龙门山构造带南段中-新生代构造变形特征研究

The NE-trending Longmenshan tectonic belt doesn’t only belong to important part of the middle-south section of the Helan-Chuandian N-S trending tectonic belt, but also to the eastern margin of the Tibetan Plateau. Moreover, Longmenshan tectonic belt extended southward to the N-S trending Sichuan-Yunnan tectonic belt, both of which form ‘Y-shaped’ tectonic pattern. At the same time the Longmenshan tectonic belt is the superposed result of Mesozoic Tethyan tectonic belt and the eastern boundary of Tibetan Plateau in the Cenozoic. Due to special tectonic setting and location, the southern section of the Longmenshan tectonic belt experienced complex superposition and presents intracontinental deformations since the indosinian period. On the basis of our previous research, the project will take the main faults, the thrust nappes between them in the southern section of the Longmenshan tectonic belt, and the Mesozoic foreland deposition as research objects. Here we propose to carry out structural geology, tectonic chronology, rock magnetism and tectonic geomorphology. Through a detailed field observation, structural analysis, isotopic dating and magnetic fabric analysis, we will closely focus on the structural deformation of the southern section of the Longmenshan tectonic belt since the indosinian period, and reveal the structural deformation characteristics and the superimposed relationship of the intracontinental tectonic under the different mechanisms in the Meso-Cenozoic. All results of this study will provide new approach for recognizing and reconstructing the superimposed characteristics of the intracontinental tectonic, and also provide important evidences for exploring the relations to conjunction and transformation of the Longmenshan and Chuandian tectonic belts, and then lead to a better understanding of the uplift of the eastern margin of the Tibetan Plateau and structural responses to its eastern boundary, as well as the composite evolution process of the Helan-Chuandian N-S trending tectonic belt.

龙门山构造带是贺兰-川滇南北构造带重要组成部分和青藏高原东部边界，向南与近N-S向川滇构造带构成一“Y”字形构造格局，也是中生代特提斯构造与新生代青藏高原东部边界构造复合叠加的结果。特殊的构造位置与背景使龙门山构造带南段印支期以来具有复杂的陆内构造变形与叠加改造过程。项目在已有研究积累之上，选择龙门山南段主干断裂、主干断裂之间的推覆体、山前沉积地层为解剖对象，综合构造地质学、构造年代学、岩石磁学、构造地貌学等多学科，运用野外地质观察与构造解析、同位素测年、磁组构等方法手段，重点突出地解析龙门山南段印支期以来的构造变形特征，揭示中-新生代不同机制下的陆内构造变形特征及叠加改造关系，为识别和重建陆内构造行迹叠加改造特征提供新思路和新途径，同时为探索解决龙门山与川滇构造带交接转换关系与过程提供有效依据，为青藏高原东缘隆升及其东边界的构造响应和贺兰-川滇南北构造带的复合形成演化奠定基础。

现今呈NE向展布的龙门山构造带是贺兰-川滇南北构造带中段主要组成部分，也是青藏高原东部边界。项目选择龙门山南段的主干断裂、推覆体以及山前构造扩展带为研究对象，通过野外详细的构造观察与解析，结合构造岩、弱变形沉积岩磁组构确定了各主干断裂和推覆体空间展布及构造几何学与运动学特征。结果表明各构造单元均经历了不同层次不同性质的多期构造叠加改造，具有由NW向SE逐渐扩展，呈前展式发展的特点；盐井-五龙断裂对龙门山南段的差异性演化具有主导控制作用，显示出边界断裂的属性；以同位素年代学实验结果和区域年代学统计结果分析为约束，将龙门山南段构造变形时序厘定为三个阶段：（1）印支晚期陆内造山阶段：主要发育挤压剪切体制下较深层次的顺层片理、顺层剪切褶皱及韧性剪切变形等构造；（2）晚中生代陆内构造变形阶段：主要发育挤压-收缩体制下地壳中浅层次的脆-韧性冲断、褶皱与滑覆构造，伴随该期构造在后山带发生了131-133Ma的构造热事件（灰绿岩墙锆石U-Pb年龄131-133Ma）；（3）新生代青藏高原隆升及其物质东向运动阶段：主要发育挤压剪切机制下地壳浅表层次的脆性逆冲走滑构造和山前带的褶皱变形，为两幕：第一幕为喜山早期具有自NNW向SSE的逆冲推覆兼有左行走滑构造；第二幕为喜山晚期具有自NW向SE的逆冲兼右行走滑构造，也是龙门山南段新生代以来的主导构造。龙门山南段整体的构造发展演化过程充分说明了龙门山构造带是古特提斯洋与新特提斯洋盆起始演化及新生代青藏高原形成与隆升扩展的的具体响应。后山带辉绿岩墙群的发现和研究为龙门山南段的大地构造环境及动力学过程提供了新的启发。上述系统的研究认识为探索解决龙门山与川滇构造带交接转换关系与过程提供了有效依据，为青藏高原东缘隆升及其东边界的构造响应和贺兰-川滇南北构造带的复合形成演化奠定基础。