Shear of simulated quartz-feldspar aggregates under conditions spanning the brittle-plastic transition

Continental earthquakes often nucleate at the brittle-plastic transition zone in the upper crust. Since the strength of the crust reaches the maximum here, it is inferred that strain is localized, leading to seismic rupture. Fault rock deformation experiments under pressure-temperature conditions simulating the brittle-plastic transition are key to unravel the processes triggering continental earthquakes. We investigated the mechanical behavior and post-mortem microstructure of simulated quartz-feldspar gouges using a Griggs-type solid medium apparatus. The samples consist of mixtures of powdered quartz: albite = 50 : 50 (wt%), which were sheared under pressure-temperature conditions simulating depths of 7 to 30 km, realizing a geothermal gradient of 30 °C/km and a lithostatic pressure corresponding to a granitoid rock density of 2700 kg/m³. Specifically, experiments were carried out at temperatures ranging from 210 °C to 900 °C and confining pressures ranging from 185 MPa to 870 MPa. The bulk shear strain rate was sequentially stepped between ~10^-3 and ~10^-4 /s. After the experiments, each sample was analyzed using optical and scanning electron microscopy.

Experimental results show a clear positive dependence of the shear strength on temperature and pressure up to 720 °C and 750 MPa, suggesting the dominance of brittle deformation. On the contrary, when the condition rises to 900 °C and 870 MPa, the strength dropped by about 550 MPa compared with that of at 720 °C and 750 MPa. This may imply that the plastic deformation gradually has taken over the deformation. Microstructural observation revealed elongated grains with their long axes intersecting with the direction of a Riedel-1(R₁) shear plane (i.e., similar to a S-C fabric). Some grains were reduced in size to the nanometer range. Our observations suggest that shear strain was highly concentrated within fine-grained zones, which, we speculate, may lead to catastrophic rupture. Crack distributions illuminated by image analysis indicate that the formation mechanism of crack changes with temperature and pressure. At the lower temperature (~ 240 °C) and pressure (~ 212 MPa), cracks are short and oriented to various directions. However, as the temperature and pressure increase to 300 °C and 265 MPa, they become longer and the ratios of R₁- and Y- shears increase. This implies that cracks coalesce in the kinematically favored orientations for slip, making it easy to cause a rapid seismic rupture. Since such microstructural changes occur at relatively low temperatures (below 720 °C), it is expected that the structures at higher temperatures (720 °C or higher) show predominance of the plastic deformation. Our results imply that the brittle-plastic transition gradually takes place at the microscopic scale, even within the range where the bulk mechanical behavior indicates brittle deformation.