| 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,has 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 zonesAlthough 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 two series of experiments with different confining pressures,the axial loading rate is between 0.04 ?m/s and 1.0 ?m/s.In the high confining pressure experiments,the velocity stepping tests were carried out under the confining pressure of 136MPa,and the pore pressure was 30 MPa.In the low confining pressure experiments,the velocity stepping tests were conducted under the confining pressure of 83MPa,and the pore pressure was 30 MPa.The frictional sliding velocity is switched between 1.22 ?m/s,0.244 ?m/s,and 0.0488 ?m/s to obtain data on the response to the velocity change.The experimental results are as follows1.In the series of high confining pressure experiments(Cp=136 MPa,Pp=30MPa),except for the quasi-static oscillations at 505? 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 temperature2.In the series of high confining pressure experiments of hornblende fault gouge(Cp=136 MPa,Pp=30MPa),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 weakening3.In the low confining pressure series of experiments(Cp=83MPa,Pp=30MPa),the hornblende fault gouge samples showed stable sliding behavior at all experimental temperatures.The steady-state friction coefficient ranges from about 0.67(607?)to about 0.73(101?),with an average value of about 0.71.After the temperature exceeds 400?,the steady-state friction coefficient decreases significantly with temperature,from 0.72 at 403? to 0.67 at 607?4.In the low confining pressure series experiment of hornblende fault gouge(Cp=83MPa,Pp=30MPa),the velocity strengthening behaviors were observed at the temperatures of 101 and 203?,and the velocity dependence changed to velocity weakening at 303?.It is still velocity weakening at 403?.It changes to velocity strengthening at 505?,and the velocity strengthening continues until the highest temperature in our experiment,607?.The absolute value of velocity weakening(b-a)is between 0.0014 and 0.0015,which is a weak velocity weakening5.In our series of low confining pressure experiments,the steady-state friction coefficient of hornblende is lower than the steady-state friction coefficient of antigorite obtained by previous studies at temperatures above 500?.Hornblende can be a candidate for shear deformation when the temperature is higher than 500?.We numerically fitted the experimental data and obtained the values of the friction constitutive parameters a,b,and Dc at each temperatureOur results of data fitting show that the slowness law adequately reproduces both the non-oscillatory rate steps and the periodical slow slips.As a result,a and b values for the two series of different confining pressures 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 both series show a growing trend in the whole temperature rangeThe average Dc was found to range from 10.8 to 23.5 ?m for non-oscillatory cases,with no systematic changes as related to temperature.Much smaller Dc of 2 ?m was inverted for the oscillatory slow slips at 505? and 607? in the high confining pressure series,indicating that it was the cause of heightened critical stiffness that approached the vicinity of the loading stiffnessSEM(ZEISS Sigma)was employed for microstructural observation on thin sections of deformed gouge samples.Preliminary impression of the microstructures is the remarkably reduced grain size after the shearing processesIn the high confining pressure series,the sample sheared at 101? formed R1 shear zones in width of 23-160 ?m.At 203?,one notable microstructural character is the prominently developed,parallel to sub-parallel,through-going R1 shears with a width of 27-134 ?m.The angle between these R1 shears and the gouge-wall rock boundary ranges from 10°-15°.At 303?,prominent R1 shears were developed with a couple of through-going ones,with width of 24-112 ?m.At 403?,the panoramic features are similar to the case at 303?,with the shear-zone width ranging from 36-106 ?m.In the temperature range up to 400?,B shears were not well developed along the gouge boundaries.In contrast to the microstructure at lower temperatures,the R1 shear zones(39-93 ?m wide)at 505? were more frequently distributed,and distinct B shears(of width of 18-40 ?m)developed at the boundaries of gouge.Back steps between neighboring R1 shears also developed at this temperature.At 607?,distribution of the shear zones is comparable to those at 505?,but the width of R1 shear zones is in a wider range(38-159?m)In the low confining pressure series,the panoramic images exhibit quite pervasive shear deformation up to 403?,with only minor shear zones developed Prominent shear localization along several R1 shears occurred above 505?.Compared with the high confining pressure series,much less B shears developed at the gouge boundariesAs seen in magnified images,submicron grains with particle size<500 nm can be commonly seen within the shear zones.Particle sizes outside the shear zones are typically 20-100 ?m,with finer grains mixed inside.The particles of large size are commonly cracked and partly rounded.In all deformed samples,a great number of particles of precipitates(in size of 50-200 nm)with platy appearance were recognized to form upon the surfaces of fragments.In the gouge sheared at low temperatures(101? for example),precipitated particles in shear zones(<200 nm)are smaller than the precipitates outside the shear zones(-200 nm).In the sample deformed at high temperatures above 400?,the precipitated particles in shear zones appear to have cemented together and much more precipitated layers can be observed than in samples deformed at low temperatures.The morphological features of the precipitates in the low and high temperature cases turned out to be quite similarThe calculated b/a values of hornblende are between 1.06 and 1.28,which indicate a minor velocity weakening.The b values in the rate and state friction constitutive relation both showed an upward trend with increasing temperature,indicating that Arrhenius type creep was activated at the friction contact point.The previous researchers have proposed two explicit models of evolution effect(b value)that are consistent with Arrhenius-type creep.One is the exponential rheological model(BE model)based on the crystal plasticity of the contact point,and the other is the IPS model related to intergranular pressure solution.We observed the microstructure of the deformed samples and found that the hornblende fault gouge,whether it was a high or low confining pressure series experiment,showed a significant reduction in particle size(crushed particles are usually reduced to 1-2 ?m or less).For all deformed samples,ubiquitous precipitated particles(50-100 nm)with a plate-like morphology were observed to adhere to the surface of the crushed particles,indicating that intergranular pressure solution generally occurs.Based on the relationship between the IPS mechanism proposed by the previous researchers,we used our experimental data to perform inference and estimation.As a result,the effective normal stress we estimated at 100? was significantly larger than the valuesat other temperatures.In the temperature range above 200?,all estimated effective real stresses show similar levels.This shows that the evolution effect of hornblende at 200?-600? is controlled by the IPS mechanism.The observation of the microstructure,coupled with the comparison between the experimental data and the model predictions,indicates that the intergranular pressure solution process at the frictional contact point may control the evolution effect of hornblendeHornblende 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. |
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