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Experimental Study on the Effect of Pre-Existing Fabric to Deformation of Granitic Gneiss under High Temperature and Pressure
详细信息   
摘要
The experimental study on the effect of pre-existing fabric on the rheological properties of rocks is a new research focus.The related experimental studies are very limited.In this paper,deformation experiments on natural granitic gneiss from a detachment fault of eastern Liaoning in the North China Craton were carried out in a Griggs-type molten salt medium triaxial apparatus at temperatures of 600~840℃,confining pressure of 800~1200 MPa and strain rate of 1×10-4~2.5×10-6/S.To investigate the effect of pre-existing fabric to rheology of rocks,samples were drilled from natural granitic gneiss with the cylinder axis parallel to the foliation and normal to the foliation.The experimental results show that the flow strength of samples with compression direction perpendicular(PER) to the foliation is stronger than that of samples with compression direction parallel(PAR) to the foliation.In temperature range of 600~700 ℃,the deformation is accommodated by semi-brittle flow,with the average of stress exponent n=6.5.In temperature range of 800~840 ℃,the deformation is primarily plastic flow,with the average of stress exponent n=2 and activation energy Q=380 kJ/mol(PER),and Q=242.6 kJ/mol(PAR).Microstructure observations show that in the samples of PER,the new deformed bands developed in the deformation process replaced the original foliation;in the samples of PAR,the new deformed bands developed in the deformation process inherited the original foliation.Dehydration melting biotite and hornblende occurred in both PER and PAR samples at temperature of 800~840℃.EBSD analysis shows that the c-axis fabric of quartz of the original granitic gneiss localized within the Z-axis domains for basal 〈a〉 slip,the c-axis fabric of quartz in samples with PER localized within the X-axis domains for prism 〈c〉 slip;the c-axis fabric of quartz in samples with PAR localized within the Z-axis domains,and Y-axis,for basal 〈a〉 slip and prism 〈a〉 slip.This indicates that deformation of quartz in samples of PER is more thorough than that of samples of PAR,which is consistent with the microstructure analysis.Obviously,the heterogeneous fabric of samples has a significant impact on the strength and c-axis orientation of quartz,but no apparent effect in brittle-plastic transition and plastic deformation mechanisms.This study can help us to comprehensively understand bulk flow strength of rocks with preexisting fabric.

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