商丹构造带中高应变剪切带的野外和数值模拟研究

The Shangdan tectonic belt, which separates the North China Block and the South China Block, is a major suture in the Qinling orogeny. High strain shear zones along the Shangdan tectonic belt are structures for constraining the tectonic evolution history, deformation mechanism, and rheological properties of the Qinling orogeny. Recent important progress in the research of the shear zones implies that there are many misunderstandings transposition foliation and lineation within shear zones in previous work and literatures misconception. There are three important scientific issues on the Shangdan shear zones, i.e. (1) the shear deformation history is unclear, (2) there are no precise geochronological constraints, (3) the boundary conditions is unclear. We choose the Shangdan high strain shear zones as the research object in Danfeng area in the project. First, through detailed geological mapping and structural analysis, we determine the ductile shear deformation stages. Second, in order to obtain a wealth of fabric data and spatial distribution of fabrics, we measure and statistically analyze foliation and stretching lination data obtained in different lithological units and different spatial locations in the high strain zones. Third, we plan to collect systematically all different stages of mylonite samples, dykes, and plutons in the high strain zones. We plan to study the microstructures and lattice preferred orientation of deformation rocks, analyze characteristics of microscopic deformation and deformational conditions in the Laboratory. We constrain the age of ductile shear deformation by dating the metamorphic mineral 40Ar/39Ar cooling ages and zircon U-Pb age. Based on the study of the above, we use a micromechanics-based Multi Order Power Law Approach (MOPLA) to simulate multiscale fabrics in the shangdan high strain zones. Through the numerical simulation, we can get the rheological characteristics and boundary conditions of the shear zones. We could provide reliable geochronology and theoretical evidence for correct interpretation of deformation mechanism and reconstruction of tectonic evolution of the Qinling orogenic belt.

商丹构造带是分隔华北和华南板块的主要缝合带，带内的剪切带对探讨构造演化历史、岩石变形机制和岩石圈流变学特征具有重要意义。高应变带研究在过去10年取得了重要进展，以往文献对商丹剪切带内构造组合的研究需要更新和重新解释。过去普遍对剪切带变形历史研究不透，缺乏精确变形时限约束和边界条件模糊三个重要问题。本课题拟选择丹凤地区商丹高应变剪切带为研究对象，通过野外大比例尺地质填图和构造解析，厘定韧性剪切变形期次。按照不同岩性单元、不同空间位置获取丰富的变形组构数据。系统采集不同期次剪切带糜棱岩样品和与变形同期或具截切关系的岩脉和岩体，在室内进行显微构造、石英组构、40Ar/39Ar和锆石U-Pb年代学研究，分析变形微观特征和温压条件，限定剪切变形时限。基于上述研究，运用基于微力学的多阶幂律法进行多尺度组构数值模拟，确定剪切带流变学特征和边界条件，为解释岩石变形机制和造山带构造演化提供年代学和变形依据。

商丹构造带是分隔华北和华南板块的主要缝合带，带内发育的高应变剪切带对探讨秦岭造山带构造演化、岩石变形机制具有重要意义。目前，关于商丹构造带仍然存在变形历史不清，构造演化缺乏详细的地质证据。本课题选择了商丹构造带东段丹凤地区高应变带为研究对象，对该地区进行了详细的野外地质填图和构造解析，查明了商丹构造带野外构造变形样式，确定了商丹构造带多期剪切变形事件，并对各期韧性剪切带进行了详细的显微构造分析和变形矿物的组构研究。通过变形矿物及其带内侵入岩体的地质年代学研究，限定了各期变形剪切变形的时限，结合带内各类线状侵入体的岩石地化、Sr—Nd微量元素特征，很好的约束了商丹洋俯冲消减、闭合，以及南北块体碰撞的构造演化过程。通过野外及室内分析，确定了商丹构造带发生了两期韧性剪切变形。早期（D1）为斜向逆冲型韧性剪切变形，发生于～413.5Ma；晚期（D2）为左行韧性剪切变形，发生于310-340Ma。商丹构造带南侧弧前沉积体与商丹构造带具有不同的变形历史，发生了挤压型韧性剪切变形，变形时限为～260Ma。商丹构造带内发育有两期岩浆事件，早期为糜棱岩化的闪长岩、辉长岩，形成于421-424Ma，晚期为弱变形-未变型的线状花岗岩脉体和辉绿岩脉体，形成于409-412Ma。早期具有俯冲地化特征的辉长岩产生于活动大陆边缘构造背景，晚期线状花岗岩具准铝质-弱过铝质特征（A/CNK<1.1），为钙碱性I型花岗岩，形成于造山带后碰撞阶段。发育同期辉绿岩脉体，具有板内构造属性。商丹带内岩石和变形记录表明，商丹带南北两侧块体在晚志留纪-早泥盆世发生碰撞（423-413），同时，伴生强烈的挤压剪切变形，之后进入板内构造演化阶段。310-340Ma，在造山带内部发生左行韧性剪切变形。商丹带南侧地质体在～260 Ma发生强烈的挤压剪切变形，最终与商丹带拼贴在一起。