青藏高原中南部岩石圈扩张应力场与羊八井地热异常形成机制
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摘要
青藏高原中南部地区有着丰富的地热资源,其中羊八井地区存在着温度高达93℃以上,热流值高达364mWm2的地热奇观.本文利用国际以及中国地震台网资料,详细分析了青藏高原中南部地震活动性和岩石圈脆性破裂深度范围,进而讨论了高地热异常区的岩石圈应力场背景及其动力学机制.研究结果表明,青藏高原地热异常区有高于高原内部平均活动水平的震级为6级左右的中强地震活动,其震中分布呈现出与地热活动相似的、沿近南北向断陷带的带状特征;并且在高热流区域内有高原内部罕见的中深地震活动,地震震源从100km左右深处到地表呈现柱状地震群活动的空间分布等特征.震源机制与应力场研究结果表明,虽然高原中南部应力场主压、主张应力方向与青藏高原的整体特征相符,但是地震发生类型与青藏高原周缘的挤压逆断层型地震完全不同,均属于东西向扩张力作用下的正断层型地震活动,特别是在羊八井高热流区域附近,东西向扩张应力场在岩石圈应力场中起到主导性作用,推测其控制深度可达岩石圈底部100多公里处.青藏高原地热异常区在强烈的近东西向扩张应力场作用下,岩石圈东西向扩张并发生一系列的大规模正断层活动,致使深部软流圈高温热流可以沿着活动正断层及其形成的深裂隙上涌,穿过岩石圈到达地表面,形成了高地热异常区.
The central and southern regions of the Qinghai_Tibetan plateau are famous for their geothermal resource in the world. The temperature of boiling springs is high to 93℃, and the heat flow value is greater than 364 (mW/m~2) in some areas in the plateau. In the present analysis the relationships among the depth of lithospheric brittle fracture, stress field and mechanism of geothermal anomaly are analyzed using the seismic dada from ISC and Chinese seismic network and other geophysical data. The results suggest that the middle sized events about M6 frequently occur in the geothermal active zones, where the seismicity is more active than the average one inside the plateau. The events are distributed along the belts nearly N_S direction coinciding to that of fault_depression zones with intensely geothermal activity. Events of middle and deep hypocenters also appear in the high heat flow zone, where the hypocenter depth stretches to about 100 km. Normal fault type earthquakes due to the extensional stress field dominate the seismicity in the southern and central plateau, differing from the reverse or strike_slip faulting events dominating the seismicity in the low edges around the plateau, although the directions of the main compressive and extensional stress field coincide with those in the low edges. Extensional action in E_W direction dominates the lithospheric stress field in and around the Yangbajain area.
The central and southern regions of the Qinghai_Tibetan plateau are famous for their geothermal resource in the world. The temperature of boiling springs is high to 93℃, and the heat flow value is greater than 364 (mW/m~2) in some areas in the plateau. In the present analysis the relationships among the depth of lithospheric brittle fracture, stress field and mechanism of geothermal anomaly are analyzed using the seismic dada from ISC and Chinese seismic network and other geophysical data. The results suggest that the middle sized events about M6 frequently occur in the geothermal active zones, where the seismicity is more active than the average one inside the plateau. The events are distributed along the belts nearly N_S direction coinciding to that of fault_depression zones with intensely geothermal activity. Events of middle and deep hypocenters also appear in the high heat flow zone, where the hypocenter depth stretches to about 100 km. Normal fault type earthquakes due to the extensional stress field dominate the seismicity in the southern and central plateau, differing from the reverse or strike_slip faulting events dominating the seismicity in the low edges around the plateau, although the directions of the main compressive and extensional stress field coincide with those in the low edges. Extensional action in E_W direction dominates the lithospheric stress field in and around the Yangbajain area.
引文
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[2]沈显杰,张文仁,管 烨等.纵贯青藏高原的亚东-柴达木热流大断面.科学通报,1989,34(17):1329～1330ShenXJ,ZhangWR,GuanY,etal.Heatflowtransectalong Yadong_QaidamthroughTibetanPlateau.ChineseScienceBulletin(inChinese).1989,34(17):1329～1330
[3]佟 伟,张铭陶,张之非.西藏地热.北京:科学出版社,1981DongW,ZhangMT,ZhangZF.GeothermalinTibetan(in Chinese).Beijing:SciencePress,1981
[4]韩同林.试论藏南活动构造与地热的关系.喜马拉雅地质Ⅱ.北京:地质出版社,1990.45～58HanTL.RelationshipoftheActiveStructuralSystemtoGeothermal ActivityinSouthernXizang(Tibet),inHimalayanGeology(in Chinese).Beijing:GeologicalPublishingHouse,1990.45～58
[5]HousemenGA,MckenzieDP.Convectiveinstabilityofathickened boundarylayeranditsrelevanceforthethermalevolutionof continentalconvergencebelt.J.Geophys.Res.,1981,86:6115～6132
[6]滕吉文.固体地球物理学概论.北京:地震出版社,2003TengJW.OutlineofSolidGeophysics.Beijing:Seismological Press,2003
[7]XuJiren.SeismogenicstressfieldandtectonicsinEastAsia.MemoirsoftheFacultyofKyotoUniversity,SeriesofPhysics,Astrophysics,GeophysicsandChemistry.1994,XXXIX(1):49～69
[8]徐纪人,赵志新,河野芳辉等.日本南海海槽区域应力场及其板块构造动力学特征.地球物理学报,2003,46(4):488～494XuJR,ZhaoZX,KonoY,etal.Regionalcharacteristicsofstress fieldanditsdynamicsinandaroundtheNankaitrough,Japan.ChineseJ.Geophys.(inChinese)2003,46(4):488～494
[9]XuJiren,KonoY.Geometryofslab,intraslabstressfieldandits tectonicimplicationintheNankaiTrough,Japan.Earth,Planes andSpace,2002,54:733～742
[10]赵志新,松村一男,尾池和夫等.东アジアにおける地震活动变化の地域特徵と连性について、(3)中国西部と接地域.地震(第2辑),1988.41:389～400ZhaoZX,MatsumuraK,OikeK&IshikawaY.Regional characteristicsoftemporalvariationsofseismicactivityineastAsia andtheirmutualrelations(3)WestChinaanditsNeigboring regions.Zisin(2)(inJapanese),1988,41:389～400
[11]ZhaoZX,OikeK,MatsumuraK,IshikawaY.Stressfieldinthe continentalpartofChinaderivedfromtemporalvariationsofSeismic activity.Tectonophysics,1990,178:357～372
[12]XuJR,ZhaoZX,IshikawaY,OikeK.Propertiesofthestress fieldinandaroundWestChinaderivedfromearthquakemechanism solutions.Bull.Disas.Prev.Res.Inst.,KyotoUniv.,Part2,1988,38(333):49～78
[13]许忠淮.东亚地区现今构造应力图的编制.地震学报,2001,23:492～501XuZH.ModernTectonicStressinEastAsia.ActaSeismologica Sinica(inChinese),2001,23:492～501
[14]石耀霖,朱守彪.中国大陆震源机制深度变化反映的地壳-地面流变特征.地球物理学报,2003,46(3):359～365ShiYL,ZuSB.Contrastofrheologyinthecrustandmantlenear Mohorevealedbydepthvariationofearthquakemechanismin continentalChina.ChineseJ.Geophys.(inChinese),2003,46(3):359～365
[15]曾融生,丁志峰,吴庆举等.喜马拉雅及南藏的地壳俯冲带-地震学证据.地球物理学报,2000,43(6):780～797ZengRS,DingZF,WuQJ.Seismologicalevidenceforthe mulitipleincompletecrustalsubductionsinHimalayaandsouth Tibet.ChineseJ.Geophys.(inChinese),2000,43(6):780～797
[16]杨国华,李延兴,韩月萍等.由GPS观测结果推导中国大陆现今水平应变场.地震学报,2002,24(3):337～347YangGH,LiYX,HanYP,etal.Currenthorizontalstrainfield inChinesemainlandderivedfromGPSdata.ActaSeimologicaSinica(inChinese),2002,24(3):338～347
[17]BourjotL,RomanowiczB.Crustanduppermantletomographyin Tibetusingsurfacewaves.Geophys,Res.Lett.,1992,19:881～884
[18]ZhaoWJ,NelsonKD,ProjectINDEPTHTeam.Deepseismic reflectionevidenceforcontinentalunderthrustingbeneathsouthern Tibet.Nature,1993,366:557～559
[19]SteinSeth,SteinCarolA.Thermo_mechanicalevolutionofoceanic lithosphere:Implicationforthesubductionprocessanddeep earthquakes.In:GrayEBeboutetal.eds.SubductionTopto Bottom.Washigtonian:AGUPress,1996.1～17
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