伸展褶劈理地质现象、成因机制及其地质意义

Occurrences, Formational Mechanisms and Significances of Extensional Crenulation Cleavage

伸展褶劈理(C′)是韧性剪切带最主要的组构之一,也是认识剪切带演化过程的重要线索。从地质现象、成因机制以及构造地质意义3个方面对30多年来C′的研究进行综述。(1)C′以脆性破裂面或韧性剪切的形式、呈间隔性分布出现在具有各向异性结构特征的岩石中(如糜棱岩),并低角度切割S面理。一般仅出现一组近平行的C′,个别情况下,还出现多组或共轭组(简写为C″)。共轭伸展褶劈理关于S面理对称,S面理平分它们的锐夹角。C′的几何参数,包括C′的密度、C′之间的间隔以及C′分别与C面理和S面理之间的夹角,均表现出接近主滑移面时增大或减小的趋势。观察结果表明,C′形成于脆-韧性转换阶段,例如韧性剪切带演化后期阶段。(2)基于C′地质现象,结合假设条件,目前有多种关于C′的成因机制,大概分为4类:①C′为脆性破裂面,并在形成之后发生旋转;②C′发生于塑性变形阶段,对应具有特定含义的物理方向,例如塑性滑移线、特征向量或最大剪切应变速率;③C′是剪切带局部应变分解,即不稳定变形的结果;④C′初始角度具有优选方位的特点,理论推导得出,C′形成于最大有效力矩方向。(3)C′是韧性剪切带运动学研究的重要标志体,其几何特征被用于指示运动学方向和估算运动学涡度(即剪切变形中纯剪切与简单剪切的相对贡献)。一般认为,C′形成于脆-韧性或韧-脆性转换变形阶段,是应变局部化的表现,可能代表了地震成核的初始过程。在区域尺度,增厚地壳的中地壳层位在重力作用下出现低角度C′,向上扩展并发育为低角度拆离断层;低角度拆离断层(韧性剪切带)内部C′可扩展为切割韧性剪切带的高角度正断层。青藏高原内部共轭走滑断裂系与共轭C′具有相似的几何样式,其挤压方向(即欧亚大陆与印度大陆碰撞方向)正对共轭走滑系钝夹角。

Extensional crenulation cleavages(C′) are one of the main kinds of fabrics present to ductile shear zones, which plays a potential role to understand the evolutional processes of a ductile shear zone. We review the progress of researches on the occurrences, formation mechanisms and structural significances of the C′ in this paper.(1)C′ usually appears as spaced cleavages in anisotropic rocks(like mylonite) and cuts the existing S-foliation at a direction of a low angle to the S-foliation. In most cases, C′ occurs in one parallel group but sometimes in conjugate groups and even in several groups. Two groups of C′ are symmetric about the S-foliation and the acute angle of the conjugates groups is bisected by the S-foliation. Statistics suggest that an increasing or lowering trend exists on the geometrical parameters of C′, such as density, space, the angles between C′ and S-foliation and C′ and C-foliation away from the principal shear plane. Most observations show that C′ forms in the ductile-brittle transition, such as the late stage of a ductile shear event.Based on the geological occurrence of the C′ and some hypothesis, multi formation mechanisms of the C′ have been proposed. These mechanisms suggest that the C′ forms in a special direction of ① brittle fracture, ②slip lines, eigenvectors and maximum shear rate during stable plastic deformation,③strain partition due to locally unstable deformation and ④ maximum effective moment.(3)The geological significances of C′ have been well discussed. C′ is a useful kinematic marker for ductile shear zones. Its geometrical characteristics can be applied to indicate the shear direction and estimate the vorticity of shear deformation of a ductile shear zone. It is well recognized that C′ usually develops during a brittle-ductile transition or ductile-brittle transition, so the C′ may represent the initial nucleation of an earthquake. At regional scale, low angle C′ forms subject to the gravity in the middle level of thickened crust can extend into a low angle detachment fault. C′ in the ductile shear zone of a detachment fault develops into high angel normal faults. There is also a geometric similarity between the striking-slip fault system in Tibet Plateau and conjugate C′ whose obtuse intersection angles face to the direction of India-Eurasian continent collision.