Friction Properties Of Brittle-ductile Transition With A Granite Gouge At Hydrothermal Conditions

Deformation of the Earth’s crust and the crust-lithosphere system is largely concentrated in localized faults or fault zones.The mechanical behaviour and evolution of these faults are important in controlling a wide range of geological and geodynamical processes,where the strength and slip stability of a fault’s brittle-ductile transition（BDT）zone has been the focus of research in fracture mechanics.Modelling such processes relies heavily on descriptions of the rheology of fault rocks,i.e.of the brittle/friction,compaction and creep behaviour of fault rocks as derived from laboratory experiments.Laboratory experiments on bulk rock rheology have led to the construction of so-called crustal strength profiles in which upper crustal behaviour is described by laboratory-based fault friction laws with plastic flow dominating the lower crust.The transition part connecting friction laws and plastic flow is called the brittle-ductile transition zone,which controls the crust’s peak strength and is also important for the focal depth and mechanism of large earthquakes in the continental crust.Several experimental and theoretical studies have been conducted on the friction and creep behaviour of brittle-ductile transition zone to explore the variation of the strength and deformation mechanism with temperature,effective normal stress,pore fluid pressure,sliding rate,etc.Progress in understanding the friction properties of the BDT transition zone of granite rocks still needs more studies,even though they are the most widespread fault-core rocks on the continent.This thesis aims to elucidate the effects of temperatures,effective normal stress(_<sup>00))),pore fluid pressure(₀)),sliding rate,shear strain and initial grain size on the strength and sliding stability of granite fault.To achieve my aims,I implemented two friction experiments with our granite gouges collected from the Anninghe Fault.（1）The first one is Saw-cut experiments,our initial grain size is 30μm,and we conducted experiments at 25-600℃,effective normal stress of 68 and 200 MPa,pore fluid pressure of dry,30 and 100 MPa,and sliding rate of 0.04-1μm/s to get the parameters of sliding stability.The friction coefficient lies in the 0.66-0.71 in dry and low pore fluid pressure tests（30 MPa）,which is no significant change with temperature increase.At high pore fluid pressure（100 MPa）and temperature is600℃,the friction curve showed slip weakening at lower velocities,and then the friction coefficient dropped to 0.56.The velocity-weakening occurred at 200-600℃at₀)=30 MPa By contrast,dry saw-cut tests showed velocity-strengthening at all temperatures investigated.The stick-slip behaviour occurred in high pore fluid pressure tests;however,the stick-slip change to velocity-strengthening with an increase in the shear displacement atσ_n^eff=68 MPa.（2）The second one is Ring shear experiments,which were carried out with an initial grain size of 30 and 3μm,respectively.The maximum displacement of approximately 28 mm,a temperature of 25 to 600℃,a pore fluid pressure of 100MPa,effective normal stress of 100 MPa and a loading rate from 0.01 to 100μm/s in four orders of magnitude,（1） When the initial grain size is 30μm,the friction coefficient is from 0.64 to0.81 at high-velocity tests（v>1μm/s）,which is no system change with temperatures either.Especially the lower friction coefficient（0.34）exists at high temperatures,high effective normal stress and pore fluid pressure with an increase in the shear displacement.As the temperature increases（25-600℃）,there are three typical velocity-dependence regimes,from-0.0054 to+0.0477.The first regime characterized by positive（a-b）values includes all sliding velocities from 25-100℃.The second regime,defined by negative（a-b）（v-weakening behaviour）,occurs from100 to 300℃at v=1-3μm/s The third regime is from 300-600℃,named high-T velocity-strengthening.This regime moved towards higher temperatures with an increase in sliding velocity.For v=30-100μm/s,the（a-b）shows negative values from 100 to 470℃.Combining the mechanical data of saw-cut tests,we found that the velocity-weakening regime moved towards lower temperatures with increasing pore water pressure,effective normal stress and shear displacement.（2） When the initial grain size is 3μm,the friction coefficient is 0.65-0.81,which shows that the coefficient of friction is insensitive to changes in high-velocity at temperatures of 25-600℃and low-velocity at temperatures below 400℃.The velocity-weakening regime ranges from 100-350℃（=1-3μm/s）,the down-dip temperature changes from 350 to 460℃with the increase in sliding rate.（3）The microstructure of deformed granite gouge developed quite pervasive R₁-shear,Y-shear and P-shear.The angle between these R₁-shear and the gouge-wall rock boundary ranges from 10°-15°,where the grain size decreases（usually to below 1-2μm）,and the deformation mechanism is dominated by fracture and granular flow,which causes high porosity.After the temperature and pore fluid pressure increase,the slower the loading rate,the more,the finer grains in the boundary shear zone are adhering to each other by microscale dissolution-precipitation progress,leading to a reduction in porosity.The microstructure of dry and 30 MPa tests is similar,in which the shape of single grains is angular.By contrast,the grain boundary is more rounded when P_f=100 MPa,in which the porosity is decreased.From this,we infer that one or more fluid-aided deformation processes are activated in this regime and operate concurrently with the granular flow.（4）Microstructure observations are performed on the three different（a-b）regimes.Based on a microphysical model previously used to explain three-regime behaviour.The processes operating at the microscale are inferred to change from predominantly granular flow controlled by velocity-strengthening friction in all minerals to competition between dilatation by granular flow and compaction by pressure solution of quartz grains.Especially the velocity-weakening behaviour occurs due to the rate of intergranular compaction and dilatation being of comparable magnitude.（5）Based on the ring shear data and combined with the geothermal gradient in South China is 21℃/km,it is inferred that the depth of the seismogenic zone is～5-23km（=100μm/s）of the Anninghe Fault,and the range is consistent with the result of～5-25 km of earthquake distribution.The results imply that the earthquake gap should be a kind of interseismic locking along the Anninghe fault.However,extrapolation of our results on the effects of effective normal stress,shear strain,sliding rate and pore fluid pressure on（a-b）suggest that potentially unstable,velocity-weakening region becoming shallower with increased effective normal stress,shear strain,pore fluid pressure and decrease sliding rate.