鲜水河断裂带构造活动性厘定及其对青藏高原东缘变形模式的制约

Research on the strike-slip rates with space and time variation is crucial to understand the fault behavior, seismicity and the kinematics mechanism of the continental strike-slip faults. The left-lateral strike-slip XSH fault system is considered as one of the most tectonically active intra-continental fault system in China, if not in the world, with >20 earthquakes of M>6.5 since 1700. While GPS and InSAR data exist in eastern Tibet, very few quantitative late Quaternary slip-rates are documented along the XSH faults despite their key location. In addition, eastern Tibet has long been the focus of crustal studies in order to understand the highly debated topic of which deformation model prevails: continuous deformation model with numerous slow-slipping faults or hidden plate tectonics model with few large, fast-slipping faults. Therefore, this study will also help understand how the XSH fault system accommodates deformation..Here, we propose to focus our research on the SE Ganzi-Yushu and the NW Xianshuihe faults, between Dengke and Kanding. By looking at the satellite images and going to the field once, we first selected several suitable sites in order to study late Quaternary slip-rates using cosmogenic dating of offset geomorphic features (glacial moraines, terrace risers or levees). In particular, we found several impressive sites along the Ganzi-Yushu segment of the XSH fault system (between Ganzi and Dengke, mostly along the Queer Mountains, but not only), and several sites along the SE segment of the XSH fault (where the XSH fault splits into the Yalahe, Selaha, and Zheduotang segments, just NW of Kangding). Once we obtain late Quaternary slip-rates at various locations along the ~1400 km-long XSH fault system, we will be able to 1) test whether the slip-rate varies with space and time (by comparing our rates with those of others, as well as with existing geodetic or geologic rates), 2) favor a deformation model for the eastern Tibetan Plateau; and 3) assess the seismic hazard in the region.

走滑速率随空间和时间变化规律的研究对理解断裂行为、地震活动性以及大陆内部走滑断裂的运动学机制至关重要。青藏高原东部~1400km长的左旋走滑鲜水河断裂系是中国陆内最活跃的断裂，自1700年以来，该断裂系已发生震级M > 6.5的地震多达20余次，目前鲜水河断裂系仅有少数晚第四纪滑移速率的记录，制约着对地震活动性和地壳变形的认识。对该地区地壳变形的研究有助于理解长期存在争议的两种地壳变形模型：大量具低滑移速率断层存在的连续变形模型和少量具快速滑移速率断层存在的块体构造模型。因此，本申请以鲜水河断裂系为研究对象，选择北段甘孜-玉树断裂邓柯—甘孜段和中段鲜水河断裂道孚—康定段为重点活动构造研究区，对沿断裂系亟需地震灾害评估的几个高度活跃区进行晚第四纪滑动速率测量，结合前人研究，查明沿断裂系滑移速率的空间变化规律，揭示其地震活动性，探讨鲜水河断裂系对印度-欧亚板块碰撞所引起的构造变形的调节机制。

左行走滑的鲜水河断裂系位于青藏高原东缘，自北西向南东分为玉树/巴塘断裂、甘孜断裂、鲜水河断裂以及磨西-安宁和-则木河-小江断裂。该断裂系活动性极强，自1700年以来已经发生16次7级以上和26次6.5级以上大地震。因此，研究其活动性，特别是其在不同时间尺度上的滑移率，是评价区域地震危险性的必要条件。本项目主要围绕甘孜断裂、鲜水河断裂以及磨西断裂利用晚第四纪构造地貌学方法展开研究，四年来发表了一些新的结论，包括已发表的三篇文章（GSAB一篇、EPSL两篇），以及一篇正在审稿中的Tectonics文章。通过对甘孜断裂、鲜水河断裂和磨西断裂上被活动断裂错开的7个冰碛物和1个冲积扇的大地测量学和宇宙成因核素测年研究，得到甘孜断裂、鲜水河断裂南东段和磨西断裂晚第四纪以来平均左行走滑速率分别为7(±1) mm/yr，10(±0.5) mm/yr和12.5(+2.3/-2.1) mm/yr，表明沿鲜水河断裂系晚第四纪走滑速率自北西向南东递增且至少延伸至磨西断裂，区域内GPS矢量观测也支持了这一结论。鲜水河断裂南东段分成右阶雁列式分布的雅拉河断裂、色拉哈断裂、折多塘断裂以及我们最新发现的木格措南断裂。通过断裂走滑速率的研究，提出甘孜断裂马尼干戈段具有7.6级大地震的危险性，鲜水河断裂南东段康定附近未来具有6.5-7.3级大地震的危险性。利用该项目的冰碛物年龄重建了西藏东部的古气候史，表明青藏高原冰川作用主要发生在MIS -2和MIS-6(其间没有MIS-3冰进作用)，这是北半球冷却周期的最冷期，表明青藏高原东南缘冰川作用对气温下降最为敏感。此外，在本项目的支持下，发表了两篇关于藏南裂谷(如亚东-谷露裂谷)的论文(JGR和Tectonophysics)。结果表明，从喜马拉雅到谷露的东西向扩张速率基本一致(1 ~ 1.3+/-0.3 mm/yr)，而向北由于崩措右阶走滑断裂的影响，扩张速率增加到~6 mm/yr；西藏南部的总扩张速率为9(+/-2) mm/yr，与GPS扩展率基本一致。本项目成果对认识青藏高原东缘的新生代构造变形机制具有重要的科学意义，同时对准确评估鲜水河断裂系地震危险性、服务城镇防震减灾和国家大型基础设施规划建设具有重要的实际意义。