Low-to High-velocity Frictional Properties Of The Longmenshan Fault Gouges Retrieved From The Jinhe Shallow Drilling Cores

The Yinxiu-Beichuan fault of the Longmenshan fault system cuts through the granitic rocks of the Penguan complex near the Jinhe outcrop.In this study,we conducted low-to high-velocity friction experiments on the three kinds of fault gouges retrieved from the the shallow drilling hole penetrating the Longmenshan fault zone near the Jinhe outcrop.Unlike previous friction experiments mainly performed on the fault gouges developed in sedimentary rocks,the experimental studies on the gouges developed in granitic rocks will help us to get a more comprehensive understanding on frictional properties of the Longmenshan fault zone in terms of variations in lithology.Moreover,BET surface area of the experimentally deformed samples was also studied to estimate the changes in surface energy of the samples after seismic slip,which may provide some constraints on the energetics of seismic faulting.Friction experiments were performed under room-humidity and pore-pressure（0.5 MPa）conditions,with effective normal stress of 1.2 MPa（roughly the in-situ condition of the retrieved fault gouges）and slip rates ranging from 20μm/s to 1.4 m/s.All the three gouges contain quartz,feldspar,calcite and phyllosilicates.The differences in mineral composition mainly lie in the type and content of phyllosilicates.Specifically,the blackish gouge（BG）contains 14%illite,14%smectite and 9%chlorite;The Gray-greenish gouge（GGG）contains 45%chlorite,2%illite and 1%smectite;The Yellow-greenish gouge（YGG）contains 56%chlorite,3%illite and 3%smectite.Results show that the frictional properties of the three gouges are similar under dry condition.At low to high velocities(V_eq=20μm/s⁰.14 m/s),the steady-state friction coefficients range from 0.55⁰.80 for the three gouges,and there is no clear velocity dependence of friction in this velocity regime.At high velocities(V_eq=1.4 m/s),all the gouges show dramatic slip weakening.However,under pore pressure condition,the frictional strength of the three gouges at low-to intermediate-velocities have the following relation:GGG>YGG>BG.This relationship together with the mineral compostion suggest that the friction at low-to intermediate-velocities is closely related to the types rather than total contents of clay minerals—smectite and illite could lower gouge friction more effectively than chlorite.All the three gouges show velocity strengthening at low-to intermediate-velocity regime,suggesting a mechanical barrier to accelerating slip during seismic rupture propagation.The yellow-greenish and gray-greenish gouges both show extremely rapid dynamic weakening that is contemporaneous with transient dilatancy of gouge layers at the slip rate of 0.14m/s and1.4 m/s under pore pressure condition.Local fluid pressurization triggered by flash heating probably could explain such mechanical behavior.The BET surface area(A_BET)of the initial BG,GGG and YGG are 42.467 m²/g,13.123 m²/g and 11.749 m²/g,respectively.At the high slip rate of 1.4 m/s,the three gouges show clear reduction in BET surface area under both dry and wet conditions,suggesting the occurrence of grain sintering.In the low-to intermediate-velocity regime,the BG samples deformed under dry condition have smaller A_BET than that of the initial samples,and the A_<sub>BET of the deformed BG decreases with the increases of slip rates.This is in contrast to the cases of the deformed GGG and YGG,both of which almost have no change after the experiments.Under pore pressure conditions,the A_<sub>BET of deformed BG almost has no change,while the A_<sub>BET of deformed GGG and YGG significantly increased（approximately doubled）.In general,the three gouges deformed under pore-pressure conditions tend to have larger A_BET than that under dry conditions.Microstructure observation of the deformed gouges reveal a possible explaination for this phenomenon:the presence of pore water tends to distribute the shear deformation within the entire gouge layer,which involves more material in shear deformation and thus consumes relatively more energy in grain crushing.Moreover,surface energy estimates reveal that the energy consumed by grain crushing is less than 8%of the frictional work in all the friction experiments,suggesting the energy partition for grain crushing is small during seismic faulting.