小江断裂带南段第四纪构造变形、起始时代与滑动速率研究

Various models proposed to describe the tectonic evolution and uplift of the southeastern margin of the Tibetan plateau acknowledge clockwise rotation in a region between the eastern Himalayan syntaxis (EHS) and the left-lateral strike-slip Xianshuihe-Xiaojiang fault system (XXFS) with respect to the south China block. The XXFS is one of the most active fault systems in China. The Xianshuihe portion of the XXFS may have formed first, with the XXFS propagating and younging to the south. Appearance and activity of the southern segment of the Xiaojiang fault (SSXF) in the Quaternary suggest final formation of the XXFS as a uniform left-lateral structural zone, which accommodates the relative motion between Sichuan-Yunnan block (fragment) and South China block. The GPS velocities indicate that the left-lateral strike-slip motion along the Xiaojiang fault extends southwestward across the Red River fault (RRF) with ～7mm/yr, even ～10mm/yr left slip rate, but it is inconsistent with a weak activity of the SSXF in the late-Quaternary concluded by some geologists, as they consider that the NWW- trending Qujiang-Shiping fault zone has absorbed and transformed the southward motion of the western side of the Xiaojiang fault through dextral strike-slip/shearing as well as transverse shortening/thrusting. Thus precise study on the fault activity is crucial in differentiating these recognitions. A promising survey conducted in the recent, correlated with the interpretation of remote sensing, provides a basis to determine several representative sections in the SSXF for studying at detail. Geological mapping with a large scale, such as 1:5,000～10,000, precise topographic measuring by equipments, such as Trimble 5800 GPS, trench detecting, systematic sampling and dating of the Quaternary deposits will be carried out at the representative sections. The purpose of the study is to acquire a complete and reliable understanding of active features of the SSXF in the Quaternary, which include deformation styles, inception time, slip rates and abutting relation with the RRF. The above results, correlated with relocation and focal mechanisms of micro-earthquakes, may be applied to obligate or test the kinematic models about the southeastern region of Yunnan Province, which will be in favor of improving the theoretical model and furthering our recognitions of the dynamic process and period. Different models suggest rather variant crustal deformation patterns. It is the theoretical model based on precise understanding of fault activity that can make a reasonable assessment about the future seismic risk. The project has both theoretical and practical meanings.

小江断裂带南段第四纪时期的构造演化和活动特征，反映了鲜水河-小江断裂系作为统一的左旋走滑构造带的最终形成、以及对川滇块体与华南块体在该区段相对运动的调整。但GPS显示的该断裂段两侧不小于～7mm/yr左旋走滑速率与目前地质上有关它晚第四纪活动性很弱的认识存在不协调。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解译，选择几个典型地段，借助大比例尺地质填图、精细的地貌测量、槽探、系统的新年代样品采集与测试等方面的工作，针对小江断裂带南段第四纪时期的构造变形特征、起始时代、滑动速率以及与红河断裂带的交切关系开展研究。结合小震重新定位和震源机制解等研究工作，对滇东南构造动力学理论模型进行约束和检验，深化对动力学过程、阶段的认识。不同的理论模型预测了不同的构造活动图像，只有在详细了解第四纪断裂活动性基础上建立的理论模型，才能对未来地震危险性做出合理评价。本项申请具有重要的理论意义和社会价值。

涉及青藏高原东南缘的构造动力学模型均认同该地区围绕着东喜马拉雅结作顺时针转动，近年来GPS观测资料清晰地勾画出这一图像，但是块体转动要求的小江断裂带南段活动性与目前地质上有关该断裂段晚第四纪活动性较弱的认识存在不协调。地质上一般认为沿着鲜水河-小江断裂系的左旋走滑运动在小江断裂带南段受到北西西向曲江、石屏-建水断裂的阻挡。本项目围绕小江断裂带南段第四纪以来的构造变形特征、起始时代、运动参数以及与红河断裂带的交切关系开展了研究。.本项目研究表明小江断裂带南段是一条有着强烈活动性的全新世活动断裂，并且发育一条地震地表破裂带。在小江断裂带南段中部、南部和北部进行了探槽开挖（T-fm、T-lc1、T-lc2和T-ls），地质剖面分析表明断裂活动已经断错了剖面上现今土壤层之外的所有晚第四纪地层，贯穿小江断裂带南段分布的最新一期地震地表破裂带应该是在同一次地震形成的，长度为75±5 km。采用无人机移动摄影测量技术和RIEGL VZ-4000三维激光扫描，进行了多个区段的构造地貌精细测量，获得42个水平位错量，确定的最大位错量为7.3±0.3m。根据地震地表破裂带长度和位错量，估算了最晚一次地震震级为7.5级。.探槽中年代测试结果表明最晚一次发生在AD 1397之后。建水县是国务院批准的中国历史文化名城，如果在AD 1397年以来发生过断错地表的地震事件，那么，在历史文献应该有明确的记载。此次事件与历史文献中记载的1606年建水地震是可以相互验证的。小江断裂带南段最晚一次活动时间发生在1606年，大震复发周期~700yr。.本项目从地质和地震两个方面对青藏高原东南缘围绕着东喜马拉雅结作顺时针转动的理论模型提供了有力的支撑。本项目用地质方法获得了小江断裂带南段的左旋走滑速率为（7.02±0.20）mm/yr；根据大震位错量和复发周期换算的滑动速率为（10.43±0.43）mm/yr。鲜水河-小江断裂系在与红河断裂带交汇区的左旋走滑速率，完成可以被小江断裂带南段所调整，鲜水河-小江断裂系应以一种相对单一的活动断裂方式切穿了红河断裂带。