Abstract:

The Qaidam Basin, as the largest continental sedimentary basin on the northeastern margin of the Qinghai-Xizang Plateau, has formed thick sedimentary strata since the Cenozoic. Its complete and continuous Cenozoic sedimentary strata not only record the sedimentary evolution of the Qaidam Basin but also reflect the uplift and erosion of the adjacent orogenic belts and regional climate-environmental changes. Furthermore, they serve as a crucial resource for understanding the development of the basin-mountain system, tectonic deformation, and the mechanisms and patterns driving the northward expansion of the Qinghai-Xizang Plateau on the northeastern margin. Therefore, it is vital to reconstruct the sedimentary evolution history of the Qaidam Basin during the Cenozoic. Currently, the detrital zircon U-Pb dating method is the most widely used provenance analysis method for reconstructing the basin’s tectonic evolution and the uplift history of the surrounding mountain ranges. However, this method usually relies too heavily on qualitative comparisons of zircon U-Pb age spectra and lacks reliable quantitative analysis. This limitation directly leads to significant differences and disputes in some key understandings, such as the erosion of the source and the transport path of sediments. To address this issue, we selected three representative Cenozoic sedimentary sections on the northern margin of the Qaidam Basin for our study: Dahongou, Hongshan, and Huaitoutala. At the same time, by systematically collecting the latest published U-Pb age data of detrital zircons and basement zircons in modern river sands, we redefined three main provenance areas, including the southern segment of the Qilian Mountains, the East Kunlun Mountains, and the Mesozoic strata in the northern part of the Qaidam Basin. The results show that the Dahongou, Hongshan, and Huaitoutala sections have 6, 2, and 3 distinct shifts in provenance, respectively. Although the sedimentary provenance evolution patterns of these sections are not completely consistent, there is a quasi-synchronous provenance change at ~12-10Ma. Around ~20Ma, the southern Qilian Mountains expanded towards the Qaidam Basin, causing the uplift and erosion of the Mesozoic strata on the northern margin of the Qaidam Basin. All three sections recorded the significant contribution of the Mesozoic provenance from the northern margin of the Qaidam Basin during this period. Although the East Kunlun Mountains experienced a rapid uplift and erosion event in the middle-late Miocene, this tectonic event had a relatively limited impact on the provenance of the Hongshan section, underscoring the limitations of single-section studies for elucidating regional tectonic and sedimentary evolution. Additionally, the simulation of provenance inversion suggests that the northern margin of the Qaidam Basin experienced influences from at least three distinct provenance regions throughout the Cenozoic. The zircon age distribution characteristics of simulated source area Ⅰ closely resemble the detrital zircon age distribution found in modern river sands across the eastern Kunlun Mountains, as per our compilation. Meanwhile, the zircon age distribution in simulated source area Ⅱ closely matches that of modern river sands in the southern Qilian Mountains. Additionally, the provenance area Ⅲ revealed by the inversion simulation exhibits a notable Proterozoic double-peak zircon age signature, although it shows a higher proportion of zircon components exceeding 550Ma, closely aligning with the detrital zircon age composition from the Mesozoic strata(including Jurassic and Cretaceous) of the northern margin of the Qaidam Basin that we compiled. Overall, the inversion simulation results reveal a substantial similarity in the zircon age distribution between the simulated source area III and the Mesozoic strata in the northern margin of the Qaidam Basin. Moreover, in most Mesozoic clastic samples, a significant proportion of the parent material is derived from sources. This suggests that during the Neogene, the relationship between the Mesozoic parent material sources in the northern margin of the Qaidam Basin is more complex than simply a competition between the two end-member source areas, namely the South Qilian Mountains and the East Kunlun Mountains, as has been previously highlighted. Additionally, the Mesozoic strata uncovered by the frontal thrust deformation are likely to be significant contributors to the material supply during the Neogene. Additionally, the provenance inversion simulation in this study is consistent with the above forward simulation results, indicating the rationality and credibility of the provenance interpretation. At the same time, it also shows that, in cases where the number of potential source areas is difficult to determine and the age composition of source-area zircons is difficult to constrain, provenance-inversion simulation will be one of the most effective methods. In conclusion, we believe that the quasi-synchronous provenance change at ~12~10Ma may represent the surface response to the deep-dynamic transformation of the plateau, namely, the delamination of the thickened lithosphere triggers regional tectonic uplift and surface deformation. Besides, the East Kunlun Mountains and the southern Qilian Mountains, the Mesozoic strata distributed in the transition zone between the southern Qilian Mountains and the northern margin of the Qaidam Basin are another important provenance area for the Cenozoic sedimentary process on the northern margin of the Qaidam Basin. The zircon age distribution of these strata is often confused with the provenance of the East Kunlun Mountains in previous studies. These insights provide reliable references for a deeper understanding of the evolution of the Cenozoic Basin-mountain system on the northeastern margin of the Qinghai-Xizang Plateau. We emphasize that, in complex basin-mountain systems, simply comparing the age distributions of detrital zircons in most provenance analyses is no longer sufficient. A thorough qualitative and quantitative analysis and assessment are more conducive to obtaining reliable and comprehensive results.

Key words: Northern Qaidam Basin, Cenozoic strata, detrital zircon, provenance modeling

摘要：

完整连续的柴达木盆地新生代地层是揭示青藏高原东北缘盆-山格局演化、 地表构造变形时间及高原向N扩展模式和动力学机制的重要研究载体。其中, 碎屑锆石物源分析又是厘清柴达木盆地构造演化过程、 重建周缘造山带隆升历史的关键研究方法之一。然而, 以往的碎屑锆石物源研究多是基于简单定性的特征对比, 缺乏定量的分析手段, 直接导致在一些关键认识上出现重大分歧。文中选取大红沟、 红山、 怀头他拉3个典型的柴北缘新生代剖面, 汇总收集了具有代表性的碎屑锆石U-Pb数据, 并重新定义了南祁连山、 东昆仑山和柴北缘中生界3个潜在物源区。基于碎屑锆石数据的正演和反演物源模拟, 我们发现柴北缘中生代地层也是新生代盆地的重要物源区之一, 其物源贡献量不能被忽视。此外, 大红沟剖面、 红山剖面、 怀头他拉剖面的物源演化规律具有明显的时空异质性, 仅在距今12~10Ma发生了一期准同步的物源变化, 可能是高原中部岩石圈拆沉的地表响应。因此, 简单的碎屑锆石年龄分布对比无法满足复杂盆-山系统下的物源分析工作, 定性和定量研究手段的联合使用和综合判断有利于得到更加可靠、 更加深刻的认识。

关键词: 柴北缘, 新生代地层, 碎屑锆石, 物源模拟