俯冲增生杂岩带变形特征、成因机制及与后期变形的区别

Deformation in subduction-accretionary complex belts:Characteristics,mechanism and differentiation from late-stage event

俯冲增生杂岩带是造山带重要的组成单元,它记录了从俯冲到碰撞以及碰撞后陆内的演化历史,具有重要的研究价值。由于增生楔形成过程复杂,而后期的碰撞以及陆内变形又会强烈改造俯冲期的变形,因此如何区分增生杂岩中俯冲期间和碰撞阶段的变形就非常重要,但明确的区分两者又是非常困难的工作。我国几乎所有地区发育的俯冲-增生杂岩都经历了后期强烈的改造,因而正确合理地筛分俯冲阶段和碰撞阶段的变形,在我国的造山带研究中日益突出。本文在详细介绍俯冲期间相关变形及其机制的基础上,从不同构造要素的分布、发育特征、形成环境、成因机制等方面综合对比了俯冲阶段和碰撞阶段以及之后构造变形的异同,提出了区别不同阶段变形的主要原则。相比碰撞阶段变形,俯冲阶段的变形主要集中在俯冲隧道中,以简单剪切或一般剪切为主(逆冲断层多见),底板垫托以及双冲构造是变形的重要特征,变形呈弥散性,断层和面理以及褶皱等具有优势的构造极性,但缺少区域尺度的大型褶皱;纯剪变形少见,主要发育在俯冲隧道上方的增生楔中。流体作用以及水岩反应强烈,直接控制变形行为,发育有从显微尺度到区域尺度的变形分解现象。而碰撞阶段主要是在陆上环境进行,主要变形集中在接触带以及大型断裂/剪切带附近。断层和面理的构造极性不明显,增生楔整体变形,出现区域尺度的大型褶皱;流体作用虽有,但不如俯冲阶段明显和强烈,以逆冲和走滑断层多见。然而很多指标和依据并不是某种环境下唯一的,因此在实际工作中需要综合各方面信息和要素进行判断,合理区分不同阶段的变形。

Subduction-accretionary complex belts are an important component of orogenic belts and have great research values. They record the evolutionary history from the subduction to the collision periods as well as the history of intracontinental evolution after the collision event. Because the formation process of accretionary wedges is complex, and the deformation occurred during the subduction can undergo strong transformation due to the late-stage collision and intracontinental deformation, it is very important to distinguish between the initial and late-stage deformation, although such a task is very difficult. The subduction-accretionary complexes developed in China have all experienced significant late-stage transformations;therefore, the need for reasonable differentiation criteria for deformation of different stages becomes increasingly important in the study of orogenic belts of China continent. Based on the detailed descriptions of subduction-related deformation and its formation mechanism, this study comprehensively compares the similarities and differences in deformations formed in the subduction, collision and subsequent intraplate orogenic stages in terms of their distribution and development characteristics as well as the formation conditions and mechanisms of different structural elements, and puts forward the main principles for distinguishing deformations of different stages. Compared to the collision-induced deformation, deformation in the subduction stage is mainly concentrated in the subduction channels as simple shear or general shear primarily(thrust faults are common), with underplating and duplexing as the important features of deformation. The collision-induced deformation is diffused in matrix during the subduction, with faults, foliations and folds exhibiting dominant structural polarity;but regional-scale large folds are lacking. Pure shear deformation is rare and mainly developed in accretionary wedges above subduction channels. Abundant fluids and strong water-rock interaction directly control the deformation behavior, causing development of strain partitioning, from micro to regional scales. The collision stage mainly occurs in onshore environments, and the main deformation is concentrated near the contact zones between different geological units and large faults or shear zones. The structural polarity of faults and foliations is not obvious, and accretionary wedges are deformed as a whole, resulting in the development of large folds on a regional scale. Although fluids also exist during the deformation, their effects are not as obvious and strong as in the subduction stage, and thrust and strike slip faults are more common. However, many deformation indicators and differentiation criteria are not unique within a given environment;therefore, in practice, it is necessary to consider all aspects of such information so that reasonable judgment can be made.