| Shallow earthquakes in the continental crust,especially big ones,often nucleate near the brittle-plastic transition zone,where brittle fracturing/frictional sliding and plastic flow could concur.It's of great significance to study the deformation processes in the brittle-to-plastic transition zone through high temperature and high pressure experiments,from which we may get a more comprehensive understanding on fault strength and stability and deformation mechanisms at different depths,and shed light on earthquake mechanisms.Previous work in rock deformation community mainly focuses on the deformation process either in frictional or steady-state creeping regimes.In order to study the deformation characteristics of rocks in semibrittle regime and the transient creeping behavior of faults that may arise during postseismic stage,we performed transient creep experiments on granite and Carrara marble.(1)Three sets of deformation experiments were performed on fine-grained granite collected from the Pengguan Complex using the solid-medium triaxial apparatus(modified Griggs rig).The temperature and confining pressure used in the experiments roughly follows the in-situ conditions of the Wenchuan Earthquake fault zone corresponding to the depths of10-30 km,which range from 190 to 490°C and 250 to 750MPa,respectively.The strain rates used are 5×10-4 and 5×10-5 s-1.(2)Two kinds of experiments were conducted on Carrara marble in a gas-medium triaxial apparatus(Paterson),including triaxial compression experiments on intact cylindrical samples and shear experiments on saw-cut samples with a angle of 45°to the axis of maximum principal stress.The temperature,confining pressure and strain rate range from 20 to 800°C,30 to 300 MPa and 10-3 to 10-6 s-1,respectively.Microstructures of the deformed samples were observed under polarized light microscope,scanning electron microscope and EBSD.The major results as follows:(1)Transient creep experiments on granite(1)The mechanical data,microstructures and analysis of deformation mechanisms demonstrate that the granite samples experience brittle,semibrittle and plastic deformation under the designed experimental conditions.Under relatively low tempetature and low pressures,the strain-stress curves are characterizied by strain hardening behavior and microstructures indicate brittle deformation is dominant.With the increase of temperature and pressure,the strain-stress curvesshow clear yielding points and the stresses tend to stabilize to constant values.Microstructures indicate that fracturing was the dominant deformation mechanism,while dynamic recrystallization and dislocation creep were also activated.Under even higher temperature and pressure(900?/750 MPa),the deformation enters steady-state flow regime.As a representative rock of the Pengguan complex,the results from granite experiments suggest that the brittle to plastic trasition of Pengguan complex may occur at the depth of 10 to 20 km and semi-brittle deformation of the Pengguan complex may gradually give way to plastic deformation at the depth of 25 to30km.(2)Based on the empirical friction-to-flow law proposed by Shimamoto and Noda(2014),the estimated depth of brittle to plastic transition of granite is 10-25 km for strain rates of 10-12 to 10-14 s-1,which is consistent with the estimation from our experiments.(3)The Mohr circle analysis reveal that,for both high(5×10-4 s-1)and low(5×10-5 s-1)strain rates,the failure strength predicted by envelop of Mohr circle,the Byerlee frictional strength and the flow strength at the depth of 10-20 km are quite close.The focal depth of the 2008 Wenchuan Earthquake concides with the depth of the brittle-plastic zone.(2)Transient creep experiments on Carrara marble(1)The mechanical data and microstructures analysis demonstrate that deformation of Carrara marble in the triaxial compression experiments experience brittle to semibrittle,semibrittle to plastic and plastic deformation under the designed experimental conditions.a)Under the conditions of temperature of 200-400°C,confining pressure of 100-300MPa and strain rate of 10-4-10-6 s-1,the samples mostly display strain hardening behavior and the strength increases with confining pressure,indicating the features of brittle deformation.At temperature of 400?,the strength tends to be weakly dependent on stain rate,suggesting the deformation is in the semi-brittle to plastic regime.b)Under temperature of 600-800°C in conbination with confining pressure of 100-300MPa and strain rate of 10-3-10-6 s-1,most strain-stress curves evolve to steady states,while the cases of high strain rates(10-3-10-4 s-1)still display strain hardening behavior and dependence of strength on confining pressure.This suggests the sample deformation remains in the transient creep regime under the combined conditions of relatively high temperature,low confining pressure and high strain rate.Moreover,based on the empirical friction-to flow-law proposed by Shimamoto and Noda(2014)and the steady-state flow properties of Carara marble reported in Schmid et al.(1980),the main deformation mechanisms of Carrara marble in transient creep regime are cataclasis,dilation,twinning.Meanwhile,some plastic deformation mechanisms,like dynamic recrystallization,dislocation creep,grain boundary slide and TWIP also have been activated.(2)Mechanical data and microstructures of the deformed samples in the saw-cut shear experiments suggest that the Carrara mable experienced brittle,brittle to semibrittle and semibrittle to plastic deformation under the tested conditions.a)In an experiment under temperature of 20°C,confining pressure of 30 MPa and strain rate of 10-5 s-1,a new fracture plane with a angle of 30-35°to the axis of maximum principal stress formed and visible shear displacments accumlated on this plane(should be stable slip as indicated by mechanical data),which definitely suggests brittle deformation is dominant.b)In an experiment under temperature of 20°C,confining pressure of 50 MPa and strain rate of 10-5 s-1,the sample slipped along the saw-cut plane.The wavy extinction of the grains near slip surface indicates the occurrence of local plastic deformation.These features together suggest the sample deformation is in the semibrittle regime.When temperature is 300°C,confining pressures are 30 to 50MPa and strain rate is 10-5 s-1,slow slip with stress drop lasting for 3s can be observed.Meanwhile,the weak dependence of strength on confining pressure and the microstructural characteristics suggest the plastic deformation was also involved.The coexistence of these features suggests the samples experienced semibrittle deformation.c)Under temperature of 600°C,the recovered samples after experiments do not show visible slip along the saw-cut planes.This together with microstructural observations indicate that the sample deformation is in semibrittle to plastic regime. |
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