Behavior of extensional crenulation cleavage (C' foliation) during progressive deformation and its effect on shear zone foliation (S foliation)

Abstract

伸展褶劈理（extensional crenulation cleavage，简称C’面理）作为韧性剪切带内常见的构造面理之一，在递进变形过程中是否旋转以及其如何影响剪切带面理（S面理）是尚未解决的构造地质前沿问题。本文统计了不同剪切带中C’面理、S面理和C面理（糜棱岩面理）的夹角。结果发现，从剪切带边缘向中心随着应变逐渐增大，C’面理与剪切平面（C面理）的夹角（α）呈近正态单峰式分布，表明C’面理在递进变形过程中不旋转。在C’面理发育的情况下，S面理与C面理的夹角（β）随着应变的增加逐渐增大，即S面理在C’面理形成后发生反向旋转，因此原用于计算剪应变的公式：tan 2β=2/γ失效。强弱互层材料的应变分配结果显示，一般剪切条件下，粘性分层与剪切平面的夹角（β）减小到20°时，强硬层的运动学涡度接近于0，即表现为纯剪切，而软弱层表现为近简单剪切；β小于10°时，强硬层发生反向旋转。由此可见，伸展褶劈理是不均匀岩层在递进变形过程中应变分配（strain partitioning）的结果。随着递进变形的进行，S面理与C面理的夹角逐渐减小，强硬层被分配以更多的纯剪切而布丁化，随后内部产生微型剪切条带并扩展形成C’面理，承担应变分配的简单剪切。至此，随着应变分配的完成，C面理上的剪切作用自行停止，因此C’面理不发生旋转，而S面理在C’面理的制约下开始反向旋转。;Extensional crenulation cleavages (C’ foliations) are common structural foliations in ductile shear zones. It is still confusing that the behavior and effects of C’ foliations on shear zone foliations (S foliations) during the progressive deformation. This paper collected and analyzed degrees of the included angles between C’ foliation, S foliation and C foliation (mylonite foliation) in different shear zones. The results show that the degrees of the angle (α) between the C’ foliation and the shear plane (C foliation) presents a nearly normal unimodal distribution with the strain increasing from the edge to the center of the shear zone, which indicates that the C’ foliations do not rotate during progressive deformation. When there are C’ foliations, the angle (β) between S and C foliation becomes larger with the increasing of strain. As a result, S foliation rotates backwards (anticlockwise in dextral shear or clockwise in sinistral shear) after the C’ foliations developed, and the formula used to calculate shear strain: tan 2β=2/γ is invalid. There is a strain partitioning in the layered materials. During general shear, when the angle (β) between the viscous layer and the shear plane decreases to 20°, the kinematic vorticity of layers with high viscosity is close to 0, that is, pure shear, while layers with low viscosity play simple shear. When β is less than 10°, layers with high viscosity rotate away from the shear plane. Thus, extensional crenulation cleavages may result from the strain partitioning during the progressive deformation of heterogeneous layered strata. During the progressive deformation, the angle between the S foliation and the shear plane becomes smaller, and layers with high viscosity are distributed with most pure shear and boudinage, and then the micro shear bands in S foliations expand to C’ foliations. At this moment, the strain is completely repartitioned. The simple shear is partitioned into the slip along C’ foliations, and C’ foliations will not rotate during the progressive deformation. S foliations begin to rotate away from the shear plane because of the slip along the C’ foliations.