南天山-拜城凹陷新生代构造演化过程的低温热年代学分析

The ancient Tian Shan, which formed in Late Paleozoic, was rejuvenated by the far field stress of the India-Asia collision since Cenozoic and then experienced a complex shortening history. Cenozoic foreland fold-and-thrust systems were formed on both the northern and southern side of the range. As one of the most active intra-continental orogens, Cenozoic deformation of the Tian Shan has attracted great attentions of numerous geologists. However, the timing and the mechanism of the Cenozoic deformation of the range are still debatable. The Baicheng Depression is one of the largest foreland basins of in the Southern Tian Shan and contains 3-7 km-thick continuous Cenozoic deposits, which makes it a critical place for understanding the uplift and shortening history of the Tian Shan in Cenozoic. Based on previous magnetostratigraphic studies in the Baicheng Depression, the detrital apatite fission track analysis and (U-Th)/He thermochronology are planed to carry out to constrain the denudation history of the Southern Tian Shan and determine the timing of the deformation of the fold-and-thrust system in the depression in this project, in order to get further understanding of the intro-continental deformation history of the Southern Tian Shan.

天山造山带是一个古生代形成造山带，新生代以来由于受到印度-欧亚碰撞远程效应的影响，造山带复活，并经历了复杂的陆内造山活动。尽管前人对其陆内造山活动已经开展了大量的研究，但是天山山脉新生代构造活动的起始时间及其构造变形机制仍存在较大的争议。拜城凹陷是天山南麓构造最为活动的前陆盆地之一，并且发育了3-7 km厚的连续的新生代沉积地层，是认识天山山脉新生代构造活动的重要窗口。本项目拟选取拜城凹陷为研究区，结合前人磁性地层学数据，对凹陷中新生代地层进行碎屑磷灰石裂变径迹分析，重建南天山新生代构造剥露历史，并通过磷灰石(U-Th)/He分析限定前陆逆冲褶皱带构造变形的时间，探讨南天山-拜城凹陷新生代构造变形机制。

天山山脉毗邻青藏高原西构造节和刚性的塔里木地块，位于中亚地区陆内构造变形的构造前锋位置，是理解陆内造山过程和欧亚碰撞远程效应的关键位置。由于天山山脉自晚白垩世以来就处在半干旱-干旱气候区，其新生代的构造剥蚀量不足3 km，该山脉基岩区所记录的构造剥露事件（24 Ma）与前陆盆地沉积相变和沉积加速事件（11-5 Ma）并不耦合，因此，天山山脉新生代陆内造山活动的起始时间和构造演化历史一直存在很大的争议。基于以上争议本研究选择南天山拜城凹陷科拉苏剖面和北天山塔西河剖面为研究区，利用碎屑矿物低温热年代学、沉积物源示踪和构造地貌学分析等方法重建天山山脉晚新生代的构造演化历史。两个地区的分析结果均表明天山山脉两侧的主逆冲断裂均在26-23Ma开始活动，而后在中新世中晚期南、北天山均经历了快速的构造剥露和地表抬升。此外，本研究还重建了帕米尔-天山汇聚带新生代以来的构造演化历史，该地区新生代沉积物的物源变化信息表明帕米尔弧形构造带在41Ma以前开始向北推挤，并于中新世中晚期移动到其现今的位置，西南天山中新世中晚期也经历了一期快速剥露事件。总的来说，整个天山山脉从东到西在中新世中晚期都经历了一期强烈的构造变形，而帕米尔弧形构造带加速的向北推挤并不能解释整个天山山脉中新世中晚期的构造变形，该期构造活动则更可能是帕米尔-塔里木加速向北移动的结果。