维西-乔后断裂晚第四纪活动、滑动速率及其在川滇块体西边界构造演化中的作用

Various models are proposed to describe the tectonic evolution and uplift of the Tibetan plateau. Sichuan-Yunnan block lies to southeastern boundary of the eastern Himalayan syntaxis (EHS), and is strongly active for a long term, with intense earthquake having occurred frequently along the boundary faults of the block historically. That the Wenchuan Magnitude 8.0 earthquake occurs warn people to focus on the block boundary study on faults activity and seismic risk to a higher degree. The Weixi-Qiaohou fault is located at the western margin of Sichuan-Yunnan block, connecting to the Red riverI fault in the south and to the Jinsha river fault in the north, respectively, is the bridge between the north and the south active faults. On the basis of pre-research, combining the interpretation on the Satellite images figures and aerial photographs, this project aims at the study on the latest active time, the slip rates, the activity features, Paleo-earthquake recurrence interval, etc., and appraising the future seismic risk of large earthquake, by approach of precise topographic measuring by equipments, such as Trimble 5800 GPS, Geological mapping with a large scale, such as 1:5,000～10,000, field investigating, trench excavating and chronology testing. Study on the tectonic topographic characteristics and evolution history, the active inception, the relationship of tectonic deformation style and the regional tectonic stress field evolution characteristics since late Cenozoic will be conducted as well. The purpose of the study is to acquire a complete and reliable understanding of active features of the Weixi-Qiaohou fault in late Cenozoic, to explore the relationship of the fault with the Red river fault and with the Jinsha river fault ,and to assess the role and influence of the fault in the tectonic evolution of the western boundary of Sichuan-Yunnan block. The research result has both theoretical and practical meanings on assessing reasonably the strong earthquakes risk along the western boundary faults of Sichuan and Yunnan block, on discussing the present dynamic circumstances around the southeastern Tibetan plateau and the boundary tectonic deformation style patterns.

川滇块体为青藏高原东缘长期强烈活动的块体，边界断裂带上历史地震活动频繁。汶川8级大地震的发生，更加警示人们重点关注块体边界断裂活动性和地震危险性的研究。维西-乔后断裂位于川滇块体西部边缘，南与红河断裂相连，北与金沙江断裂相接，是连接南、北两条活动断裂(红河和金沙江断裂)的桥梁。本项目拟在已开展的预研究基础上，结合高分辨率遥感影像和航空遥感图像解译，借助精细的地貌测量、槽探、年代样品测试等方面的工作，研究断裂的最新活动时代、滑动速率、活动特征及其古地震复发间隔等，评价未来强震的危险性；研究分析断裂构造地貌特征及演化历史、断裂活动的起始时代；分析晚新生代以来构造形迹特征和区域构造应力场演变；探讨其与红河断裂和金沙江断裂的关系，断裂在川滇块体西边界构造演化中的作用和影响。研究成果对合理评估川滇块体西边界断裂的强震危险性，对探讨青藏高原东南缘现今动力学环境、块体边界构造变形具有重要理论和实际意义。

川滇块体是青藏高原东缘侧向挤出最强的也是最具代表性的活动块体，地震频繁。川滇块体西南边界红河断裂是一条大型走滑剪切带，主要表现为右旋走滑运动。西北边界金沙江断裂曾是一条多期活动的缝合线构造，后期表现出右旋走滑兼逆冲性质，其中北段被认为属晚更新世~全新世活动段，然而，德钦-中甸-大具断裂以南其活动迹象不明显。维西-乔后断裂位于川滇块体西部边缘，南与红河断裂相连，北与金沙江断裂相接。问题是此断裂是否具有与红河、金沙江同样的晚第四纪活动特征？是否属于川滇块体边界的重要组成部分？加强这些问题的研究，无疑对深入理解块体边界构造变形机制和变形样式，对认识青藏高原周边地壳变形的运动学与动力学特征等具有重要的理论和实际意义。本项目应用地震地质、地貌学等方法重点研究晚更新世-全新世活动的地质地貌证据、最新活动时代及其活动特征，估算滑动速率，探讨古地震活动历史并评价未来强震的危险性等，分析该断裂与红河、金沙江断裂的关系及其在川滇块体运动中的作用和意义。.项目组共开挖玉狮场、德胜、石岩村、佛堂村等大中型探槽9个，位错地貌测绘4处，并编制了1：5万条带状活动断裂图。研究揭示出断裂晚更新世~全新世活动迹象和古地震事件。对断裂沿线T1~T4阶地、洪积扇和探槽物质共完成测年数据95个（14C和OSL）。共发表论文12篇（SCI论文3篇），会议交流论文2篇。维西-乔后断裂全长280km，自北而南分为北段、中段和南段。北段长约60km，为逆冲兼右旋走滑性质，晚更新世以来运动速率为0.3~0.4mm/a；中段长180km，以右旋走滑运动为主，走滑速率1.8~2.4 mm/ a；南段长40km，以正断为主，其滑动速率为0.18~0.38 mm/a。盖场探槽揭示出28000a.左右的古地震事件，玉狮场揭示出1500 ~1400a.，1190a之后和600a.左右3次地震事件。维西-乔后断裂晚第四纪活动特征明显，以右旋走滑为主，与红河、金沙江断裂具有相似的运动学特征，是红河活动断裂带的北延部分，它们共同构成川滇块体西边界，在川滇块体新构造演化中扮演着重要角色。