Abstract: The Youjiang Fault zone is located in the Guixi (Western Guangxi) fault block region. Since the beginning of seismic records,22 earthquakes of magnitude 4.0～6 occurred in this region,among which the largest one is the magnitude 6/2 earthquake occurred in the area between Leye,Guangxi Autonomous Region and Luodian,Guizhou Province in 1875. Of these events,15 earthquakes of magnitude 4.0～5.0 occurred on the Youjiang Fault zone. The Tianlin Bagui M5.0 earthquake of 1962 and the Pingguo M5.0 earthquake of 1977 had caused certain damages of basic installations in the regions. Obviously,the Guixi region is an active region of moderate earthquake,and the Youjiang Fault zone is an active belt of moderate earthquake,which plays an important role in the seismicity in Guixi fault block region and in the territory of the Guangxi Autonomous Region. Based on the interpretation and analysis of satellite images,aerophotos,and large-scale topographic maps,as well as field investigation,a line of geologic geomorphic evidence of late Pleistocene activity of the Youjiang Fault zone have been obtained,and the left-lateral displacements on the fault zone have been measured. This paper presents all these results and provides the horizontal and vertical slip rates of the fault zone since mid-late Pleistocene. The Youjiang Fault zone can be divided into 3 segments. They are the west of Bose,Bose-Silin and Silin-Tanluo segments,each of which can be subdivided into several sub-segments. The offset of late Pleistocene terrace deposit and talus can be observed along each segment of the fault zone. The ages of the deposits have been dated to be (3.28±0.25)×10⁴a BP～(10.16±0.79)×10⁴a BP. Geomorphically,the fault zone controlled the development of the Bose-Tiandong late Quaternary basin. A series of fault valleys,troughs,and scarps were developed along the fault strand,while the drainage system crossing the fault zone was left-laterally offset. According to the comparison of the amplitudes of vertical and horizontal displacements on the fault zone,it is inferred that the activity since late Pleistocene of the fault zone has been dominated by left-lateral strike-slipping accompanied by extensional differential motion. The horizontal displacement rate since late Pleistocene on the fault zone has been determined to be 1.47～1.98 mm/a,the vertical displacement rate since middle Pleistocene is 0.74～0.76 mm/a,and the vertical displacement rate since late Pleistoce is 0.1～0.35mm/a. All these values are significantly lower than those on the fault zones surrounding or within the Chuandian fault block. The recent results of GPS observation support also this conclusion.

Key words: Youjiang Fault zone, fault activity, displacement rate

摘要： 右江断裂带地处桂西断块区,有记载以来沿带曾发生4.0～5.0级地震15次,属中强地震带。笔者在室内卫片、航片、大比例尺地形图解译和分析的基础上,经野外实地调查,获得了断裂带晚更新世活动的若干地质地貌证据,实测了断裂的左旋位移数据。文中介绍了有关证据,并根据年龄数据,计算了断裂中、晚更新世以来的水平和垂直位移速率。断裂带在平面上分3大段,即百色以西段、百色—思林段、思林—坛洛段,各大段又可进一步分为若干个小段。断裂断错了距今(3.28±0.25)×10⁴a～(10.16±0.79)×10⁴a的阶地堆积物和残坡积物,控制着百色—田东晚第四纪盆地的发育,地貌上形成断层谷和槽地、断层崖和陡坎,横穿断裂的水系发生同步左旋位移,其活动性质以左旋走滑为主,兼有张性差异运动。晚更新世不同时段以来断裂的水平位移速率为1.47～1.98mm/a,中更新世以来的垂直位移速率为0.74～0.76mm/a,晚更新世以来为0.1～0.35mm/a。该断裂的位移速率明显低于其西的川滇断块内部断裂,更低于川滇断块周边断裂。

关键词: 右江断裂带, 断裂活动, 位移速率