拉萨那林拉卡断裂带晚第四纪古地震研究
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摘要
拉萨地区最主要的一条晚更新世活断层为那林拉卡断裂带,是走向NWW、倾向SSW、高陡倾角的左旋逆走滑断层,全长33km。通过对该断裂带的研究,发现其从晚更新世以来有明显的活动,并在地表保留了断错地貌现象。其中,在次角林西沟及谢村最为明显,断层错断河流、山脊、阶地等地质体,形成断头沟、断层陡坎等断错地貌,并且两处晚更新世以来的水平位错分别为54～87m和20～67m。通过对断裂带上4个探槽的研究,发现距今约7万年以来,沿那林拉卡断裂带可能发生了5次古地震事件,各次事件的大致发生年龄为距今68.53,54.40,<41.23,21.96,9.86ka,事件平均复发间隔为14.67ka。由于探槽有限,并且各探槽揭露出的地震事件有限,没有一个探槽能完全揭露5次事件,因此文中对一些事件上下限年龄的确定存在一定的不确定性。
As the most important fault of Late Pleistocene in the Lhasa area,the Nalinlaka Fault is a left-lateral thrust fault,striking NWW,dipping SSW with a high dip angle,and extending over 33km.According to the studies on the latest strata on the Nalinlaka Fault zone,this fault zone has been obviously active since Late Pleistocene and the movement left behind some geomorphologic phenomena on the earth's surface,especially at the sites of the gully west of Cijiaolin and around Xiecun village.For example,some rivers,ridges and terraces are dislocated,forming beheaded gullies,fault escarps and so on.The horizontal displacements since Late Pleistocene at the above two places are 54～87m and 20～67m,respectively.Based on the studies on the 4 trenches along the fault using progressive constraining method,we conclude that there might have occurred 5 paleoearthquake events along the Nalinlaka Fault since 70ka BP,the ages of each paleoearthquake are 8.53,54.40,<41.23,21.96,and 9.86 ka BP,and the average recurrence interval is 14.67ka.Because of the limits of trenches and earthquake events exposed by each trench,no single trench revealed completely all the 5 events.So,there may be some errors in determining the upper and lower limits of some events in this article.
As the most important fault of Late Pleistocene in the Lhasa area,the Nalinlaka Fault is a left-lateral thrust fault,striking NWW,dipping SSW with a high dip angle,and extending over 33km.According to the studies on the latest strata on the Nalinlaka Fault zone,this fault zone has been obviously active since Late Pleistocene and the movement left behind some geomorphologic phenomena on the earth's surface,especially at the sites of the gully west of Cijiaolin and around Xiecun village.For example,some rivers,ridges and terraces are dislocated,forming beheaded gullies,fault escarps and so on.The horizontal displacements since Late Pleistocene at the above two places are 54～87m and 20～67m,respectively.Based on the studies on the 4 trenches along the fault using progressive constraining method,we conclude that there might have occurred 5 paleoearthquake events along the Nalinlaka Fault since 70ka BP,the ages of each paleoearthquake are 8.53,54.40,<41.23,21.96,and 9.86 ka BP,and the average recurrence interval is 14.67ka.Because of the limits of trenches and earthquake events exposed by each trench,no single trench revealed completely all the 5 events.So,there may be some errors in determining the upper and lower limits of some events in this article.
引文
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纪占胜,杨欣德,臧文栓,等.2006.拉萨地区林周盆地设兴组上部(古近系)年代研究的新进展及意义[J].古生物学报,45(2):277—282.JI Zhan-sheng,YANG Xin-de,ZANG Wen-shuan,et al.2006.Advance of geochronological dating of Shexing formationin the Lhunzhub Basin,Lhasa area and its significance[J].Acta Palaeontologica Sinica,45(2):277—282(inChinese).
毛凤英,张培震.1995.古地震研究中的逐次限定法与新疆北部主要断裂带的古地震研究[A].国家地震局地质研究所编.活动断裂研究(4).北京:地震出版社.153—164.MIAO Feng-ying,ZHANG Pei-zhen.1995.Progressive constraining method in paleoseismic study and paleoearthquakesalong the major active faults in northern Xinjiang[A].In:Institute of Geology,SSB(ed).Research on ActiveFault(4).Seismological Press,Beijing.153—164(in Chinese).
吴珍汉,叶培盛,胡道功,等.2003.拉萨地块北逆冲推覆构造系统[J].地质评论,49(1):74—80.WU Zhen-han,YE Pei-sheng,HU Dao-gong,et al.2003.Thrust system of the north Lhasa block[J].Geological Re-view,49(1):74—80(in Chinese).
张培震,闵伟,邓起东,等.2003.海原活动断裂带的古地震与强震复发规律[J].中国科学(D),33(8):705—713.ZHANG Pei-zhen,MIN Wei,DENG Qi-dong,et al.2003.Paleoearthquake and strong earthquake recurrent rules alongthe Haiyuan active fault,north-central China[J].Science in China(Ser D),33(8):705—713(in Chinese).
Armijo R,Tapponnier P,Han T.1989.Late Cenozoic right-lateral strike-slip faulting in southern Tibet[J].J GeophysRes,94:2787—2838.
Armijo R,Tapponnier P,Mercier J L,et al.1986.Quaternary extension in southern Tibet:Field observations and tec-tonic implications[J].J Geophys Res,91(13):13,803—13,872.
Ren J,Shen J,Cao Z,et al.2000.Quaternary faulting of Jiali Fault,Southeast Tibet[J].Eos Trans AGU,81(22):West Pacific Geophysics Meet(Suppl),G52A202.
Yeats R S,Sieh K,Allen C R.1997.The Geology of Earthquakes[M].Oxford University Press,Oxford.
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