龙门山南段晚新生代地壳缩短变形定量研究

The Longmen Shan (LMS) range of the eastern margin of the Tibetan Plateau (TP) is characterized by young high-topography but relatively low horizontal shortening rate. The uplift mechanisms and topographic evolution has been the subject of considerable debate. To investigate late Cenozoic crustal shortening and its direct contribution to uplift provide critical solution for the debates. Focus on that purpose, this study investigates the southern LMS by using balanced cross sections, interpreting high-resolution two- and three-dimensional seismic profiles, conducting field investigation, and considering fault-related folding to determine accurately total shortening during the late Cenozoic. Growth strata are showed and analyzed by shallow seismic reflection profiles, and activities of oblique thrusting are constrained by cosmogenic nuclide burial dating. Moreover, by comparing the forward-modeling of oblique thrusting and the three-dimensional structural interpretation and modeling of Dayi and Xiongpo anticlines to build structural kinematic models of oblique thrusting, try to use oblique thrusting to interpret the contradiction between the major topographic and structural features, and expect providing a new perspective on the uplift mechanisms of eastern margin of the TP.

青藏高原东缘龙门山边界“年轻高地形但低水平缩短速率”的特征引发了对其隆升机制（地形演化）的争论，确定晚新生代地壳缩短量及其对隆升的直接贡献是解决这一争论的关键。本项目将选取龙门山南段晚新生代地壳缩短变形展开定量化研究，基于野外地质调查和新的二维、三维地震剖面，辅以断层相关褶皱理论，建立该地区平衡地质剖面，更准确定量出龙门山南段晚新生代地壳缩短量；基于浅层地震剖面和宇宙成因核素埋藏测年，厘定斜向逆冲作用活动时间。在此基础上，对比斜向逆冲正演数值模拟和大邑、熊坡背斜区三维地震构造解译和建模结果，建立斜向逆冲作用的构造运动学模型，并尝试用斜向逆冲作用解释龙门山边界地貌和构造之间的矛盾，为青藏高原东缘的隆升机制提供新的视角。

青藏高原东缘龙门山边界“年轻高地形但低水平缩短速率”的特征引发了对其隆升机制（地形演化）的争论，确定晚新生代地壳缩短量及其对隆升的直接贡献是解决这一争论的关键。本研究基于野外地质调查和新的二维、三维地震剖面，辅以断层相关褶皱理论，确定该地区的构造几何学特征，准确定量出龙门山新生代以来的地壳缩短量及其对垂向抬升和地形生长的贡献。研究结果表明，晚新生代龙门山地壳缩短量>30 km；结合区域上的剥蚀速率和剥蚀量，确定晚新时代地壳缩短变形使得龙门山隆升2-8 km，与现今3-5 km的地形相匹配。同时，我们观测到龙门山中南段山前发育的N-S向和NE向构造的重新活动反映龙门山地区区域的E-W向地壳缩短变形过程，斜向逆冲于NE走向的龙门山；基于构造、沉积、年代学约束和2008年汶川地震同震滑移特征，确定斜向逆冲作用的开始时间~5-2Ma并持续至今。我们提出青藏高原东缘斜向逆冲作用下的两种应变分配模型：青藏高原向东扩展中，E-W向斜向汇聚下的应变分配分别被龙日坝走滑断裂，龙门山和岷山褶皱冲断带所吸收。我们认为地壳缩短和斜向逆冲作用可用来解释龙门山边界地貌和构造之间的矛盾，为青藏高原东缘的隆升机制提供新的视角。这些结果提高了对松潘-甘孜和四川盆地两大块体构造关系的认识，为青藏高原东缘的地球动力学过程提供了基础的科学数据。