摘要
地表调查发现,位于西藏南部的错那-沃卡裂谷带包含了3个相对独立的地堑-半地堑——沃卡、邛多江和错那-拿日雍错地堑（从北到南）,并构成了该区重要的近SN向控震构造带。该裂谷带整体的展布方式及其中各地堑主边界断裂带的正断层活动指示了100°±2°的区域伸展方向。各边界断裂带的活动强度分析表明,断裂的平均垂直活动速率介于0.3-1.9mm/a。其中,末次盛冰以来合理的活动速率估算值为1.2-1.5mm/a,而末次间冰期以来的活动速率只有（0.6±0.3）mm/a,暗示该裂谷带的断裂活动行为可能类似于地震的丛集活动,存在间歇期与活跃期交替出现的特点。综合分析认为,中-下地壳物质的近EW向伸展或流动所导致的上地壳均匀拉张模式可能是该裂谷带的主要成因。
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 loeated around 92°E and between 27°40＇N and 29°40＇N and euts the south Tibet Detaehment into High Himalaya bloek to the south and strides aeross Yarlung Tsangpo Fault（or Great Counter Thrust fault）into eastern Gangdese batholith zone to the north. There are 3 independent grabens eontained in the rift. They are Oiga graben, Qungdo＇gyang graben and Cona graben from north to south. The earthquake aetivity is very prominent along the rift. There are two large earthquakes of M7.5 and M7.0 oeeurring at northern Cona in 1806 and southern Oiga in 1915, respeetively. In the rift,late Quaternary tills and fluvioglaeial deposits may be divided into 4 sets eorresponding to the so-ealled Nyanyxungla Iee Age,Jilongshi Iee Age, Ronbushi Iee Age and Holoeene glaeiations respeetively 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 seeond tills formed during about 200- 140ka BP,the third and fourth tills formed during the last Glaeial maximum（ about 28 - 15ka BP）and Holoeene glaeiations respeetively. The Oiga graben is loeated 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, indieating that the extension direetion is 108 ± 1° in the region. Field survey shows that the master boundary fault is the eastern margin fault of Oiga Basin whieh has been aetive from Quaternary or Plioeene to Holoeene. The boundary fault shows obvious aetivity during late Pleistoeene. Based on measurement of fault searp,the vertieal displaeements 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 sinee MIS6,24 -18ka BP and 15 -llka BP sinee middle-late Holoeene, respeetively. Given sueh displaeements and ages of fault searps, the average throw rates are limited between 0. 4 -0.9mm/a sinee MIS6 ,and about （1.2 ± 0. 3）mm/a sinee MIS2. The Qungdo＇gyang graben is a N18°E-trending and 11 -20km-wide basin. It euts aeross the eastern segment of Yala Xiangbo gneiss dome and is a half-graben limited by the west boundary fault whieh is a N 18 ± 1°E-trending,east dipping and about 40km long normal fault. It has been aetive sinee Plioeene. Based on measurement of late Quaternary fault searp,the vertieal displaeements are （29.8 ± 1.0）m and （ 12.0 ±0. 5）m sinee about 28 - 15ka BP, （4.6 ± 0.4） m and （7.0 ± 0. 7 ） m sinee middle-late Holoeene, respeetively, and the most probably values of throw rates of main boundary fauh 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 - 18ka BP and 15 - 1 l ka BP, 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.
出处
《地震地质》
EI
CSCD
北大核心
2008年第1期144-160,共17页
Seismology and Geology
基金
国家自然科学基金(40501006)
中国地质调查局项目(1212010541404)
科技部国际合作项目(2006DFA21320)资助
作者简介
吴中海，1974年出生，1997年毕业于长春地质学院，2004在中国地质科学院获构造地质专业博士学位，副研究员，电话：010—68486987，E—mail：wzhly@sina．com。
