STUDY ON LATE-QUATERNARY ACTIVITY AND DISPLACEMENT OF DRAINAGE SYSTEMS ALONG THE WANDING FAULT

Wanding Fault lies along the border of west Yunnan,China and Myanmar,striking near EW on the west and NE on the east of Bengdong,dipping to N and NW,with a length of 170km. Recent studies show that there are neotectonic landforms along the fault,represented as clear fault facets,straight fault troughs,fault scarps,linear ridges,multi-level waterfalls,and etc. The fault has played an obvious control role in the development of Quaternary basins,such as Wanding,Manhai,Menglianba,Jiucheng and Wandian Basins etc.,which show a beaded distribution along the fault. The Quaternary faults near the Longzhen bridge and the displacement of the alluvial fans at Gaijiazhai reveal that the Wanding Fault has offset the late Pleistocene deposits with TL age of(17.60±1.49)kaBP to(38.24±3.25)kaBP,indicating that the fault was obviously active in late Quaternary. The Nujiang River extends along the fault in the Bengdong area,the main river course was offset left-laterally about 9.5km,forming a hairpin-like bend. The Gongyanghe River,Zhongshanhe River,Wanmahe River,Manxinhe River,Mengguhe River,Wandinghe River and their tributaries all run along the Wanding Fault,and there are clear evidences that these river segments are partially or entirely dominated by the fault,suggesting that these segments were formed by tracking erosion along the fault fracture zone after the fault's latest activity. Synchronous left-lateral displacement of drainage systems was observed at many parts along the Wanding Fault,such as between Pingziting to Gongyanghe,where 11 streams were synchronously displaced left-laterally,and more than 10 streams between Zhongshan to Wanmahe displaced. The displacements can be divided into 5 levels,i.e.40～50m,90～100m,200～250m,300～400m and 600～1100m. Comb-form rivers are developed along the fault. The synchronous left-lateral displacement of drainage systems and the displacement of terraces and alluvial fans suggest that the Wanding Fault was dominated by horizontal left-lateral slip in late Quaternary,with an average rate of 1.7～2.2mm/yr. Several moderate earthquakes occurred along the fault,which are assumed to be the aftershocks of the May 29,1976 Longling M_S7.3 and M_S7.4 earthquakes,e.g.the January 26,1946 Myanmar M_S6.0 earthquake,and the June 1,1976 Anding M_S6.0 earthquake,and the July 4,1976 M_S6.0 earthquake at east of Mengbang,Yunnan,etc. These aftershocks are not distributed in a band-like pattern along the Longling-Ruiling Fault-the seismogenic fault,but concentrated in an area of 55km in length and 32km in width between the Longling-Ruili Fault and the Wanding Fault. So,it is inferred that the extensive distribution of the aftershocks is the result of the successive activities of the Longling-Ruili Fault and Wanding Fault.