Frictional behaviour of simulated quartz fault gouges under hydrothermal conditions: Results from ultra-high strain rotary shear experiments

Numerous experimental studies have been conducted to explore the mechanical behaviour and strength properties of rock materials. Most of these studies, however, do not take the effects of fluids into account, though it has long been recognized that fluids might play an important role in the deformation of crustal rocks under brittle-ductile conditions. Moreover, all these studies are limited in the total shear strain that could be reached. In this study we report on ultra-high strain experiments performed on simulated quartz fault gouges in a rotary shear apparatus under hydrothermal conditions (effective normal stresses of 20100 MPa, a fluid pressure of 200 MPa, temperatures of 400-600 degrees C, sliding velocities of 0.01-1 mu m/s and total shear strains up to 50). The experiments show strain hardening up to a shear strain gamma of 0.6-1.8, followed by strain weakening of up to 30% towards a steady state value at a strain gamma of similar to 8-12. This strain weakening effect is much higher than previously reported for quartz gouge. The steady state shear strength increases with decreasing grain size, increasing sliding velocity, and decreasing temperature. The microstructure of the deformed quartz gouge is characterized by the presence of a through-going boundary-parallel Y-shear. Some samples also show Riedel shears oriented oblique to the shear zone boundary. Deformation in these tests was largely by cataclastic processes, with most displacement being accommodated along the boundary-parallel Y-shear, causing the marked weakening observed. Intergranular pressure solution did not accommodate significant shear strain but appears to play a role in smoothing and weakening the localized slip surface and in controlling gouge compaction. (c) 2008 Elsevier B.V. All rights reserved.