普通角闪石的速率依赖性及其对俯冲带慢滑移机制的启示

RATE DEPENDENCE OF FRICTION OF HORNBLENDE AND IMPLICATIONS FOR UNSTABLE SLIPS

为了解俯冲带中常见含水矿物角闪石在水热条件下的摩擦滑动行为,以普通角闪石作为模拟断层泥样品的材料,在温度为101~607℃、围压为136MPa的条件下进行了速率阶跃滑动试验,孔隙压力为30MPa,轴向载荷速率为0.04~1.0μm/s。实验结果表明摩擦系数为0.70~0.72,与温度无系统性关系。在303℃、505℃和607℃时均发生了速度弱化行为,b-a<0.0014,b/a值为1.06~1.18。速率与状态摩擦本构关系中的b值随着温度升高而从0.0013升至0.0102,呈上升趋势,表明在摩擦接触点激活了Arrhenius型蠕变。普通角闪石的速度弱化程度很弱,这种微弱的速度弱化特性可能导致在闭锁的孕震带下方的俯冲界面下倾部分上发生慢滑移事件,例如日本西南部、卡斯卡迪亚北部和墨西哥的情况。

Since the 21st century,the occurrence of tremor and slow-slip events in the subduction zones has increasingly attracted researchers’attention.It seems that minerals in the subduction zone which may be related to tremor and slow-slip events,have become a topic of concern.Hydrous minerals are generally lower in strength than anhydrous minerals,so the hydrous minerals in the subduction zone(e.g.,hornblende,serpentine,talc,etc.)may control the frictional sliding behaviors in the subduction zones.Although many geophysical data indicate that serpentinization may exist in the depth range around the mantle wedge,ocean drilling results indicate that hornblende is a common hydrous mineral in the mantle.In order to understand the frictional behaviors of hornblende as a common hydrous mineral in the subduction zones under hydrothermal conditions,we used pure hornblende as the material for simulating fault gouge samples.In a series of experiments with a certain confining pressure,the axial loading rate is between 0.04μm/s and 1.0μm/s.In these experiments,the velocity stepping tests were carried out under the confining pressure of 136 MPa,and the pore pressure was 30 MPa.The frictional sliding velocity is switched between 1.22μm/s,0.244μm/s,and0.0488μm/s to obtain data on the response to the velocity change.The experimental results are as follows:(1)In these experiments(C_P=136 MPa,P_P=30 MPa),except for the quasi-static oscillations at505℃and 607℃,at most experimental temperatures,hornblende fault gouge samples all show stable sliding behaviors.The steady-state friction coefficient ranges from about 0.70(607℃)to about 0.72(403℃),with an average value of about 0.71,and does not show systematic changes with temperature.(2)In these experiments of hornblende fault gouge(C_P=136 MPa,P_P=30 MPa),velocity strengthening behaviors were observed at temperatures of 101 and 203℃.It changes to velocity weakening at 303℃.It is in transition state at 403℃,and changes to velocity weakening at 505℃,and the velocity weakening continues until the highest temperature in our experiment,607℃.The absolute value of velocity weakening(b-a)is between 0.00051 and 0.0014,which is a weak velocity weakening.We numerically fitted the experimental data and obtained the values of the friction constitutive parameters a,b,and D_c at each temperature.Our results of data fitting show that the slowness law adequately reproduces both the nonoscillatory rate steps and the periodical slow slips.As a result,a and b-values for the series exhibit a similar trend up to 203℃.The maximum of a averaged over steps(~0.009)occurred at 101℃,associated with a step-averaged b of 0.0013.As temperature increased to 203℃,the step-averaged a decreased rapidly to a level of 0.0068,with the corresponding b-value of 0.0053~0.0055.The temperature at 303℃is a turning point for the a-value from the decreasing trend to a monotonic increasing trend up till 607℃.In contrast to the a-value,the average b in the series shows a growing trend in the whole temperature range.The average D_c was found to range from 12~23.5μm for non-oscillatory cases,with no systematic changes as related to temperature.Much smaller D_c of 2μm was inverted for the oscillatory slow slips at 505℃and 607℃in the series,indicating that it was the cause of heightened critical stiffness that approached the vicinity of the loading stiffness.Hornblende has a weak velocity weakening,with(b-a)-value below 0.0015,which is less than the(b-a)-value used in the numerical simulation of the slow-slip in the Cascadia subduction zone,indicating that the velocity weakening is very weak,and this weak velocity weakening is conducive to the generation of slow slip.Therefore,the degree of velocity weakening of hornblende is in line with the appropriate conditions for the occurrence of slow slip in the subduction zone.The slow slip event may occur in a wider range of effective normal stress.