Fluid-assisted Healing Processes in Gouge-bearing Faults: Insights from Experiments on a Rock Analogue System

On the basis of both experimental and field studies, solution transfer processes are expected to be important in contributing to fault strength recovery (healing) under the hydrothermal conditions prevailing in large parts of the seismogenic zone. However, most experimental work on healing effects in faults has been done using quartzo-feldspathic sliding surfaces or fault gouges, under conditions in which solution transfer processes are very slow. Mechanisms of fluid-assisted fault healing are accordingly rather poorly understood. We have performed slide-hold-slide fault healing experiments on simulated faults containing brine-saturated granular halite as a fault rock analogue. Halite was used because solution transfer processes are known to be rapid in this system under easily accessible conditions. We studied the effects of hold time duration, shear stress during hold, pre-hold sliding velocity and porefluid composition on strength evolution. The results show rapid fluid-assisted compaction and stress relaxation during hold periods, and a significant hold time-dependent strengthening upon re-shear. The data reveal that healing resulted from both a packing density increase and a contact area/strength increase during hold periods. The general type of behaviour observed is similar to that observed in quartzofeldspathic gouges at room temperature, although important differences were observed as well. Notably, the time-dependence of healing deviates from the log-linear trend observed in gouges where solution transfer processes are absent. This means that if pressure solution is an important healing mechanism in natural faults, applying a log-linear trend will underestimate natural fault healing rates.