Abstract: The Longmenshan fault zone is divided into three sections from south to north in the geometric structure. The middle and northern segments are mainly composed of three thrust faults, where the deformation of foreland is weak. The geometric structure of the southern segment is more complex, which is composed of six fault branches, where the foreland tectonic deformation is very strong. The Wenchuan M_S8.0 earthquake occurred in the middle of the Longmenshan in 2008, activating the bifurcation of two branches, the Yingxiu-Beichuan and the Guixian-Jiangyou faults. In 2013, the Lushan M_S7.0 earthquake occurred in the southern Longmenshan, whose seismogenic structure was considered to be a blind fault. After the Lushan earthquake, the seismic hazard in the southern Longmenshan has been widely concerned.
At present, the studies on active tectonics in the southern Longmenshan are limited to the Dachuan-Shuangshi and the Yanjing-Wulong faults. The Qingyi River, which flows across the southern Longmenshan, facilitates to study fault slip by the deformation of river terraces. Based on satellite imagery and high-resolution DEM analysis, we measured the fluvial terraces along the Qingyi river in detail. During the measurement, the Sichuan network GPS system (SCGNSS)was employed to achieve a precision of centimeter grade. Besides, the optical luminescence dating (OSL)method was employed to date the terraces' ages. And the late Quaternary activities of the six branch faults in the southern Longmen Shan were further analyzed.
The Gengda-Longdong, Yanjing-Wulong and the Xiao Guanzi faults (west branch of the Dachuan-Shuangshi fault)all show thrust slip and displaced the terrace T₂. Their average vertical slip rates in the late Quaternary are 0.21-0.30mm/a, 0.12-0.21mm/a and 0.10-0.12mm/a, respectively. Since the Late Quaternary, vertical slip of the east branch of the Dachuan-Shuangshi fault was not obvious, and the arc-like Jintang tectonic belt was not active. Crustal shortening rate of the southern Longmenshan thrust fault zone in the late Quaternary is 0.48-0.77mm/a, which equals about half of the middle segment of the Longmenshan. Based on the previous study on the tectonic deformation of the foreland, we consider that the foreland fold belt in the southern Longmenshan area has absorbed more than half of the crustal shortening. The three major branch faults in the southern Longmenshan are active in the late Quaternary, which have risk of major earthquakes.

Key words: Longmenshan, Qingyi river, river terraces, thrust, crustal shortening

摘要： 继2013年芦山M_S7.0地震发生之后，龙门山断裂带南段的地震危险性得到了广泛的关注。为了深化对龙门山断裂带南段晚第四纪活动性的认识，我们对横跨该断裂带的青衣江上游河段开展了河流阶地调查与测量。在卫星影像和高分辨率DEM分析的基础上，基于SCGNSS（Sichuan Global Navigation Satelite System，四川省卫星定位连续运行基准服务平台）对河流阶地进行了精细测量和对比，开展了河流阶地的光释光测年，建立了青衣江上游河流阶地纵剖面图。耿达-陇东断裂、盐井-五龙断裂和小关子断裂（大川-双石断裂西支）均垂直断错了青衣江二级以上阶地，表现为逆冲活动，其晚第四纪平均垂直错动速率分别为0.21～0.30mm/a、0.12～0.21mm/a和0.10～0.12mm/a。晚第四纪以来，大川-双石断裂东支垂直错动不明显，金汤弧形构造带没有活动。通过青衣江河流阶地变形得到龙门山断裂带南段冲断带晚第四纪地壳缩短速率为0.48～0.77mm/a，该缩短速率约为龙门山断裂带中段的一半。结合前人对前陆区构造变形的研究，认为龙门山南段前陆褶皱带可能吸收了一半以上的地壳缩短量。龙门山断裂带南段3条主要分支断裂均为晚第四纪活动断裂，具有发生强震的危险性。

关键词: 龙门山, 青衣江, 河流阶地, 逆冲活动, 地壳缩短