龙门山断裂带脆塑性转化带内花岗岩的微量水分布特征及其在地震孕育中的作用

The distribution of the trace water in granites at the brittle-plastic transition zone of the Longmen Shan fault and its role on the seismic energy accumulation

本文采集了映秀—北川断裂带南段出露的发生韧性变形的花岗岩样品,分析了脆塑性转化带内的岩石变形特征、变形温度和流动应力,并利用傅里叶红外吸收光谱仪(FITR)重点对石英和长石进行了测试.研究表明,龙门山断裂带深部发生过局部化塑性流动,以中、高温位错蠕变机制为主,重结晶细粒石英的粒度约15~100μm,估算得到的流动应力约15~80MPa.石英和长石内的微量水由晶体缺陷水、颗粒边界水和少量的包裹体水构成,两者的红外吸收光谱特点非常相似,主要吸收峰出现在波数3400cm^(-1)附近,次要吸收峰主要分布在波数3050cm^(-1)、3200cm^(-1)、3300cm^(-1)、3380cm^(-1)、3430cm^(-1)、3600cm^(-1)、3650cm^(-1)和3730cm^(-1)附近.花岗片麻岩内石英的水含量0.004~0.019wt%,长石的水含量0.013~0.043wt%;花岗初糜棱岩中石英的水含量0.004~0.018wt%,长石的水含量0.029~0.069wt%,愈合片麻岩中裂缝的早期石英脉中的细粒石英的水含量0.003~0.014wt%,发生半脆性-脆性变形的晚期石英脉的水含量0.016~0.032wt%.通过与前人的研究对比,认为龙门山地区韧性剪切带内微量水含量随变形程度增加而升高,弱化了脆塑性转化带内断层中心岩石流变强度,提高断层的应变速率,加强了脆塑性转化带向脆性域底部的闭锁断层的应变传递作用.对于难以滑动的高角度逆断层,这有利于强震在中地壳深度附近的孕育和发生.

In order to understand the role of the brittle-plastic transition zone on large earthquakes such as the MW 7.9Wenchuan earthquake,this study focused on the trace amount water in granites and the rock status at the brittle-plastic transition zone of the Yingxiu-Beichuan fault in the past.The samples occurred ductile deformation were collected at an outcrop in the southern segment of the Yingxiu-Beichuan fault.The optical observation showed inhomogeneous ductile deformation occurred in the brittle.The quartz in gneiss was deformed by SGR and that in protomylonite was deformed by GBM.The dominant grain size of recrystallized quartz is estimated to be 15 to 100μm,and the flow stress is about 15-80 MPa.Using the Fourier transform infrared spectroscopy（FTIR）,we measured the trace amount water contents in quartz and feldspar for deformed granites.The absorbance spectra suggest that water in quartz and feldspar occur mainly as hydroxyl in crystal defects,grain boundaries and fluid inclusions,and the similarity between the absorbance spectra of quartz and feldspar and new peak were determined. Water content of quartz in granitic gneiss is 0.004~0.019wt%,and that of feldspar 0.013 ~ 0.043 wt%. The granitic protomylonite shows water contents of 0.004~0.018wt%in quartz and 0.012~0.036wt%in feldspar.Water contents of fine-grained quartz in two quartz veins are respectively 0.003~0.014 wt% and 0.016~0.032 wt%.The water contents increase with the deformation suggest fluid flow and weakening exist in the fault core at the brittle-ductile transition zone of the Longmen Shan fault,which could enhance the ductile shear deformation and accelerate the elastic strain accumulation at the bottom of the locked brittle fault transmitted from the plastosphere,and this indicates large earthquakes are favorable to occur at the middle crust for a high-angle reverse fault.