西秦岭临潭-宕昌断裂第四纪最新活动特征

STUDIES ON NEW ACTIVITY OF LINTAN-DANGCHANG FAULT,WEST QINLING

临潭-宕昌断裂是西秦岭造山带内一条重要的分支断裂,其最新活动特征是分析西秦岭构造变形的重要依据。临潭-宕昌断裂的新构造活动强烈,中强地震频繁,但目前对于断裂的新活动特征研究程度较低,未见有其全新世活动地质地貌证据的报道。文中基于遥感解译、宏观地貌分析研究断裂的长期活动表现和分段性;同时通过地质地貌考察、无人机摄影测量、差分GPS和放射性碳测年等方法定量研究断裂的新活动特征;最后基于研究结果探讨了断裂及附近区域的地震危险性和区域构造变形。结果表明:根据断层迹线收敛程度和宏观地貌差异,可将临潭-宕昌断裂分为西、中、东3段;断裂的运动性质以左旋走滑为主,兼具逆冲分量,左旋走滑使洮河及其支流、冲沟和山脊等发生同步左旋拐弯,最大左旋位移可达3km,逆冲分量使新近纪盆地边缘和内部形成300~500m的垂向位移;断裂的最新活动时代为全新世,限定了1次2 090~7 745a BP(置信度为2σ)的全新世古地震事件;全新世早期以来,临潭-宕昌断裂东段主干断裂的左旋走滑速率为0.86~1.65mm/a,垂直滑动速率为0.05~0.10mm/a。临潭-宕昌断裂分配了约2mm/a的左旋走滑分量,是东昆仑-西秦岭阶区变形分配的关键断裂之一。

Located in the intervening zone between Tibetan plateau and surrounding blocks,the LintanDangchang Fault(LDF)is characterized by north-protruding arc-shape,complex structures and intense fault activity.Quantitative studies on its new activity play a key role in searching the seismogenic mechanism,building regional tectonic model and understanding the tectonic interaction between Tibetan plateau and surrounding blocks.The LDF has strong neotectonic activities,and moderate-strong earthquakes occur frequently(three M6~7 earthquakes occurred in the past 500 years,including the July 22 nd,2013,Minxian-Zhangxian MS6.6 earthquake),but the new activity of the fault is poorly known,the geological and geomorphological evidence of the Holocene activity has not been reported yet.Based on remote sensing interpretation and macro-landform analysis,this paper studies the long-term performance of LDF.Based on the study of fault activity,unmanned aircraft vehicle photogrammetry and differential GPS,radiocarbon dating,etc.,the latest activity of LDF is quantitatively studied.Then the research results,historical strong earthquakes and small earthquake distribution are comprehensively analyzed for studying the seismogenic mechanism and constructing regional tectonic models.The results are as follows:Firstly,the fault geometry is complex and there are many branch faults.According to the convergence degree of the fault trace and the fault-controlled macroscopic topography,the LDF is divided into three segments:the west,the middle and the east.The west segment contains two fault branches(the south and the north)and the south Hezuo Fault.The south branch of the west segment mainly dominates the Jicang Neogene Basin,and the south Hezuo Fault controls the south boundary of Hezuo Basin.The middle segment has more convergent and stable trace,consisting of the main fault and south Hezuo Fault,and these faults separate the main planation surface of the Tibetan plateau and Lintan Basin surface geologically and geomorphologically.The fault traces in the east segment are sparsely distributed,and the terrain is characterized by hundreds of meters of uplifts.The branch faults include the main fault,Hetuo Fault,Muzhailing Fault and Bolinkou Fault,each controlling differential topography.Secondly,the motion property of the LDF is mainly left-lateral strike-slip,with a relative smaller portion of vertical slip.The left-lateral strike-slip offset the Taohe River and its tributaries,gullies and ridges synchronously,and the maximum left-lateral displacement of the tributary of Taohe River can reach 3 km.Meanwhile,the pull-apart basins and push-up ridges associated with the left-lateral fault slip are also developed in the fault zone.The performance of vertical slip includes tilting of the main planation surface,vertical offsets of the boundary and interior of Neogene basin and hundred meter-scale differential topography.The vertical offset of the Neogene is 300~500 m.Thirdly,one fault profile was newly discovered in Gongqia Village,revealing a complete sequence of pre-earthquake-coseismic-postseismic deposition,and this event was constrained by the radiocarbon ages of pre-earthquake and post-earthquake deposition.The event was constrained to be 2090~7745 a BP(confidence 2σ),which for the first time confirmed the Holocene activity of the fault.Fourthly,a gully with two terraces at least on the west side of Zhuangzi Village in the east segment of the main fault retains a typical faulted landform.The T2/T1 terrace riser of the gully has a left-handed dislocation of 6.3~11.8 m,and the scarp height on terrace T2 is 0.4~0.7 m,the radiocarbon age of the terrace T2 is 7170~7310 a BP,so the derived left-lateral strike-slip rate since the early Holocene in the east segment of the main fault is0.86~1.65 mm/a,and the vertical slip rate is 0.05~0.10 mm/a.The derived slip rates are in line with the regional tectonic model proposed by the predecessors,so the LDF plays an important role in the internal deformation of the West Qinling.The clockwise rotation of the middle to east segments of the LDF acts as an obstacle to the left-lateral strike-slip motion,which inevitably leads to the redistribution and rapid release of stress,so earthquakes in the middle-east segment of the LDF are unusually frequent.