菱形石香肠简单剪切成因模式及其构造流变计

菱形石香肠虽常被作为指示剪切作用的标志,但其形成过程和流变学意义并未完全清楚。纯剪切模式作为一种已被认可的成因模式表明菱形石香肠也可能在简单剪切作用下形成。本文从石香肠与基质的流变差异出发,提出了菱形石香肠的简单剪切成因模式,建立了简单直观的平面模型,即方形石香肠置于方形基质中,后将其简单剪切变形分解为旋转和纯剪变形两步,并利用有限变形几何学知识,计算出理论的菱形石香肠剪切角和层面转动角,其与有限元数字模拟的实验结果还是较接近的。进一步的理论分析给出了利用菱形石香肠几何学参数(剪切角和层面转动角)求取石香肠与基质粘度比的方法,即菱形石香肠构造流变计,并运用它在湖北铁山地区获得石香肠的粘度是基质的4.76倍,菱形石香肠构造流变计扩展了香肠构造流变计的适用范围,提供了在野外快捷测算岩石粘度比的途径。

Simple Shear Mode and Rheology Gauge of Rhombic Boudins

Rhombic boudins are commonly used as shear sense indicators, but their development and rheologic significance are incompletely understood. Pure shear mode as a considered formation mode suggests that rhombic boudins may form under the simple shear mechanism. Based on rheologic contrast between matrix and boudins, this paper presents a simple shear mode of rhombic boudins, sets up a simple and intuitional plane model with square boudins embedded in square matrix, resolves the distortion associated with simple shear into rotation and pure shear by geometry of finite deformation, and then calculates shear angle of rhombic boudins and rotation angle of the layer, with the two angles close to the results of finite element numerical modeling. Further theoretical analysis provides a method for estimating the viscous ratio of rhombic boudins to their matrix by means of geometrical parameters of rhombic boudins (shear angle and rotation angle of the layer), namely, rheology gauge of rhombic boudins. It was applied to the calculation of the viscous ratio of a boudin to its matrix, and the result was 4.76 in Tieshan, Hubei. Thus the adapted range of rheology gauge is expanded, and a way is introduced to measure and compute the viscous ratio of rocks swiftly in the field.