青藏高原东缘大凉山构造隆升及四川盆地的最终封闭

The Daliang Shan tectonic belt and adjacent southwestern Sichuan basin are located at the transition zone between the West Pacific and the Tethys-Himalaya tectonic domains, and thus a key to decipher the continental tectonics of the South China Block since the late Mesozoic and the eastward growth of the Tibetan Plateau during the Cenozoic. Methods and techniques involved in this study comprise dis- and un-conformities investigation, facies and paleo-current analyses, clastic composition analysis of sandstones, element and isotope geochemistry of fine-grain sediments, zircon U-Pb dating, Ar-Ar dating, fission track and (U-Th)/He dating. Based on the Cretaceous to Plaeogene sedimentary records within the southwestern Sichuan basin and Xichang basin, structural analysis and multiple thermochronologic study, we may reconstruct systematically the deformation history and exhumation process of the Daliang Shan tectonic belt and the eventual closure of paleo-Sichuan basin from an endorheic basin to an exorheic basin. Research results will give crucial constrains on both the origin and development of the southwestern basin-mountain margin of Sichuan basin, which would not only better our understanding of the tectonic framework and evolution of Sichuan basin as a whole, but also enhance out knowledge of both the continental tectonics of the the South China Block in the setting of westward subdution of the Pacific Plade and the tectonic evolution of eastern plateau margin.

四川盆地西南缘及其南侧的大凉山构造带位于特提斯-喜马拉雅构造域和滨太平洋构造域的交接转换部位，其形成演化对于理解西太平洋俯冲背景下中国南方大陆构造和青藏高原东缘构造具有重要意义。本项目拟在四川盆地西南部和西昌盆地白垩系-古近系剖面的细致测量基础上，通过碎屑组分、碎屑锆石U-Pb年龄和Hf同位素、泥质岩沉积地球化学指标等的精细对比，结合沉积相、不整合面、古流向等的分析，明确限定四川盆地由大型敞流盆地最终演变为封闭性内陆湖盆的时间；通过横穿四川盆地西南缘的构造剖面测制和构造解析，查明其构造几何学-运动学特征、构造叠加关系和构造期次等，再现其构造变形过程，进而探讨其形成和隆起方式；通过短距离、大高程差地表剖面样品的多重热年代学测试和热史模拟，恢复四川盆地西南边框的隆升-剥露历史和机制，最终恢复四川盆地西南缘盆山边界构建的地质过程，为整体把握四川盆地构造格局和完整认识其形成演化过程提供关键约束。

本项目通过构造解析、多重低温热年代学、碎屑沉积记录的综合研究，揭示了青藏高原东缘大凉山构造带的隆升历史，明确了四川盆地与西昌盆地分隔的过程。大凉山构造带地处特提斯-喜马拉雅构造域和滨太平洋构造域的交接转换部位，晚中生代以来在这两大动力体系交互作用下形成了近南北向、北西向和北东向三个主要方向的构造。详细的构造解析表明研究区主要经历了四期变形：早白垩世晚期至晚白垩世早期NW-SE向挤压、晚白垩世晚期至始新世NE-SW向挤压、渐新世至中新世早期近EW向挤压、中新世晚期以来的NW-SE向左旋挤压。地表路线和高程剖面的多重低温热年代学研究揭示大凉山构造带主要经历了晚白垩世（80-70Ma）、渐新世末至早中新世（30-20Ma）和中新世（~10Ma）三期热构造事件，其中最重要的是渐新世末至早中新世（30-20Ma）的热构造事件；经历了~80Ma之前的埋藏增温、80~30Ma间的缓慢冷却和~30Ma以来的快速冷却三个阶段。四川盆地与西昌盆地白垩系至古近系沉积序列、岩相组合、碎屑组成、碎屑单矿物和泥质岩地球化学等揭示其在早白垩世中期和晚白垩世晚期经历了两次重要的沉积环境和物源转变，响应于青藏高原东缘白垩纪两次重要的冷却事件。构造变形、隆升历史和沉积记录共同表明四川盆地与西昌盆地经历了晚白垩世晚期以前的完全连通、晚白垩世晚期的部分连通以及新生代以来的完全分隔三个阶段。大凉山构造隆升以及四川盆地与西昌盆地的分隔主要受控于青藏高原东缘晚白垩世以来多期向东挤压，但在早白垩世晚期还受到华南褶皱带向西扩展的影响。