天水盆地及周缘新生代构造-地貌演化的碎屑热年代学证据

The Cenozoic basin successions and modern rivers sediments of the Tianshui Basin, which are located at the junction of the Liupan Shan and Western Qinling orogenic belts, the northeasternmost margin of Tibetan Plateau, record the processes and details of the far-field effects of India-Asia collision and Tibetan uplift extended to the eastward of the Western Qinling syntaxis. To detect and deciphering these documents can provide a central constrain to understanding the uplift history of the Tibetan Plateau. However, they are still poorly studied.The detrital fission-track (FT)thermochronology is a sensitive and precise tool to deciphering them. Base on the previous study of the magnetostratigraphy, we try to conduct an integrity detrital zircon and apatite FT thermochronology in this region. First，analysis the apatites and zircons from the sediments of the Weihe, Xihanshui Rivers and their branches, which are the most directly products of the formation of the modern structural geomorphology, combining with the analysis of the regional deformation structures, to clarify that how and when the modern framework of the geomorphology formed along the Tianshui basin and its neighboring regions. Moreover, by comparing the FT population ages with the present bedrock FT ages surrounding the basin, the erosion-migration regularities of the regional detritus (apatite and zircon) are studied, which are the key reference for the analysis of FT stratigraphy. Second, conduct the detrital FT study on the relatively complete Cenozoic successions whose depositional ages have been precious constrained by the paleomagnetism and fossil mammal ages. Correlation of age populations among the different samples should be resulted into several peak-age groups, which belonging to three peak-types: "static peaks", "moving peaks" and "mixing peaks". The occurrance age of the first one, the age-range of the second one and the change point of the last one are all indicate a tectonic episode. By analysis these peak-age groups with the above rules, combing with the analysis of the paleocurrent, clastic composition and tectonic- sedimentary deformation, a series of significant Cenozoic tectonic events should be yielded. Finally, we attempt to unravel the Cenozoic morpho-tectonics of the northeasternmost margin of Tibetan Plateau, by combining analysis the changes and/or Hierarchical combination of the typical geomorphology（Drainage pattern, source terrane, etc.）corresponding to episodic tectonism, the published results of the Cenozoic deformation, stratigraphic sequence and tectonic geomorphology in the neighboring regions. This study can provide a crucial and entire FT thermochronology evidences of the northeasternmost margin of the Tibet which are contributed to improve the understanding of the long-term uplift and deformation history of the Plateau.

位于青藏高原最东北缘六盘山与西秦岭两重要构造带交汇处的天水盆地充填了较完整新生代沉积序列及现代河流沉积物，它们记录着印度-亚洲大陆碰撞远程效应及高原隆升跨越西秦岭构造结向东部发展的过程。"发掘"和"解译"这些记录可为全面的认识和理解高原隆起过程提供高原最东北缘这一关键点位的地质地貌约束。然而至今该区新生代构造-地貌演化方面研究十分薄弱。本申请拟充分利用我们前期盆地新生代磁性地层年代序列控制，重点通过该盆地长序列、精细的系列新生代地层和现代河流沉积物中锆石和磷灰石裂变径迹热年代学研究，结合古流向及砾石成分分析、沉积-构造变形和地貌响应，揭示天水盆地及其周缘物源区新生代相对完整的重大构造事件序列，探讨区域构造-地貌演变过程，为认识和理解高原整体构造变形过程提供高原最东北缘关键的、相对完整的裂变径迹热年代学证据。

位于青藏高原最东北缘六盘山与西秦岭两重要构造带交汇处的天水盆地充填了较完整新生代沉积序列及现代河流沉积物，它们记录着印度-亚洲大陆碰撞远程效应及高原隆升跨越西秦岭构造结向东部发展的过程。“发掘”和“解译”这些记录可为全面的认识和理解高原隆起过程提供高原最东北缘这一关键点位的地质地貌约束。然而至今该区新生代构造-地貌演化方面研究十分薄弱。鉴如此，本项目主要开展天水盆地南峪和喇嘛山剖面以及现代河流沉积物的系列沉积物裂变径迹热年代学研究，获取该区新生代重大构造事件序列。在国家自然科学基金委的支持下，经过几年的深入研究，不仅按原定计划完成了研究内容，而且还增加了天水盆地典型剖面的磁性地层学和哺乳动物年代的研究（部分成果已发表SCI期刊《Journal of Geophysical Research》和《Journal of Asian Earth Science》），以及盆地周缘基岩样品的裂变径迹热年代学研究。实现了原先设定计划目标。取得了一些成果和认识：我们确定了新生代以来青藏高原东北缘天水盆地高精度的地层年代序列，并建立了区域盆地沉积演化模式以及沉积速率变化序列（成果发表在SCI期刊 《Journal of Earth Science》）。并且根据天水盆地沉积物（包括河流阶地以及现代河流沉积物）和周缘山体的裂变径迹热年代学研究，揭示出区域重大构造事件本次研究共获得区域新生代构造变形活跃事件七次，分别发生在49-46、34-29、23-18、14-13、10、8、3.6 Ma（成果发表在SCI期刊《Tectonophysics》）。印度-欧亚板块碰撞的远程效应早在始新世已经同步传递到高原最东北边缘，导致区域盆山系统重组，有些地质体隆起成山遭受剥蚀，成为物源区。而天水-喇嘛山-成县一带下陷成盆，开始接受沉积。随后又发生了几个期次的构造事件，导致个别山体隆升成为新的剥蚀源区，而盆地也开始逐步瓦解。直到最近期次3.6 Ma的构造事件，导致区域最大规模的构造变形并隆升，区域盆山体系重新组合，奠定了现代地貌格局的雏形（已投SCI期刊《GSA Bulltin》）。.这些研究结果，不仅为下一步深层次认识和理解高原整体构造变形隆起过程以及构造-气候相互作用提供了高原最东北缘关键的、相对完整的低温同位素热年代学证据，而且它对正确理解亚洲内陆干旱化和季风形成演化的构造背景具重要意义。