The Activity Features During Late Quaternary Of Yema River-Danghe Nan Shan Faults In Western Qilian Shan

The western Qilian Shan is the key area of tectonic transformation between Altyn Tagh fault trending NE and Northern Qilian Shan fault trending NW. The four faults studied in this thesis which locate in the western of Qilian Shan are:North and South margin faults of the Yema River, North and South margin faults of the Danghe Nan Shan respectively. Through interpreting the Satellite and aerial photos, carrying out field investigation, measuring and dating the samples, we obtained these faults’ quantitative parameters on the latest tectonic activity in late Quaternary.(1). Northern margin fault of Yema River contains both east and west segment, and its trending is SW-NE. The length of the western segment is～27km, and its movement is mainly left-lateral strike slip and strike-slip rate is～2.2mm/a. The eastern segment is reverse fault, and its total length is-30km. Two trenches which across the fault imply at least two paleo-earthquake events, which occurred in7.7±0.7aB.P. and4.5±0.3a B.P. respectively. The recurrence interval age is-3ka.(2). The Southern margin fault of Yema River locates in the north piedmont of Danghe Nan Shan and Yema Nan Shan. The trending of the fault is sub-EW and length is～60km. The movement of the fault is mainly strike-slip with a little thrust component in west, and its sinistral strike slip rate is～1.8mm/a in the Late Quaternary. The trenches show several paleo-earthquake events. The last one occurred7879±113a B.P. ago, hence its elapsed time is much long.(3). The Northern margin fault of Danghe Nan Shan locates in the north piedmont of Dandhe Nan Shan and contains three segments. Its total length is～110km and trending is sub-EW, According to the differential GPS measurement and the sample dating on different terraces, we obtained the slip rates as follows:the vertical rates are0.4±0.1mm/a,0.8±0.1mm/a and1.0±0.1mm/a, and the shortening rates are0.5±0.1mm/a,1.2±0.1mm/a, and±0.1mm/a in west, center and east segments respectively. We also found earthquake scarps, which were formed between1000-2000aB.P, near the Dahong Valley and the Maya Valley. The total length of these scarps is nearly～40km.(4). The Houtang segment of the Southern Danghe Nan Shan fault, which is60km in length, is the eastern extension of the Southern Altyn Tagh fault, and locates in the south piedmont of Danghe Nan Shan. The fault mainly performs left lateral. There are a series of gullies, valleys and terraces, which are dislocated by the sinistral strike slip fault. The sinistral strike slip rate is～4.55±0.6mm/a in Holocene. The rupture zone containing earthquake grooves and bump et al. was also found along the fault. Combining the historical earthquakes data with the field investigation, we inferred that the earthquake scarps and surface rupture zone maybe caused by one or two of four historical earthquakes occurred in384a A.D.,419a A.D.,1000a A.D. and1297-1303a A.D respectively. The intensities of the above earthquakes are VI near the Dunhuang.(5). The sinistral strike slip rate is～10±2mm/a in the central segment of Altyn Tagh fault, but gradually reduce to1-2mm/a in the east segment (Zhang et al.,2007). The slip rate reduced in the east segment is transferred to the Southern and Northern Danghe Nan Shan faults and the Southern and the Northern Yema River faults. The difference of the slip rates between west and east of Subei triple junction agrees with the amount of shortening rate in Northern Danghe Nan Shan fault and sinistral strike slip rate in Northern and Southern Yema River faults. Our results support the continuous deformation model of Qinghai-Tibet block.(6). We analysis the active characteristics and historical, paleo-earthquakes of faults in the western Qilian Shan, which includes the Altyn Tagh fault, the Southern and Northern Danghe Nan Shan faults, the Southern and Northern Yema River faults, Changma fault, Daxue Shan fault. We suppose that the east segment of Altyn Tagh fault will occur strong earthquake in the future, because the elapsed time of the last earthquakes in east segment of Altyn Tagh fault and the Yema River faults is too long.