鄂东南铁山梯形石香肠构造基质层中的应变测量与分析

鄂东南铁山是全球第二个梯形石香肠构造的发现点。以该区成层分布、形态相近的梯形石香肠构造为研究对象,利用惯量椭圆法对其基质层进行有限应变测量,获得真应变差、岩层厚度比、运动学涡度分布等。结合梯形石香肠几何形态学、岩石学特征,对所得数据综合分析研究表明:剪切带中垂直剪切方向的厚度比值与其所受真应变差趋于负相关;该梯形石香肠构造是由其上、下相邻基质层厚度差异,基质层中总体上的纯剪切,局部相对集中的简单剪切及相对富集于张裂隙处角岩层中的热液流体等综合作用形成的;发育成层分布且形态相近的梯形石香肠构造所需特征性条件是能干层上、下相邻基质层厚度相差较大,且受总体上持续的平行层面拉伸、垂直层面压缩的纯剪切与局部的简单剪切共同作用。该类石香肠构造是较好的岩石流变学标志。

Strain Measurement and Analysis of Matrix of Trapezoidal Boudinage in Tieshan, Southeast Hubei, China

Tieshan in southeast Hubei is the second place in the world where the trapezoidal boudinage was discovered.Using trapezoidal boudins with similar shape and locating in the same layer in this area as studying objects,this study acquired the distribution of true differential strain(ε-1-ε-2),stratum thickness ratio(S),kinematic vorticity number(Wk)and major long axe of finite strain ellipse(λ1)through finite strain measurement at different positions of its matrix with inertia moment ellipse method,then studied the data combined with its morphological and petrologic features.The results shows that the thickness ratio(S)of shear zone in the direction perpendicular to its shear direction tends to be negatively related to differential strain.This study also proved that trapezoidal boudinages in this location were formed due to comprehensive effects of difference in thickness between the upper and lower matrix layers,pure shear with extension parallel to strata and with contraction perpendicular to strata in most parts of matrix layers,and simple shear distributed mainly in partial matrix.Besides,the hydrothermal flow concentrated relatively in hornfel layers near wedge-shaped cracks also played a significant role.Characteristic features during the formation of trapezoidal boudinages similar in shape and continuous in competent layer manifest in two aspects,namely,on the one hand,a large difference in thickness between matrix layers,and on the other hand,the combined action of the sustained pure shear with extension parallel to strata and with contraction perpendicular to strata in most parts of matrix layers,and the simple shear concentrated locally in matrix layers.The above results indicate that this sort of trapezoidal boudinages is a good rheological indicator of rocks.