先存组构对各向异性岩石流变强度的影响

EFFECT OF PRE-EXSITING FABRIC ON RHEOLOGICAL STRENGTH OF ANISOTROPIC ROCKS

地壳深部岩石普遍存在变形组构,花岗质岩石中的变形组构不仅影响岩石强度,而且对后期变形具有显著控制作用。近年来,先存组构对各向异性岩石的流变强度影响成为高温高压实验研究的热点之一。文中对前人给出的各向异性岩石(包括云母片岩-片麻岩、石英-钙长石均匀混合体与层状组构样品)半脆性-塑性流变实验数据进行了重新整理与分析,结合作者开展的不同组构条件下花岗片麻岩与糜棱岩流变实验结果,讨论了先存组构对各向异性岩石流变强度的影响。实验数据表明:1)各向异性岩石的面理与最大主应力方向的角度是影响强度的主要因素。在半脆性破裂域,样品压缩方向垂直于面理(PER)和平行于面理(PAR)的强度基本相同,在压缩方向与面理呈30°夹角时,岩石破裂强度最小;在塑性流变域,垂直于面理方向的强度显著高于平行于面理方向的强度,当面理与最大主应力方向的角度为45°时,岩石强度最小。2)后期变形对原有组构的继承与改造程度,决定了各向异性岩石强度高低。3)样品中矿物的含量、分布与粒度对各向异性岩石强度有显著影响。理论模型预测结果与云母片岩实验结果比较吻合,但其他类型各向异性岩石的流变比理论模型结果要复杂得多。因此,进一步开展具有先存组构的各向异性岩石的流变实验,并将实验变形与实际地质条件下更为复杂的岩石变形进行对比分析,是认识各向异性岩石流变和变形机制最有效的方法。

Deformed fabric has been broadly observed from middle to lower crustal rocks.The deformed fabric of granitic rock not only affects the strength of rock,but also controls the later deformation,which transforms the former deformation.The experimental exploration on effect of pre-exsiting fabric on the rheological strength of rocks is thus a new research topic.In this paper,we re-analysed the semibrittle-plastic creep data of anisotropic rocks（mica schist-gneiss,synthetic layers and particulate quartz-anorthite （50：50） composites）,combined with the rheological experimental results of granitic gneiss and mylonite by the authors under different fabric conditions and discussed the effect of preexsiting fabric on rheological strength of anisotropic rocks.The experimental results show that the angle between the compression direction and the original foliation of anisotropic rocks is an important factor,which controls the variation of strength.In the semi-brittle deformation regime,the flow strength of rocks with compression direction perpendicular （PER） to the foliation is basically similar with that of rocks with compression direction parallel （PAR） to the foliation.The fracture strength is minimum when the angle between the compression dircction of experiment and the original foliation is 30 degrees.In the plastic deformation regime,the flow strength of rocks with PER is significantly stronger than that of rocks with PAR.The strength has a minimum value when the angle between the foliation and the orientiation of the maximum principal stress is 45 degrees.The degree of replacement of former deformed fabric by that of later deformation determines the strength of anisotropic rocks.Besides,the content,distridution and grain size of minerals have a significant effect on the strength of anisotropic rocks.Prediction by previous theories is consistent with experimental results of mica schist,but the rheological model of other types of anisotropic rock seems more complex than such theoretical models.Therefore,further investigation on the effect of pre-existing fabric on rheology of rock with comparison of the deformation between geological and experimental conditions is the most effective method to understand the rheology and deformation mechanism of anisotropic rocks in tectonic settings.