Deforming while cooling: The effect of gradual stress increases on the microstructures of experimentally deformed quartz

The interpretation of rock microstructures often involves an assumption that deformation occurred at a steady-state. Since ductile creep rheologies are strongly temperature dependent, and many rocks deform during exhumation and cooling, it is likely that this assumption is to some extent invalid in many deformed rocks. The ability to detect and interpret stress increases using microstructural observations is hindered by a lack of experiments simulating deformation during cooling.

We carried out a series of shear deformation experiments on synthesized quartz aggregate as temperature decreased at rates of 10 ˚C/hr, 4 ˚C/hr, and 2 ˚C/hr from 900 ˚C to 800 ˚C at a constant shear strain rate of 5×10^-5 s^-1 and confining pressure 1.1 GPa. The temperature ramps scale to geologically-relevant values of strain-per-temperature-change. Temperature ramp and control experiments were analyzed using Electron Backscatter Diffraction (EBSD) to derive textural characteristics and grain statistics. Quantitative microstructural analyses demonstrate that continuous cooling during deformation has a discernable effect on the evolution of recrystallized grains by producing a larger standard deviation of grain size compared to experiments carried out at a constant temperature. Crystallographic preferred orientation (CPO) for control experiments are relatively strong at 900 °C and weak at 800 °C, respectively, suggesting an increased component of diffusion creep at colder temperatures. In cooling ramp experiments, the stronger CPO developed during the earlier phase of deformation are diminished—more so at slow cooling rates than at fast. Our results suggest that the grain size distribution and other microstructural aspects of continually deforming natural shear zones (particularly those at faster cooling rates and slower strain rates) should differ in quantifiable ways from rocks deformed at a constant temperature.