【摘要】 青藏高原内部的近东西向伸展变形在藏南地区形成了一系列近南北向裂谷带,对其中最东端的错那-沃卡裂谷带南段错那-拿日雍错地堑调查结果表明,它是第四纪活动明显的半地堑式断陷盆地。控制该盆地发育的主边界断裂带是近南北走向、倾向东侧和长80~110km左右的盆地西缘正断层,是区域内N90°~92°E方向伸展变形的产物。断裂活动速率的多种估算结果表明,该断裂带百万年时间尺度的平均垂直活动速率介于0.2~0.9mm/a之间;MIS 8和MIS 6以来的断裂平均活动速率的合理估计值是0.6±0.2mm/a;但末次冰期盛冰阶以来的断裂平均活动速率明显增加,介于1.0~2.1mm/a之间,合理的估计值为1.3±0.3mm/a,末次冰盛期以来断裂垂直活动速率的增加极可能是该断裂带进入地震丛集期的反映。该地堑近于严格的南北走向及其与当雄-羊八井地堑相似的活动强度,指示了下地壳物质侧向流动所产生的纵向拉张作用最有利于该区近南北向裂谷带的形成。更多还原

【Abstract】 The Cona-Nariyong Co graben,forming the southern section of the NNE trending Cona-Woka rift zone which is the only rift located at the east of the Yadong-Gulu rift zone in Southern Tibet,is a north-trending extensional basin in eastern Himalaya Mountains.The basin is about 80km long and 1～10km wide and contains three small paleo-dammed lakes called Nariyong Co,Yang Co,and Yaba Co from north to south respectively.There are 4 sets of tills and fluvioglacial deposits,corresponding to the Nyanyxungla Ice Age,Jilongshi Ice Age,Ronbushi Ice Age,and Holocene glaciations respectively from oldest to youngest.Eight samples of calcareous sands and coats for U-series and coarse-fine grained sands for TL datings respectively were collected from natural and digging exposures either within or associated with the moraines and fluvioglacial deposits.The dating results show that the first(oldest)set of tills was formed before marine isotope stage MIS6;the second set of tills was formed during MIS6,and the third and fourth moraines were formed during the last glacial maximum and the Holocene glaciations respectively.Based on remote images and field survey,the main boundary fault of the graben is found to be located at the western margin of the Cona-Nariyong Co Basin.It is a N-trending and east dipping normal fault and about 110km long extending from 15km south of Bangla pass to 15km north of Wushankou pass of Cona County,and is the seismogenic fault of 1806 M 7.5 Cona earthquake.It extends in the Tethyan Himalya Block to the north and cuts the south Tibetan Detachment into the High Himalaya Block to the south.In that region,the fault prominent offset Tethyan fold-thrust belt containing Triassic-Cretaceous sedimentary rocks,and the 80km long central segment of normal fault offset Quaternary deposits showing obvious activity during Quaternary or Late Quaternary.Based on restoration of geological structure and paleo-topography and measurement of fault scarp offset moraine platform and fluviaoglacial terraces,the total vertical throws is about 1.9～4.2km since about 12～5MaB.P.and the displacements are 140±20m,42～80m,27±1m,and 15.5±0.5m since MIS8 or earlier,MIS6,24～18kaB.P.,and 15～11kaB.P.respectively.Given such displacements of fault and ages of rifting onset and tills filling,the most probable values of throw rates on differential time scales are deduced.The vertical throw rate on the Cona-Nariyong Co west margin fault must be more than 0.2mm/a and less than 0.9mm/a since rifting onset,the Late Quaternary throw rates are between 0.4～0.8mm/a,the most probable value is 0.6±0.2mm/a,and the Holocene throw rate is about 1.3±0.3mm/a(lower than 2.1mm/a).The Late Quaternary and since rifting onset throw rates are mutually consistent and constrain the uniform long-term slip rate along the west margin fault of the Cona-Nariyong Co graben.The obvious increase of throw rate during Holocene is most probably resulted from earthquake cluster in Holocene.This new observation on the Cona-Nariyong Co graben shows that the development of trends of grabens or rifts is strictly limited in Southern Tibet.The newly obtained long-term and short-term slip rates from the Cona-Nariyong Co graben are also distinctly similar to the inferred throw rate of normal faults bounding the west margin fault of the Damxung-Yangbajain in northern section of the Yadong-Gulu graben system in Southern Tibet.The pronounced similar and consistency on magnitude and geometry of faults and grabens in Southern Tibet suggest that normal faulting in Southern Tibet isn’t resulted from the mechanisms largely controlled by local processes,including oroclinal bending of Himalaya orogen,right-laterally oblique convergence of India continental lithosphere,right-lateral motion along the Karakorum-Jiali Fault Zone,a change in boundary conditions along the eastern margin of Asia,topographic collapse of an overthickened crust or advective removal of thickened Tibetan mantle lithosphere,and radial spreading of the Himalayan arc on India plate.The most reasonable interpretation might be that the uniform extension controlled by regional processes be resulted from lower crust lateral flow in South Tibet or longitudinal extension be resulted from lower crustal shorten vertical orogen.更多还原