Dislocation Creep Flow Laws of Wet Quartzite: the Significance of Pressure and Slip Systems.

Abstract:

An accurate flow law for dislocation creep of quartzite is critical for the understanding of continental rheology and geodynamic models. Despite many years of effort, existing creep experiments have yielded very different quartz flow law parameters. We demonstrate that the difference can be explained by considering the pressure effect on the activation enthalpy and the slip system dependence of the stress exponent. We carefully examine high-quality experimental data of wet quartzite corresponding to steady-state regimes 2 and 3 dislocation creep together with related quartz c-axis fabrics and identify two end-member quartz flow laws corresponding to dominant prism <a> slip and dominant basal <a> slip respectively. The parameters of two flow laws are determined from experiments through a self-consistent iterative approach. The flow law for dislocation creep by dominant prism <a> slip is ε=2.5×10^-14f_w^2.6exp(-(132000+35.8P)/RT)σ⁴ and that by dominant basal <a> slip is ε=6.3×10^-12f_w^1.7exp(-(126000+23.1P)/RT)σ^2.5. Quartz c-axis from both experiments and nature suggest that wet quartzites commonly flow by a continuous combination of prism <a> and basal <a> slips. In such a case, quartzite is a polycrystal aggregate in which the slip system varies among grains. On a suitable representative volume element, the overall rheology of a quartzite must be obtained through a self-consistent micromechanics approach. We propose a numerical method to determine the creep behavior where both basal <a> and prism <a> slips are significant and plot the overall strength versus temperature and contributions of dominant slip systems in a 3D profile. We also discussed the significance and implications of multiple flow behaviors for continental strength.