北天山准噶尔盆山过渡带构造演化的低温热年代学制约

The transitional belt between the northern Tianshan Mountains and the Junggar basin (transition zone) is a roughly east-trending complex tectonic deformation zone. The southern Junggar basin, contained up to approximately 6000 m of Cenozoic sediments and Mesozoic sedimentary strata, is the northern foreland fold-and thrust belt of the actively uplifting Tian Shan ranges. The southern Junggar fold-and thrust belt is composed of three east–west trending imbricated fault-related folds. This proposal puts emphasis up on the relationship between the thermal evolution and the structural analysis of the Tianshan basement and sedimentary series of southern Junggar basin in the transition zone. We will select several sections across the northern Tianshan basement and southern Junggar basin transitional zone to carry out detailed field structural geometry analysis and fault kinematics analysis, determinate the boundary position and characteristic of different thrust sheets, utilize low-temperature thermochronometry for the different thrust sheets, and establish tectonic thermal geochronology cross section to quantify the cooling history of rocks in different thrust sheet settings. The combination of thermochronometry and numerical modeling provides an approach for quantifying the thermal history, denudation and tectonic deformation in the transition zone. This work finally set up the tectonic evolutionary model that may provide insights into coupling between uplift exhumation, tectonic deformation and the thermal evolution of the different thrust sheets along the transition zone. This research not only is very important to understand the tectonic evolution of the northern Tianshan Mountains and the southern Junggar basin transitional zone, but also has important scientific significance for the understanding of Cenozoic tectonic evolution and geodynamic process of the Tibetan Plateau.

北天山-准噶尔盆山过渡地区具有独特的地质构造特征，既有中新生代盆地的沉积作用，又有山脉的强烈隆升作用和大规模的不同逆冲岩席构成的褶皱冲断带。针对盆山过渡带热演化及其与构造变形间关系的问题，本项申请强调北天山基岩断隆带和准噶尔盆地第一排构造带构造解析、热演化相结合，以及隆升剥蚀与构造变形相结合。选择横穿北天山-准噶尔盆山过渡地区数条剖面，通过详细的野外构造解析和断裂运动学的分析，确定过渡带地区不同逆冲岩席的边界和性质；开展不同逆冲岩席的系统的低温年代学分析，建立构造热年代学剖面；采用热史模拟和数值模拟相结合的方法，反演盆山过渡带地区热史演化、侵蚀剥露和构造变形过程，探讨山脉隆升剥露、构造变形以及不同逆冲岩席热演化之间的关系。这些研究不仅对认识北天山-准噶尔盆山过渡带地区的构造演化具有十分重要的意义，而且对认识青藏高原新生代构造演化和地球动力学过程也具有重要的科学意义。

本项目以野外地质调查为主，通过野外地质剖面测量、构造变形分析、开展磷灰石裂变径迹和(U-Th)/He分析、河沙碎屑磷灰石裂变径迹定年分析，研究了北天山准噶尔盆山过渡带构造演化特征。观察薄片100 余片，完成了盆山过渡带基岩隆起带和盆地内第一排构造带中生界地层磷灰石裂变径迹年代学分析114件、磷灰石(U-Th)/He分析15件、现代河沙碎屑磷灰石裂变径迹定年7件和侏罗系煤岩镜质体反射率测定37件。主要成果有：北天山基岩断隆带样品AFT年龄分布在88±10–246 ± 23 Ma，封闭径迹主要分布在9.28±1.81–13.98±0.43 m，大部分样品的长度分布呈单峰态势，反映单一的冷却降温过程，热史模拟结果主要揭示了晚三叠世-侏罗纪和晚侏罗世-白垩纪的冷却剥露历史；磷灰石（U-Th）/He年龄分布在8.57±7.34–78.83±4.75 Ma，大部分介于10–30 Ma，记录了新生代的冷却剥露过程。（U-Th）/He年龄和高程大体呈现负相关，即海拔越高，样品年龄反而越年轻，可能与断裂活动的掀斜作用有关。磷灰石（U-Th）/He数据热历史模拟结果表明研究区新生代至少经历了4期快速隆升剥露事件，分别为58–40 Ma、24–20 Ma、15–10 Ma和～3 Ma，其中2期隆升剥露事件（24–20 Ma和15–10 Ma）在北天山基岩断隆带和准噶尔盆地第一排构造带广泛分布。现代河沙碎屑磷灰石裂变径迹年龄具有7组峰值，分别为7.9–18.7 Ma、28.4 Ma、51 Ma、67–82 Ma、119–124 Ma、156–176.8 Ma和243–305 Ma，反映上游地区存在冷却年龄更小的地质体。准噶尔盆地第一排构造带磷灰石裂变径迹分析结果指示存在150–125 Ma; 120–105 Ma; 100–80 Ma; and 75–45 Ma四期冷却年龄。热史模拟结果表明该构造带侏罗系地层经历早中侏罗世的埋藏作用，以及150–110 Ma 开始的缓慢冷却和晚中新世（20–10 Ma）以来快速冷却过程。根据北天山断隆带基岩磷灰石裂变径分析和（U-Th）/He结果以及河沙碎屑磷灰石裂变径迹年龄数据，花状挤出抬升为区域性的大范围抬升模式，而掀斜式抬升模式为局部范围的抬升模式。