【摘要】 遥感解译和地表调查结果发现,西藏桑日县的沃卡盆地构成了藏南近南北向裂谷带中最东端的错那-沃卡裂谷的北段,它是在该区近东西向的逆冲构造带停止活动之后,上地壳沿N108±1°E方向发生区域性的伸展变形所形成的第四纪活动明显的地堑式断陷盆地。晚第四纪期间,控制该盆地发育的主边界断裂带为整体呈北北东走向、倾向西侧、长50～60km的盆地东缘正断层。该断裂带也是1915年桑日M7.0级地震的控震断裂。断裂活动速率的估算结果表明,该断裂带MIS6以来的垂直活动速率介于0.4～0.9mm/a之间,末次冰期盛冰期以来断裂平均活动速率最合理的估计值为1.2±0.3mm/a。Q3晚期以来活动速率的明显增加可能标志着该断裂带全新世进入了地震丛集期。更多还原

【Abstract】 The Oiga graben,belonging to the northern section of the NNE trending Cona-Oiga(or Cona-Sangri)rift zone which is the only rift located at the east of the Yadong-Gulu rift zone in southern Tibet,is a N18°E trending basin located around 92°15’ E and between 29°10’N and 29°40’N,and cutting across the eastern Gangdese batholith zone at the north of the Yarlung Tsangpo fault(or Great counter thrust fault).It is about 50 km long and widens from 3～5 km in the south to 15～18 km in the north.Four sets of tills and fluvioglacial deposits are found in the basin,which are corresponding to the so-called Nyanyxungla Ice Age,Jilongshi Ice Age,Ronbushi Ice Age and Holocene glaciations respectively from oldest to youngest.The Thermal Luminescence(TL)age results associated with the deposits show that the first(or the oldest)tills formed before marine isotope stage(MIS)6(about 200～140 ka BP),the second set of tills formed during MIS6,the third and fourth moraines formed during the last Glacial maximum(about 28～15ka BP)and Holocene glaciations respectively.The graben is limited to the east and west by two N18±1°E-trending boundary faults of opposite dips about 50～60 km long and indicates that the extension direction is N108±1°E in the region.The field survey shows that the eastern margin fault of the Oiga basin,a west dipping normal fault,is the master boundary fault of the graben.It had been active from Quaternary or before that to Holocene,and is the seismogenic fault of 1915 M7.0 Sangri earthquake.In this region,it offset the Gangdese batholith zone,containing Jurassic-Cretaceous sedimentary rocks,early Tertiary volcanic sediment and late Cretaceous-early Tertiary granites,and is limited between the Great Counter fault in the south and the Modro Gongkar-Kongpo Gyamda fault in the north.The master boundary normal fault offset late Quaternary deposits showing obvious activity during the late Pleistocene and Holocene.Based on measure of fault scarp offset moraine platform and fluviaoglacial terraces,the displacements are 50～90 m,24.0±1.5 m,16.0±1.0 m or 13.7±0.5 m and 3.7±0.4 m since MIS6,24～18 ka BP,15～11 ka BP and middle-late Holocene respectively.Given such displacement of fault and ages of tills filling,the most probably values of throw rates on differential time scale are deduced.The Late Quaternary throw rates are limited between 0.4～0.9 mm/a,with the most probably value being 0.7±0.2 mm/a,and the throw rate is about 1.2±0.3 mm/a since MIS2 and during the Holocene.The Late Quaternary throw rates maybe constrain the uniform long-term slip rate along the eastern margin fault of the Oiga graben.The obviously increasing of the throw rate during the Holocene is most probably resulted from earthquake cluster in the Holocene.The new observation on the Oiga graben shows that the development of trends of the grabens or rifts is strictly limited in southern Tibet.The new long-term and short-term slip rates from the Oiga graben are also distinctly similar to the throw rate of inferred for normal faults bounding the west margin fault of Damxung-Yangbajain in the northern section of the Yadong-Gulu graben system.The strict geometry,pronounced similar and consistent on the active magnitude and trend of faults and grabens in southern Tibet suggests that the normal faulting in southern Tibet did not result from the mechanisms controlled largely by local processes,including oroclinal bending of the Himalaya orogen,right-laterally oblique convergence of the 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 the India plate.The most probably interpretation is that the uniform extension controlled by regional processes resulted from lower crust lateral flow in south Tibet or longitudinal extension resulted from lower crustal extension normal orogen.更多还原