渭河盆地晚中新世-上新世水系变化的沉积和锆石年龄谱证据

Multiple evidences show that approximate coeval tectonic deformation occurred at 10-5 Ma in the northeastern Tibetan Plateau, which directly influenced the drainage pattern evolution and the formation of major rivers in the surrounding areas. The Weihe River, one of the major tributary of the Yellow River, originated in the northeastern Tibetan Plateau. The Weihe Basin, located in the lower reaches of the Weihe River, preserved relatively complete Cenozoic deposition since late Eocene. The evolution of sedimentary environment and provenance of Cenozoic sediments in the Weihe Basin contains information about the evolution of the Weihe River. Previous results reveals that the detrital zircon U-Pb age of the ~11-7.2 Ma Bahe Formation shows peaks at ~150-140 Ma and ~500-400 Ma. This U-Pb age distribution pattern is similar to that of the North Qinling. The sedimentary analysis shows that the Bahe Formation is mainly alluvial facies. These two lines of evidence indicates that the Bahe Formation is the alluvial fan of the North Qinling Mountain. On the other hand, ~6.7-2.6 Ma fluvia-lacustrine sediments synchronous with the Lantian Formation shows detrital zircon U-Pb age much more peaks than that of the Bahe Formation, at 300-200 Ma, 500-400 Ma, 1200-800 Ma and ~1500 Ma. These characteristics are similar to that of the modern sand bar sediments of the Weihe River, the upper part of the Yellow River, the West Qinling and other parts in the northeastern Tibetan Plateau, as well as the modern river sediments among the Ordos Basin. The fluvia-lacustrine sediments at 6.7-2.6 Ma are recognized as fluvial facies. Therefore, these sediments are probably derived from a wider source region than previous ones. The West Qinling or even much further region in the NE Tibetan Plateau is probably included in the drainage of the Weihe Basin. In this project, more detailed sedimentary analysis and around 40 detrital zircon U-Pb age analysis will be carried out on fluvia-lacustrine deposits in Late Miocene to Pliocene (the Bahe Formation and heteropic fluvia-lacustrine sediments synchronous with the Lantian Formation). Based on comprehensive survey of geology background setting of surrounding regions of the Weihe Basin, the evolution of sedimentary environment and sediment provenance will be obtained. Furthermore, the change of drainage system will be recognized accordingly. Combined with the tectonic evolution history of the surrounding regions of the Weihe basin in the late Cenozoic, the possible mechanism of the water system pattern change and geomorphological evolution of the Weihe basin in the late Miocene-Pliocene will be surveyed.

青藏高原东北部及周缘在晚中新世-上新世发生了影响深远的构造变形，对区域的水系格局以及大河的形成演化有直接影响。现今发源于青藏高原东北部的渭河是黄河的重要支流，渭河盆地新生代沉积环境和物源变化是渭河流域水系演化的良好记录。初步研究表明，盆地南部陕西蓝田地区~11-7.2 Ma灞河组沉积碎屑锆石年龄谱与北秦岭相似；~6.7-6 Ma河湖相沉积与现代渭河沉积相似，这一物源变化原因尚不清楚。本项目拟选取蓝田地区有详细定年结果的~11-7.2 Ma灞河组和~6.7-2.6 Ma河湖相沉积剖面，分析两套地层的沉积相和碎屑锆石U-Pb年龄，建立剖面~11-2.6 Ma沉积环境演化和锆石年龄谱变化的时间序列；在全面收集区域构造资料的基础上，揭示~11-2.6 Ma渭河盆地蓝田地区沉积环境、物源和水系的变化，讨论水系变迁对区域构造活动的响应过程。该研究有望对这一阶段渭河盆地水系演化过程研究提供重要参考。

认识晚中新世以来渭河流域地貌发育和水系演化过程，有助于理解晚新生代青藏高原向东北部扩展的过程和机制。本项目围绕晚中新世以来渭河盆地沉积、物源分析和流域水系连通过程开展。开展了4次野外考察，新增超过30,000个锆石U-Pb年龄数据、26个样品的矿物组合定量分析数据。本项目在渭河流域晚中新世以来沉积环境演化、物源变化、水系连通过程等方面取得了一些进展。主要研究内容包括：分析了渭河盆地蓝田地区晚中新世以来沉积剖面的沉积环境、古水流方向。晚中新世以来，渭河盆地蓝田地区沉积物记录了几次从山前洪泛平原、河流相到浅湖-滨湖相，再到洪泛平原沉积的旋回。测试了渭河流域现代沉积物碎屑锆石U-Pb年龄谱和矿物组合，提取了不同潜在源区物质组成特征。北秦岭山前沉积锆石年龄具150 Ma特征峰，鄂尔多斯地块沉积物锆石以太古代-元古代年龄为主，西秦岭地区锆石年龄以200-500 Ma为主；此外，石英-长石-岩屑三角图可有效区分不同源区沉积物矿物组合特征。获得了渭河盆地、三门峡盆地晚中新世以来河湖相沉积碎屑锆石U-Pb年龄、矿物组合特征变化的时间序列，分析了其反映的盆地物源变化、区域水系连通过程。研究表明，早期渭河的雏形可能在晚中新世-上新世已经形成。探讨了渭河流域晚中新世以来黄土地貌发育过程，认为主要受到晚新生代全球变冷控制。研究结果对于认识晚中新世以来渭河流域水系连通过程和流域地貌发育过程等有积极意义。本项目共发表论4文篇，其中Science子刊Science Advances 1篇，Palaeogeography, Palaeoclimatology, Palaeoecology、《中国科学》和《沉积学报》各1篇。发表在Palaeogeography, Palaeoclimatology, Palaeoecology的论文为邀请研究论文。项目负责人相关成果入选第七届中国科学优秀科技论文和2021年度“中国地理科学十大研究进展”。