MR42A-02
Water weakening during semibrittle flow and faulting of experimentally deformed quartz sandstone

Thursday, 17 December 2015: 10:35
301 (Moscone South)
Taka Kanaya, Brown University, Providence, RI, United States and Greg Hirth, Brown Univeristy, Providence, RI, United States
Abstract:
Triaxial compression experiments were conducted on Fontainebleau sandstone at temperatures to 900°C and effective pressures to 175 MPa with varying water contents. Both yield and peak strengths associated with semibrittle faulting decrease linearly with an increase in intragranular water concentration (COH); COH is determined from infrared spectroscopy. Microstructural observations and the influence of strain rate, temperature, and COH on peak strength suggest that transient semibrittle flow is accommodated through cataclasis assisted by stress-corrosion microcracking. The roles of the experimental variables on the constitutive behavior are similar to those reported for subcritical cracking of quartz single crystals. At high COH, microstructural observations indicate an increase in the relative contribution of mm-scale distributed shear fractures (bands) to axial strain, reflecting a reduction in grain-scale fracture toughness. This is consistent with the inference that highly dissipative shear fractures lead to the observed reduction in strength at high COH. Stress vs. strain rate data for transient semibrittle flow show temperature-dependent rate behavior, and are well fit by an exponential law with an activation enthalpy of 185 to 250 kJ/mol and Peierls stress of 2.5 to 7.5 GPa. Using these constraints, we infer that stress-corrosion cracking is rate-limited by the dislocation activity at crack tips. Correlation of microscale COH maps to microstructures suggests that intragranular water in the undeformed sandstone is associated mainly with clusters of fluid inclusions, resulting in a highly nonuniform distribution of COH both within and between grains. Axially deformed samples show a reduction in the median and variability of COH over a range of length scales. We observe that a local reduction in COH correlates with fluid inclusions that are decrepitated and crosscut by intragranular fractures. We conclude that intragranular fracture is the primary mechanism of water transport in our experiments.