Experiments of water effects on fault stability and reactivation of Carrara marble
To explore the effect of fluid(water)on fault reactivation and stability across the brittle-ductile transition,we performed frictional experiments on saw-cut samples of Carrara marble using a gas medium triaxial apparatus to simulate fresh faults.The experimental temperatures(T)were 70～500 ℃,the confining pressure(Pc)were 60 to 130 MPa,pore fluid pressure(Pp)was constant 30 MPa,and the slip velocity(V)was switched among 0.08 μm·s-1,0.4 μm·s-1,2 μm·s-1 and 10 μm·s-1,respectively.The mechanical data and microstructure analysis of postmortem fault surfaces showed five types of fault behaviors:stable sliding,slow sliding,tremors,stick-slips,and fault closure.Under low effective confining pressure(30 MPa)and 70 ℃,the fault exhibited stable sliding,tremors and slow sliding,with a transition from velocity strengthening behavior to velocity weakening behavior;at temperatures of 100～400 ℃,the fault showed slow sliding and stick-slips,with a velocity weakening behavior;at 500 ℃,the fault showed stable sliding,and transitioned back to velocity strengthening again.However,at high effective confining pressure(100 MPa),the closured fault did not show any sliding at temperatures of 70～300 ℃.In this study,the Carrara marble fault experienced unstable sliding under pore fluid(water)pressure within the temperature range of 100 to 400 ℃,which was wider than the temperature range of 200 to 300 ℃ for dry samples.This indicates that pore fluid(water)plays a significant role in the reactivation and sliding behavior of Carrara marble faults,and can promote the unstable fault sliding effectively.The primary deformation mechanisms for all samples included fracturing,dilation,and crystal plastic deformation.With increased temperature and confining pressure,the dissolution effect of pore fluid(water)and dynamic recrystallization were enhanced,especially at low effective confining pressure of 30 MPa and high temperature of 400 ℃.Several processes,such as,recrystallized calcite and dilation caused by calcite dissolution,the lubrication of particles by pore fluid(water),as well as fault healing induced by pressure solution,contributed to the intense unstable sliding.Assuming a geothermal gradient of 25 to 30 ℃/km,it is inferred that carbonate-dominated faults start to experience unstable sliding at a depth of approximately 2 to 4 km in the crust,followed by a transition to stable sliding at a depth range of approximately 13 to 20 km,and eventually fault closure.
Carrara marbleWaterFrictional slidingFault stabilityDissolution and precipitation
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