Deformation in the Presence of Fluids and Mineral Reactions

Natural and experimental deformation of fault rocks show that fluid flow and mineral reactions are linked to fracturing in a nonlinear feedback relationship that potentially affects the displacement and stress histories of large faults. These interactions spawn instabilities that are expressed as episodic seismic events involving cataclasis, which alternate with slow, aseismic deformation involving pressure-solution creep, as well as healing and sealing by fluid-assisted mass transfer. This chapter focuses on the timescale of these processes during the earthquake cycle, with special emphasis on the evolution of rheological and transport properties of fault rock during the interseismic period. Fracturing weakens faults dramatically by enhancing the kinetics of pressure-solution creep and of mineral reactions. Therefore, during the postseismic period and initial part of the interseismic period, weakening is faster than fault strengthening by healing and sealing of fractures. During the interseismic period, mass transfer associated with fluid-assisted chemical reactions smoothes asperities on fault surfaces, heals fractures and enhances the formation of a foliation parallel to the fault plane, and decreases permeability. If advective fluid inflow is significant, this can increase pore-fluid pressure and reduce effective shear strength, at least locally within the fault. In the long term, however, the combined effect of fracturing, pressure-solution creep, and sealing is to restore the rheological and transport properties of the fault during the interseismic period, setting the stage for renewed stress build-up and seismicity. We demonstrate the salient characteristics of fluid-assisted fault weakening and strengthening with a one-dimensional model of an idealized fault zone undergoing simple shear at constant velocity. The model shows that the kinetics of the weakening and strengthening processes determine the relative rates of shear stress decrease and increase during the interseismic period. The kinetics of dissolution precipitation and mineral reactions are therefore expected to exert an important control on the recurrence time of earthquakes.