脆-韧性转换带的变形规律及转换过程与机制研究

A mid-crustal brittle-ductile transition zone is situated between overlying brittle upper crust and underlying ductile lower crust. Deformation characteristics of the brittle-ductile transition zone is crucial for understanding continental deformation and rheology. However, present knowledge for these is very limited. The Zhangbaling Group, exposed in the southern segment of the Tan-Lu fault zone and southern margin of the Dabie Orogen, shows various ductile to brittle structures that were developed in a strike-slip or detachment zone during one deformation event. They belong to naturally exposed brittle-ductile transition zones without overprinting or reworking of later ductile structures and experienced a transition from earlier ductile, intermediate ductile-brittle to later brittle deformation. We will choose these deformation zones as research objects and take detailed structural observation and analysis as main study means. We will undertake studies on both ductile and brittle structure characteristics of the transition zones, overprinting and transition characteristics from ductile to brittle structures, the ductile-brittle transition mechanism as well as correlation of different brittle-ductile transition zones. This project aims at understanding deformation characteristics of the transition zones as well as detailed transition processes and mechanism, and setting up a deformation model for the brittle-ductile transition zone. Scientific significances of this project are helping for understanding not only deformation characteristics of the brittle-ductile transition zone, but also continental tectonics, deformation and rheology as well as mechanism triggering continental earthquakes.

介于脆性上地壳与韧性下地壳之间的中地壳脆-韧性转换带，其变形规律对认识大陆变形与流变学具有重要意义，但人们目前对其了解却十分有限。郯庐断裂带南段与大别造山带南缘的张八岭群，保存着同一期丰富多样的韧性至脆性构造，分别呈现为走滑型或拆离型变形带，属于天然出露的脆-韧性转换带。这些变形带未发生过多期韧性构造叠加或改造，主变形期皆出现了早阶段韧性、中间阶段韧-脆性至晚阶段脆性变形的转换。本项目将以此为研究对象，以精细的构造观察与分析为主要手段，开展韧性及脆性构造变形特征与规律、脆-韧性变形叠加与转换规律、脆-韧性转换机制及脆-韧性转换带变形规律对比等方面的研究。项目的研究目标是理解中地壳脆-韧性转换带的变形规律及转换的精细过程与机制，建立脆-韧性转换带的变形模式。该项目的科学意义不仅在于提高人们对中地壳脆-韧性转换带变形规律的认识，还可以促进对大陆构造、变形与流变学及大陆地震发生机制的深入认识。

脆-韧性转换带是强、弱地壳之间解耦的关键层位，是岩石圈塑性流变行为启动/终止的转换层位，也是岩石圈内最重要的强震、多震孕育带，因而是地壳内最重要的结构过渡带。但目前人们对脆-韧性转换带的认识是十分有限的，包括其变形特征、转换过程及转换机理等，均缺乏应有的研究。.本课题选取中国东部现今暴露地表的典型脆-韧性转换带作为研究对象，分别以发育脆-韧性变形的陡立的走滑剪切带和平缓的拆离剪切带两类脆韧性转换带为例，系统探讨了脆-韧性转换带内的韧性变形特征与变形规律、脆性构造变形特征与规律、脆-韧性变形叠加与转换规律、脆-韧性转换机制等问题。.研究发现，脆-韧性转换带内的变形往往具有早期的韧性变形向脆-韧性变形、脆性变形逐渐过渡的层次结构特征，不同层次的构造变形具有相似的组构方位和运动学方向。走滑型与拆离型脆-韧性转换带均具有相似的脆-韧性转换历程，即早期以发育片内a型褶皱、透入性面理和矿物拉伸线理为特征的韧性变形为代表，晚期以发育膝折、皱纹线理、褶劈理、石英脉、张裂隙、b型剪切褶皱等脆-韧性变形为代表。.脆-韧性转换带在早阶段初始应变及剪切升温下发生剪切分异及流体活动，促进了岩石的应变软化而产生广泛的韧性变形；晚阶段在降温及分异后期产生应变硬化，岩石在屈服滑移结束后开始发生局部脆性破裂，形成脆-韧性变形，并逐渐向后期的脆性变形扩展。研究表明，脆-韧性变形带内，岩石由韧性向脆性变形的转换是在同一深度带内完成的。一系列的变形现象指示，特定机制下的应变软化向应变硬化转变是脆-韧性转换的根本原因。.本次工作阐明了走滑剪切带型及拆离剪切带型两类脆-韧性转换带的构造特征、脆-韧性转换的一般过程及诱发脆-韧性转换的机制，该成果不仅加强了人们对中地壳脆-韧性变形规律的认识，还对深入认识大陆流变学结构与流变学行为具有重要的意义，同时在大陆灾害性地震研究与预测中也具有十分重要的意义。