) with respect to the shear zone boundary. A central c-axes maximum evolves at 900 °C next to the peripheral maximum. At 1000 °C, only the central maximum exists.
The differential stress of hot pressed samples is significantly higher than stresses of samples without hot pressing. The grain size distribution is the main difference between these materials. The initial powder has a broad grain size range. This range decreases with hot pressing at the expanse of the smallest and largest fraction. Hence, dissolution- precipitation processes are less effective because they are initiated in fine-grained zones.
Flow laws are used to relate crystal plasticity in experiments with natural conditions. A high stress exponent (n = 6.4 ± 1.3) at 650 °C indicates power law breakdown. A stress exponent of n = 1.9 ± 0.7 is calculated between 800 °C and 1000 °C. The corresponding activation energy is 170 ± 72 kJ/mol. The stress exponent is below the theoretical value and often used experimentally determined stress exponents for dislocation creep as well as above values for diffusion creep.
The transition from semi-brittle deformation to viscous dominated deformation is often marked by an S-C’ fabric. The occurrence of C’ shear bands is not necessarily an indication for high shear strain and crenulation of an earlier foliation. In fact, C’ bands can be generated at low shear strain without an initial foliation. They are often associated with dissolution-precipitation processes. In the viscous dominated field, dislocation creep as well as dissolution-precipitation processes occur. Apparently, the rate limiting step is unchanged between 800 °C and 1000 °C. Grain size sensitive diffusion processes are active in fine-grained material while grain size insensitive dislocation creep dominates in larger grains.