Rock fabric and hydromechanical evolution from Quartz to clay rich faults and implications for slip stability

• Hydromechanical coupling strongly controls fault stability. • Dilation hardening is a valuable mechanism to control slip stability. • Enhanced frictional stability is favored by undrained conditions. Fault rock heterogeneity has a dominant effect on frictional and fluid flow properties of faults, yet the study of how fault hydromechanical properties are coupled, evolve with fabric and influence slip behaviour is still in its infancy. Here, we show that the increase in clay content within a quartz-bearing experimental fault promotes a fabric evolution from a load-bearing granular framework to an interconnected foliated and clay-rich network. This fabric evolution causes a significant reduction in frictional strength, a marked decrease in permeability, and enhanced frictional stability. Fault stability is favoured by contact area saturation (mechanical effect) acting in concert with fault dilation associated to a reduction in pore fluid pressure promoted by transient undrained conditions within the low permeability, clay-rich faults (hydromechanical effect). Our results indicate that fault rock fabric and associated hydromechanical properties exert a primary control on fault slip behaviour. The enhanced stability within pressurized clay-rich faults matches well with observations of aseismic slip during hydraulic stimulations of shales and models proposing dilation strengthening as a mechanism for the occurrence of slow slip events in clay-rich sediments at shallow crustal levels.