藏北高原近南北向地堑群晚新生代正断层活动的时空特性与规律研究

The diffuse graben groups in central-northern Tibet and normal faulting along the south-north trending rift systems in southern Tibet are dominant active tectonic character in Qinghai-Xizang Plateau. The characteristics and dynamic mechanism of normal faulting are very important focus on understanding the timing and manner of the Qinghai-Xizang Plateau uplift. But many evidences for the models of extension across the Tibetan Plateau during late Cenozoic or Quaternary have been from studies on active north-south trending normal faults in southern Tibet, only seldom researches have related to active normal faulting in central Tibet. So, it is clearly, more data on the spatial and temporal distribution of normal faulting across the Tibetan Plateau, particularly within the central Tibet, are needed to understand the causes of EW extension across Tibet and constrain the relation with uplift of Tibet. Based on previous studies on active normal faulting in central Tibet, in this research project, we select the Amdo graben group which activity and scale are very prominent in the central-northern Tibet as the main study area. We will mainly study the geometric and kinematic characteristic, Quaternary activity, typical paleo-earthquake events and their ages, especial to unitize the multi-generation fault scarps which are formed by normal fault offset late Quaternary (about since 250ka before present) various geomorphic units to study late Quaternary normal faulting along the Amdo graben system. In the study, the vertical slip rate of normal faults in various time-scale including millenary, 10ka, 100ka and 1Ma, will be obtained by field surveying, tectonic geomorphology and Digital elevation model across grabens analysis, detailed fault scarps measuring and late Quaternary deposits dating using various chronological techniques including carbon-14, OSL, U-series, ESR, Fission track and U-Th-He so on. Because the various time-scale slip rates of normal faults will reveal the temporal and spatial active character of the main boundary normal faults along Amdo graben group, and constrain the kinematic and dynamic mechanism on extension deformation of the central-northern Tibet graben systems. Father, studying the multiple time-scale slip rates of normal faulting along the Amdo graben group also is an important approach to understand the behavior of normal faulting of intraplate rift system, the law of large earthquake migration along the south-north trending graben system and active characteristic of rift system in active collision orogenic belt between Indie and Eurasia plate. Finally, the comparative study on S-N trending normal faulting between southern and central in Tibet will be carry out to understand and constrain the causes of east-west extension deformation across Tibet during Quaternary and relationship with uplift of Tibet Plateau.

围绕藏北高原近南北向地堑群的时空发育特征、成因机制及其与高原隆升关系这一关键科学问题。本项目拟以其中典型的安多地堑群为重点研究对象，针对地堑主边界正断层的几何学、运动学特征及第四纪活动的时空特性等内容，综合应用高精度遥感解译、基于数字高程的构造地貌分析、地表活动断裂调查、断层崖高度测量和低温热年代学等多种方法，并结合14C、OSL、U 系、ESR、FT和U-Th-He等年代学测试分析手段，定量约束近南北向地堑及其边界正断层的演化过程与百万至万年不同时间尺度的活动习性，从多角度和多时空尺度揭示该区晚新生代正断层作用的时空差异性、变化特征及规律。同时结合已有研究，开展与藏南近南北向正断层的对比分析，进而系统总结高原内部正断层作用的时空特征与规律，深入剖析其成因机制，提升对青藏高原正断层作用动力学机制及其与高原隆升关系的认识，并增进对活动造山带中板内裂谷作用特性的理解。

青藏高原内部第四纪构造变形的显著特征是近南北向藏南裂谷和藏北地堑群，这也是调节印度与欧亚板块汇聚变形的重要形式。通过对藏北近南北向安多地堑群中典型第四纪不同时代断层崖的详细观测，结合第四纪年代学方法等，重点约束了多个典型地堑主边界正断层第四纪期间从百万年至万年不同时间尺度的垂直活动速率，揭示了藏北近南北向正断层第四纪低滑动速率与长地震复发间隔的时空活动特性，并利用低温热年代学垂直剖面定量限定了藏北安多、懂错和达如错地堑等邻侧隆升区的地表剥蚀速率特征，揭示藏北高原的安多地堑群隆升区自晚白垩世以来长期处于低剥蚀速率状态，这与正断层的低活动速率相一致性。而对藏南亚东-谷露裂谷中格达地堑和帕里地堑的低温热年代数据显示，该区隆升区的地表剥蚀速率明显更大，并存在约10-15Ma和2-4Ma两个阶段的快速剥露过程，推断早期剥露可能与裂谷的初始裂陷有关，而晚期快速剥露可能反映了裂谷最近的一次加速伸展过程。同事还开展了尼泊尔地震对藏南裂谷的影响以及震后藏南多庆错湖泊异常干枯的构造成因分析、藏南裂谷初始裂陷事件的沉积-构造记录、藏南裂谷典型边界正断层的晚第四纪垂直活动速率时空变化等方面的研究。通过对藏北和藏南的近南北向正断层活动对比研究发现，两者的第四纪活动存在显著的时空差异性，主要体现在两个方面：① 与藏南裂谷主边界正断层相比，藏北多数正断层为低滑动速率，一般为约0.1-0.3mm/a左右或更低，比藏南低约5~10倍；② 藏北正断层型地震（M≥7）复发间隔可长达近万年（8~12ka），明显长于藏南裂谷主边界正断层（一般为约1~2ka）。此外，还分析提出了关于藏南多庆错湖水异常消失而后快速恢复现象的地质构造成因及与2015年尼泊尔大地震之间的动力学联系的新认识。项目共培养研究生2名（含博士研究生1名），资助发表论文、成果进展、会议报告及摘要等28篇/次，其中包括SCI、EI检索与成果进展等论文11篇，并另有后续成果在撰写和投稿中。