Abstract: The Longmenshan Fault zone is an important thrust fault on the eastern margin of the Qinghai-Tibet Plateau.The Cenozoic activity of the fault zone has attracted great attention of many scientists at home and abroad.The Longmenshan Fault zone consists of the Back-range,the Central and the Front-range Faults,which differ from each other in size and activity.Meanwhile,the activity of the whole fault zone is characterized by segmentation.Previous studies on the middle and southern segments of the fault zone showed that these two segments have been active since late Pleistocene.However,little work had been done on the activity of the northern segment of the fault zone so far.In this study,we made a detailed field investigation on the northern segment of the fault Fault and collected many samples from the strata covering the fault or from fault zone materials for TL and ESR dating.According to the observation of the northern segment of the Longmenshan Fault zone and dating results,it can be concluded that the Back-range Fault had once been active in early-mid Quaternary,but has been inactive since late Pleistocene,the Central Fault was active during early Quaternary or pre-Quaternary,and the Front-range Fault was active during pre-Quaternary.They all have been inactive since late Pleistocene.However,the middle and southern segments of the Longmenshan Fault zone have been active since late Quaternary,controlling the development of late-Quaternary basin,and many strong earthquakes occurred in history along these two segments.What has caused the different activities of the different segments of the Longmenshan fault? The main reason might be the change of the construction of the block boundaries due to the variation of the regional stress field in this region.Since mid-Pleistocene,affected by the southeastward moving of the Tibet Plateau,the stress field of the region where the Longmenshan fault zone is located was changed,causing the change of the boundary faults that bound the active blocks.At present,the northern segment of the Longmenshan Fault zone is no longer the boundary fault of the active block,while the uplifting of the Minshan Mountains acts as a protective wall hampering the movement of the Longmenshan fault zone.All these factors have caused the weakening of the activity of the northern segment of the Longmenshan Fault zone.However,the southern and middle segments of the Longmenshan Fault zone,together with the Minshan uplift tectonic zone constitute the eastern border of the compressional system,controlling the development of present topography and strong earthquakes in this region.This recognition may provide useful information for the study of the geodynamics of the Qinghai-Tibet Plateau.

Key words: Longmenshan Fault zone, seismic activity, late Pleistocene

摘要： 在野外考察的基础上,结合所采集的各条断裂之上的覆盖物或断层带物质的热释光(TL)或电子自旋共振(ESR)样品年龄,对龙门山断裂带北段的晚第四纪活动性进行了分析,认为 :后山断裂在第四纪早 -中期曾有过活动,晚更新世以来已不再活动;中央断裂早更新世或前第四纪是活动的;前山断裂在白龙江以北变成一些小的、零星分布的断裂,它们在第四纪早期以前有过活动。而已有研究表明龙门山断裂带中段和西南段晚第四纪以来仍在活动。造成龙门山断裂带不同段落新活动时代不同的主要原因,可能是区域应力场的变化所导致的活动地块边界的变化。龙门山断裂带的北段现在已不构成活动块体的边界,加之岷山隆起对龙门山断裂带东北段的屏障作用,使得龙门山断裂带北段活动减弱。而龙门山推覆构造带中南段和岷山隆起构造带共同成为块体持续挤压作用的东界。这为研究青藏高原的运动学及动力学等问题提供了重要信息。

关键词: 龙门山断裂带, 活动性, 晚第四纪