水对Carrara大理岩断层激活及滑动稳定性影响的实验研究

Experiments of water effects on fault stability and reactivation of Carrara marble

为探究脆塑性转化带流体(水)对断层激活及滑动稳定性的影响,本文采用Carrara大理岩预切断层(saw-cut)光面样品来模拟新鲜断层,在气体介质三轴高温岩石力学实验系统上开展了摩擦实验研究,实验温度(T)为70~500℃,围压(P)为60和130 MPa,孔隙水压(Pp)为30 MPa.为获得摩擦滑动的稳定性参数(A-B)值,位移速率在0.08μm·s^(-1)、0.4μm·s^(-1)、2μm·s^(-1)、10μm·s^(-1)之间切换.实验力学数据和滑动面微观结构分析表明,Carrara大理岩断层在实验温度和围压条件范围内出现了断层稳滑、慢滑、震颤、黏滑、断层闭合等5种滑动行为,其中,低有效围压(P_(eff)=30 MPa)下,T=70℃时,断层被激活表现为稳滑、震颤和慢滑,摩擦强度的速率依赖性表现出速度强化向速度弱化过渡的特征;T=100~400℃时,断层被激活表现为慢滑和黏滑,摩擦强度的速度依赖性为速度弱化;T=500℃时,断层被激活,但表现为稳滑,摩擦强度的速度依赖性重新转变为速度强化.然而,高有效围压(P_(eff)=100 MPa)下,T=70~300℃时,断层均未见明显滑动,原有断层处于闭合状态.本研究获得的含孔隙流体(水)条件下Carrara断层发生不稳定滑动的温度范围为100~400℃,大于相同温度压力条件下干燥样品激活断层不稳定滑动的温度范围200~300℃,充分说明孔隙流体(水)对大理岩断层激活及滑动行为等具有影响,可以有效地促进断层发生不稳定滑动.所有样品主要变形机制为碎裂、扩容、及晶体塑性变形,且随着温度和围压升高,孔隙流体(水)的溶蚀作用、颗粒的重结晶作用增强,特别在P_(eff)=30 MPa、T=400℃时,滑动面上发育方解石菱形颗粒、溶蚀孔等,外加孔隙流体(水)对颗粒的润滑作用,以及压溶作用引起的断层愈合,共同促进了断层的重新激活及强烈的不稳定滑动.假定地温梯度为25~30℃·km-1,推测以碳酸盐岩为主的断层在地壳大约2~4 km深度即开始出现不稳定滑动,随后在深度13~20 km范围内断层重新过渡为稳定滑动,之后逐步闭合.

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 0.08μm·s^(-1),0.4μm·s^(-1),2μm·s^(-1) and 10μm·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.