帕米尔－南天山构造汇聚带新生代构造演化过程的古地磁学制约

The Pamir-Tian Shan convergence zone is located in the northwestern most part of the India-Eurasia collision zone. It comprises the northeastern Pamir Range in the south, the southwest Tarim Basin in the middle, and the Tian Shan Range in the north. These tectonic domains have formed at a complex triple junction where two convergent zones meet a dextral strike-slip boundary. The convergence resulting from far-field effects of the India-Eurasia collision have reactivated mountain building in the late Cenozoic and affected pre-existing structures in the Tian Shan Mountains. Focused study of this key juncture is critical to understand the nature of the demise of the Neotethys Ocean and ensuing crustal deformation at this western syntaxis of the India-Eurasia collision zone. In the Pamir-Tian Shan convergence zone over 3000 m of Cenozoic strata consisting of marine deposits merging upwards into continental clays and sands and then into molassic deposits record the retreat of the Tethys Sea and subsequent intracontinental deformation resulting from the India-Eurasia collision. A reliable chronological record is required to correctly exploit the sedimentary record with magnetostratigraphy providing the most suitable tool in this predominantly-terrestrial Cenozoic succession where there is typically a poverty of faunal content. A number of magnetostratigraphic studies have focused on dating Cenozoic sediments in foreland regions of the Tian Shan and east to the Talas-Fergana Fault (TFF). The Pamir-Alain Basin-Tian Shan junction has received much less attention than other parts of the India-Eurasia collision zone and study of the sedimentary record in the foreland region west of the TFF junction has only rarely been addressed. By study of key Cenozoic sections in the Kashi Depression in the southwest Tarim Basin and applying paleomagnetic, magnetostratigraphic, rock magnetic and magnetic fabric methods we aim to unravel the history of Cenozoic tectonic deformation in the Pamir-Tian Shan convergence zone. A high resolution magnetostratigraphy of these deposits will provide a precise chronology for the marine regression and subsequent Pamir and Tian Shan collision, whilst analysis of the magnitude, and potentially the timing, of vertical-axis rotations along the eastern margin of the Pamir should permit a better understanding of the tectonic evolution of Pamir-Tian Shan convergence zone under the ongoing influence of the India-Eurasia collision.

中、新生代的特提斯构造域和太平洋构造域叠加在古生代的古亚洲洋构造域之上，不同阶段地球动力学体系的叠加、复合，使多旋回构造-岩浆和成矿作用成为中国地质最突出的特征。帕米尔-南天山构造汇聚带正好位于三大构造域的叠加复合部位上，是研究新生代印度板块与欧亚大陆碰撞导致的陆内变形向北扩展的重要地区。但是，有关新生代塔里木盆地新特提斯海的退出时间和帕米尔构造演化的运动学模型一直争议颇大，极大地限制了学界对该区新生代构造演化历史的认识。本项目拟以塔里木盆地以西的新生代沉积序列为主要研究对象，开展系统的古地磁学集成研究，厘定帕米尔和南天山碰撞对接时限、以及新特提斯海从塔里木盆地的退出时间；定量分析帕米尔弧形构造带北向扩展过程中的旋转和运动学特征，为帕米尔构造演化的运动学模型的重建提供可靠的古地磁制约。并为特提斯构造域的成矿模式和环境变迁，以及探讨青藏高原北缘的地球动力学提供基础依据。

西南天山、西昆仑和塔里木盆地三大构造单元在塔里木盆地西北缘交汇，不仅记录了帕米尔弧形构造带北向扩展的过程，而且其构造变形也受到天山褶皱系活动的控制，又与塔里木盆地构造活动相关，是研究印度板块与欧亚大陆碰撞导致陆内变形向北扩展的重要地区，也是研究构造运动山脉隆升环境变化耦合过程的典型区域。本项目以西南天山前陆盆地的大山口剖面、帕米尔西南天山构造汇聚带的三棵树剖面以及喀什阿图什逆断裂褶皱带最西端的明尧勒背斜为研究对象，通过构造古地磁、磁组构和环境磁学限定帕米尔和西南天山晚新生代演化和对接拼合过程，探讨亚洲内陆干旱化过程和机制。研究内容包括：（1）帕米尔西南天山构造汇聚带构造运动的历史；（2）帕米尔弧形构造带北向扩展过程中的旋转和运动学特征；（3）与造山带隆升和剥蚀紧密相关的沉积环境的变化。取得的重要研究结果如下：.（1）定量约束了帕米尔和西南天山晚新生代以来的构造演化过程。在高精度的年龄框架基础上利用磁组构记录和沉积速率变化限定了帕米尔和西南天山的晚新生代演化和对接过程，为解密青藏高原隆升与生长历史提供新视角。.（2）获得了帕米尔东北缘晚新生代构造旋转新证据。喀什凹陷在上新世之后相对于稳定欧亚大陆整体发生了刚性的逆时针旋，这可能与印度板块向北对欧亚大陆的俯冲，导致帕米尔高原向北持续挤压有关。.（3）建立了帕米尔西南天山构造汇聚带晚新生代古环境演化序列。7 Ma塔里木盆地旱化加剧与西南天山的阶段性隆升、帕米尔西南天山的拼合在时间上有较好的吻合，提出构造驱动主导晚中新世以来亚洲内陆干旱化过程。.（4）塔里木盆地新生代特提斯海退出时间及其动力机制。塔里木与塔吉克盆地在始新世晚期发生了“准同期”的海水退出事件，提出帕米尔高原向北的俯冲导致的构造隆升和造山带的剥蚀充填，是最终海退的控制因素。.该项目研究成果为亚洲内陆干旱化的主控机制研究提供了可靠的运动学证据，证明了晚新生代亚洲内陆干旱化的构造驱动机制。