Late Quaternary Activity Of The Bangda Segment Along The Nujiang Fault Zone

Controlled by the collision of the Indian-Eurasian plates since the Cenozoic and the continuous northward pushing and wedging after the collision,as well as the blocking effect of the surrounding rigid blocks,extensive and strong deformation has occurred in the interior of the Tibetan Plateau.The Three-Parallel-River Region of the Hengduan Mountains are located on the eastern side of the top of the Eastern Himalayan syntaxis.This area is at the heart of the northward push of the Indian plate,the eastward extrusion of Plateau material,and the clockwise rotation.This region is also one of the most tectonically and seismically active regions in the plateau,and active tectonic of the region have attracted widespread attention.However,due to the strong erosion,harsh natural and traffic conditions,and extremely short written historical records,the activity data of the faults in the Nujiang-Lantsang-Jinsha Rivers Region and its surrounding blocks are extremely scarce.In particular,the Nujiang fault zone,as a plate suture zone that has developed and evolved for a long time in the geological history,the existing geological evidence is more inclined to believe that this fault zone was inactive or weak activty in the late Quaternary.Historical records show that two strong earthquakes occurred along the Nujiang fault zone,which should be related to the activity of the fault.However,there has been a lack of evidence from the late Quaternary stratigraphic profile of the fault.Based on the above,this paper mainly focuses on two scientific issues: the late Quaternary activity of the Bangda segment of the Nujiang fault zone and the current tectonic deformation mode of the Three-Parallel-River Region in the Hengduan Mountains Region.Based on traditional geological methods,combined with the results of GNSS observation and seismic activity analysis of the current crustal movement characteristics,the activity of the eastern and western boundary faults of the Nujiang suture zone and the Yangda-Yaxu fault that obliquely cuts the eastern boundary fault were carried out.The investigation and research have determined the late Quaternary activity of these faults,constructed and improved the active fault image of the Three-Parallel-Rivers Region in the Hengduan Mountains Region,and on this basis,analyzed and discussed the tectonic deformation mode of this area.Main conclusions are drawn as follows:(1)The Bianba-Luolong fault,the western boundary of the Nujiang suture zone,is a Holocene active fault.Its movement is dominated by left-lateral strike-slip with a certain dip-slip component.The latest event occurred after(3310±30)yr BP,possibly corresponding to the 1642-1654 Luolong M ≥ 7 earthquake.The horizontal slip rate of the fault since the late Quaternary is about 1-1.7 mm/yr.(2)The Bangda segment of the Nujiang fault,the eastern boundary of the Nujiang suture zone,is a Holocene active fault,and its movement is dominated by left-lateral strike-slip with obvious thrust property.The latest strong earthquake event faulted the(1720-2000)yr BP black soil layer,with a coseismic horizontal displacement of about 2 m and a vertical offset of about 0.3 m.It is estimated that the horizontal slip rate and vertical slip rate of the fault since the Holocene are > 1 mm/yr and > 0.15 mm/yr,respectively.(3)The Yangda-Yaxu fault is a Holocene active fault with a total length of about120 km.The movement is dominated by left-lateral strike-slip,and the horizontal slip rate is about(0.6-0.8)mm/yr.Through trench excavation and section cleaning,five paleoearthquake events have been revealed,named from youngest to oldest as E1 to E5 with contraints of corresponding occurrence ages as 1025±118,4690±802,11507±705,13698±558,and before 14946±195 yr BP.There are two groups of major earthquake recurrence intervals of about 2000-3000 years and about 6000 years,which do not follow the quasi-periodic recurrence pattern.(4)The Yangda-Yaxu fault cuts through the Nujiang fault and the Lancangjiang fault,while the surface trace of the late Quaternary activity of the Bianba-Luolong fault is generally discontinuous,and the obviously active segment is the NWW trend and its geometric distribution is not completely consistent with the early western boundary fault of the Nujiang suture zone,revealing that the current active deformation of the Nujiang fault zone has obvious newness in inheriting the original structural trace.(5)In the Three-Parallel-River Region of the Hengduan Mountains Region,the Jiali fault zone and the Xianshuihe fault zone have been active since the Holocene,and the tectonic deformations on both sides are quite different,showing the activity characteristics of rigid block boundary zones.The Holocene active faults between the two fault zones are mainly divided into two groups: the NEE-trending and the NW-trending.These faults are relatively small in scale,low in slip rate,and discontinuous,showing the tectonic deformation characteristics of faults caused by the change of local stress direction within the Qiangtang block.This deformation style supports the "combination of block rotation,fault dislocation,and internal strain" deformation mode.Main research progresses are drawn as follows:(1)Some new evidences and parameters of the Holocene activities of the Bianba-Luolong fault,the Bangda segment of the Nujiang fault and the Yangda-Yaxu fault were obtained.(2)The active tectonic image of the Three-Parallel-River Region in the Hengduan Mountains supports the "combination of block rotation,fault dislocation and internal strain" deformation mode.The Jiali fault and the Xianshuihe fault have the deformation characteristics of active faults at the block boundary,and the two are dominated by continuous strain within the block,and also have local fault dislocation deformation.The results of this study can accumulate basic data and cognition for further understanding of the tectonic deformation model of the Tibetan Plateau,and provide scientific basis for earthquake risk assessment in this region and the planning and construction of major engineering projects such as the Sichuan-Tibet Railway.