大巴山弧形构造形成机制研究

The arc is a typical structure in the most of orogenic belts, also popular in the orogenic belt of Western China. Dabashan arcuate fold-thrust belt between Sichuan basin and Qinling Orogeny belt is key region. Its origin and evolution play an important roles in the Intracontinent deformation in China Mesozoic-Cenozoic basin. There are many molds for Dabashan arcuate fold-thrust belt, for example, obstruct on the two ends, sinisitral strike slipping, detachment layers, superposed tectonic, and so on, but the main controlling factor is not clear. Because it’s very specificity, complex and controversy on Dabashan arcuate fold-thrust belt, the formation mechanism of Dabashan arcuate fold-thrust belt should be further researched. The study is that comparison between structure characteristics and fabrics are carried out along striking and dipping by structure analysis based the modern structure geology. Rotation vector of two limbs of Dabashan arcuate fold-thrust belt are calculated from palomagnetism and joint in sedimentary rock, and then the arc type is determined. Based comprehensive study, the kinematic and formation mechanism of Dabashan arcuate fold-and-thrust belt are built, and then the controlling factors are discussed. Our results provide not only the constrains on tectonic evolutions of Sichuan basin, Qinling orogeny belt and the collision of North and South China plates since Mesozoic, but also promote the oil and gas exploration in the northeastern Sichuan basin.

弧形构造是世界上许多造山带的典型构造，中国西部造山带弧形构造发育。大巴山弧形构造带位于四川盆地和秦岭造山带的过渡地带，其形成演化对中国中新生代陆内造山作用和大陆变形具有重要意义。已有的研究对大巴山弧形构造建立了两端砥柱阻挡、右旋剪切、多层滑脱层、构造叠加等成因模型，弧形构造的主控因素存在分歧。鉴于大巴山弧形构造的独特性、复杂性以及不同的认识，因此有必要明确大巴山弧形构造形成机制。本项目以现代构造地质理论为指导，采用构造解析方法，沿倾向和走向上开展多尺度下构造特征与组构的比较性研究；结合古地磁和地层节理共同约束弧形构造两翼相对旋转矢量，厘定大巴山弧形构造类型，建立构造运动学模式，阐明大巴山弧形构造的形成机制和主控因素。本项目不仅可以为中-新生代四川盆地、秦岭造山带，乃至中国南、北大陆后碰撞演化过程提供关键约束，而且可以推动川东北地区的油气勘探开发。

大巴山弧形构造带位于四川盆地和秦岭造山带的过渡地带，其形成演化对中国中新生代陆内造山作用和大陆变形具有重要意义。已有的研究对大巴山弧形构造建立了两端砥柱阻挡、右旋剪切、多层滑脱层、构造叠加等成因模型，弧形构造的主控因素存在分歧。本项目通过构造解析、岩石磁学、砂箱物理模拟的综合研究，明确了大巴山弧形构造带宏微观构造特征，建立了大巴山变形期次及序列，构建了大巴山弧形构造运动学模型，探讨其弧形构造的形成与演化的主控因素。构造解析揭示了早古生代秦岭洋的开启可能导致扬子北缘被动大陆边缘发育城口-房县正断层，它呈现弧形几何学形态。三叠纪构造反转，北东向挤压主要局限于北大巴山，城口-房县断层逆冲为主，东西段分别兼具左行和右行走滑。晚侏罗－早白垩世北北西向挤压导致城口断裂以逆冲兼具向东段减弱的右行走滑。随后，受北东向和南东向联合构造应力作用，南大巴山主体卷入变形向盆地递进形成了一系列与城口-房县断裂平行的断裂和褶皱。岩石磁学研究发现磁组构主要记录了早期LPS缩短，局部构造叠加形成复合磁组构，磁线理优势方位在变形过程中由西向东发生了轻微递减旋转。砂箱构造物理模拟揭示了弧形构造边界控制着应力传递方向，滑脱层对大巴山弧形构造具有重要的影响作用。综合研究认为，大巴山弧形构造带是早古生代拉张受两端基底和（或）不规则海岸线影响形成弧形主控边界，经中生代构多期造挤轻微改造，并叠加多层次滑脱和两端基底砥柱效应，形成现今的弧形褶皱冲断带。该研究成果不仅可以为中-新生代四川盆地、秦岭造山带，乃至中国南、北大陆后碰撞演化过程提供关键约束，而且可以推动川东北地区的油气勘探开发。