关键词: 西藏, 错那-沃卡裂谷带, 第四纪, 正断层作用, 近SW向伸展

Abstract: The Cona-Oiga rift zone is N10～12°E trending,about 220km long,and is the only rift located at the east of Yadong-Gulu rift in southern Tibet. It is located around 92°E and between 27°40'N and 29°40'N and cuts the south Tibet Detachment into High Himalaya block to the south and strides across Yarlung Tsangpo Fault(or Great Counter Thrust fault)into eastern Gangdese batholith zone to the north. There are 3 independent grabens contained in the rift. They are Oiga graben,Qungdo'gyang graben and Cona graben from north to south. The earthquake activity is very prominent along the rift. There are two large earthquakes of M7.5 and M7.0 occurring at northern Cona in 1806 and southern Oiga in 1915, respectively. In the rift,late Quaternary tills and fluvioglacial deposits may be divided into 4 sets corresponding to the so-called Nyanyxungla Ice Age,Jilongshi Ice Age,Ronbushi Ice Age and Holocene glaciations respectively from oldest to youngest. Their TL and U-series ages show that the first(or the oldest)tills formed before marine isotope stage(MIS)6,the second tills formed during about 200～140ka BP,the third and fourth tills formed during the last Glacial maximum(about 28～15ka BP)and Holocene glaciations respectively.The Oiga graben is located at the north of Yarlung Tsangpo Fault. It is about 50km long and widens from 3～5km in the south to 15～18km in the north. The graben is limited to the east and west by two N 18±1°E-trending boundary normal faults of opposite dips, indicating that the extension direction is 108±1° in the region. Field survey shows that the master boundary fault is the eastern margin fault of Oiga Basin which has been active from Quaternary or Pliocene to Holocene. The boundary fault shows obvious activity during late Pleistocene. Based on measurement of fault scarp,the vertical displacements are 50～90m, (24.0±1.5)m, (16.0±1.0)m or (13.7±0.5)m and (3.7±0.4)m since MIS6,24～18ka BP and 15～11ka BP since middle-late Holocene, respectively. Given such displacements and ages of fault scarps,the average throw rates are limited between 0.4～0.9mm/a since MIS6,and about (1.2±0.3)mm/a since MIS2. The Qungdo'gyang graben is a N18°E-trending and 11～20km-wide basin. It cuts across the eastern segment of Yala Xiangbo gneiss dome and is a half-graben limited by the west boundary fault which is a N 18±1°E-trending,east dipping and about 40km long normal fault. It has been active since Pliocene. Based on measurement of late Quaternary fault scarp,the vertical displacements are (29.8±1.0)m and (12.0±0.5)m since about 28～15ka BP, (4.6±0.4)m and (7.0±0.7)m since middle-late Holocene, respectively,and the most probably values of throw rates of main boundary fault aren't less than about 0.5mm/a,and the average throw rate is about 1.0～1.5mm/a since MIS2. The Cona graben is a north-trending basin,about 80km long and 1～10km wide. Its main boundary fault is located on the western margin of basin. It is a N-trending and east dipping normal fault, about 110km long. Based on measurement of fault scarp offset tills and fluviaoglacial terraces,the displacements are 44～80m, (27±1)m and (15.5±0.5)m since MIS6,24～18kaBP and 15～11kaBP, respectively. The average vertical throw rates are between 0.3～0.8mm/a and about (1.3±0.3)mm/a since MIS6 and MIS2 respectively. The late Quaternary throw rates show consistency and constrain the uniform long-term slip rate along Cona-Oiga rift. The obvious increase of throw rate during Holocene most probably results from earthquake cluster in Holocene along the rift.This new observation on Cona-Oiga rift shows that the extension direction of rifts is strictly limited to 100° in southern Tibet,and is parallel with Yadong-Gulu rift. The long-term and short-term slip rates of main bounding normal faults of Cona-Oiga rift are also distinctly similar to the throw rates of Yadong-Gulu graben system. The strict geometry pattern,pronounced similar and consistent to the active magnitude and trend of bounding normal faults in southern Tibet suggests that the N-trending rifts most probably result from the uniform extension deformation controlled by middle-lower crust lateral flow or extension parallel to orogen caused by India lithosphere insert under south Tibet.

Key words: Tibet, Cona-Qiga rift, Quaternary, normal faulting, east-west extension