青藏高原北部异常SKS分裂成因的初步探讨——被熔体强化的岩石圈各向异性
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摘要
当人们试图解释青藏高原异常的剪切波分裂成因时 ,以下的问题让人们感到困惑 :( 1 )为什么异常大的SKS分裂延时 ( 1 .91— 2 .4s)出现在青藏高原北部Sn波缺失区 ;( 2 )为什么分裂延时突变 ( 1 .47s和 1 .0 9s)出现在Sn波缺失区的边缘 ;( 3)为什么快波极化方向(FPD)与地表大规模的构造走向之间存在约 2 0°— 30°的偏差 .本文在综合分析流变学实验和岩石物理学实验研究成果、青藏高原地质和地球物理资料的基础上 ,提出青藏高原北部地震波各向异性受岩石圈地幔主要矿物的晶格优选方位 (LPO)和熔体的定向分布 (MPO)的双重控制 ,并模拟计算了MPO对青藏高原北部岩石圈地幔各向异性强度的贡献 .研究结果表明 ,由MPO强化的青藏高原北部岩石圈地幔各向异性强度可达 1 0 % ,相应的各向异性层厚度平均为 94km .该结果为研究区SKS分裂的成因解释以及造山带深部地质过程的研究提供了新的约束条件 .
While we were trying to understand the origin of subcontinental mantle anisotropy by SKS splitting measurements in Qinghai-Xizang plateau, we were perplexed for answers to the following questions: (1) why the inefficient Sn propagation region correlates with the large delay time (1.91-2.40s) of SKS splitting, (2) why very strong δt variations occur at the edges of this region, and (3) what results in the abnormal delay time. We propose that the abnormal SKS splitting is attributed to the combination of lattice preferred orientation (LPO) of major minerals in upper mantle with melt pocket preferred orientation (MPO). Estimation shows that the total anisotropy magnitude of the lithosphere beneath north-central Qinghai-Xizang plateau is about 10%, which results in the abnormal large delay time of SKS splitting observed in geophysical experiments. The thickness of the anisotropic layer beneath the north-central Qinghai-Xizang plateau is estimated to range from 70km to 113km, which is in comparable well with those in its adjacent terrenes (approximately 100km thick). We therefore propose to attribute the abnormally large delay time to MPO enhanced anisotropy other than to a very thick anisotropic layer.
While we were trying to understand the origin of subcontinental mantle anisotropy by SKS splitting measurements in Qinghai-Xizang plateau, we were perplexed for answers to the following questions: (1) why the inefficient Sn propagation region correlates with the large delay time (1.91-2.40s) of SKS splitting, (2) why very strong δt variations occur at the edges of this region, and (3) what results in the abnormal delay time. We propose that the abnormal SKS splitting is attributed to the combination of lattice preferred orientation (LPO) of major minerals in upper mantle with melt pocket preferred orientation (MPO). Estimation shows that the total anisotropy magnitude of the lithosphere beneath north-central Qinghai-Xizang plateau is about 10%, which results in the abnormal large delay time of SKS splitting observed in geophysical experiments. The thickness of the anisotropic layer beneath the north-central Qinghai-Xizang plateau is estimated to range from 70km to 113km, which is in comparable well with those in its adjacent terrenes (approximately 100km thick). We therefore propose to attribute the abnormally large delay time to MPO enhanced anisotropy other than to a very thick anisotropic layer.
引文
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[26] Mo
[2] NicolasA ,ChristensenNI.Formationofanisotropyinuppermantleperidotites:Areview,incomposition,structureanddynamicsofthelithosphere asthenospheresystem.In:FuchsKed.Geodyn.Ser.,16.WashingtonDC :AGU ,1987.111—123.
[3] SilverPG ,ChanWW .Shearwavesplittingandsubcontinentalmantledeformation.J .Geophys.Lett.,1991,96:16429—16454.
[4] SilverPG .Seismicanisotropybeneaththecontinents:probingthedepthsofgeology.AnnualRev.EarthPlanetarySci.,1996,24:358—432.
[5] 金振民,JiS C ,金淑燕.橄榄石晶格优选方位和上地幔地震波速各向异性.地球物理学报,1994,37(4):469—477.JINZhenMin,JiS C ,JINShuYan.Latticepreferredorientationofolivineandseismicanisotropyofuppermantle.ChineseJ.Geophys.(ActaGeophysicaSinica),1994,37(4):469—477.
[6] 金淑燕.上地幔的岩石组构和各向异性.地质科技情报,1993,12:32—38.JINShuYan.Rockfabricandanisotropyoftheuppermantle.Geol.Sci.Tech.Info.,1993,12:32—38.
[7] 金淑燕.大陆岩石圈各向异性和动力学意义.见:张炳熹等主编.岩石圈研究的现代方法.北京:原子能出版社,1997.79—88.JINShuYan.Continentallithosphericanisotropyanditsdynamicimplication.In:ZhangBXetal.,eds.Modernmethodsintheinvestigationsoflithosphere,Beijing:AtomicEnergyPublicationHouse.1997,79—88.
[8] McNamara.DE ,OwensTJ,SilverPG ,etal.ShearwaveanisotropybeneaththeTibetanPlateau.J.Geophys.Res.,1994,99:13655—13665.
[9] HirnA ,JiangM ,SapinM ,etal.SeismicanisotropyasanindicatorofmantleflowbeneaththeHimalayasandTibet.Nature,1995,375:571—574.
[10] GuilbertJ,PoupinetG ,MeiJ.AstudyofazimuthalPresidualsandshearwavesplittingacrosstheKunlunrange(NorthernTibetanPlateau).Phys.EarthPlanet.Inter.,1996,95:167—174.
[11] 吕庆田,马开义,姜 枚等.青藏高原南部下的横波各向异性.地震学报,1996,18:215—223.L QingTian,MAKaiYi,JIANGMei,etal.ShearwaveanisotropyoftransversewavebeneathsouthernTibet.ActaSeismologicaSinica,1996,18:215—223.
[12] 史大年,董英君,姜 枚等.西藏定日—青海格尔木上地幔各向异性研究.地质学报,1996,70:291—297.SHIDaNian,DONGYingJun,JIANGMei,etal.SherawaveanisotropyoftheuppermantlebeneaththeDingriofTibettoolmudofQinghai.ActaGeologicaSinica,1996,70:291—297.
[13] LaveJ,AvouacJP ,LacassinR ,etal.SeismicanisotropybeneathTibet:evidenceforeastwardextrusionoftheTibetanlithosphere.EarthPlanet.Sci.Lett.,1996,140:83—96.
[14] McNamaraDE ,OwensTJ,WalterWR .ObservationsofregionalphasepropagationacrosstheTibetanPlateau.J .Geophys.Res.,1995,100:22215—22229.
[15] ArnaudNO ,VidalPh,TapponnierP ,etal.ThehighK2OvolcanismofnorthwesternTibet:Geochemistryandtectonicimplications.EarthPlanet.Sci.Lett.,1992,111:351—367.
[16] TurnerS ,HawkesworthC ,LiuJ,etal.TimingofTibetanupliftconstrainedbyanalysisofvolcanicrocks.Nature,1993,364:50—54.
[17] SandvolE ,NiJ,KindR ,etal.SeismicanisotropybeneaththesouthernHimalayanTibetcollisionzone.J.Geophys.Res.,1997,102:17813—17823.
[18] GaoS,DavisPM ,LiuH ,etal.SKSsplittingbeneathcontinentalriftzones.J.Geophys.Res.,1997,102:22781—22797.
[19] LehmannI.VelocitiesoflongitudinalwavesintheupperpartoftheEarth’smantleAnn.Geophys.,1959,15:93—118.
[20] DrummondB .J.,MuirheadK .J.,HalesA .L .Evidenceforaseismicdiscontinuitynear200kmdepthunderthecontinentalmargin.Geophys.J.R .astr.Sco.,1982,70:67—77.
[21] KaratoS.OntheLehmanndiscontinuity.Geophys.Res.Lett.,1992,19:2255—2258.
[22] MontagnerJP ,TanimotoT .Globaluppermantletomographyofseismicvelocitiesandanisotropies.J .Geophys.Res.,1991,96:20337—20351.
[23] MolnarP ,EnglangP ,MartinodJ.Mantledynamic,upliftoftheTibetanPlateauandtheIndianmonsoon.Rev.Geophys.,1993,31:357—396.
[24] PeltzerG ,TapponnierP .Formationandevolutionofstrike slipfault,riftsandbasinsduringtheIndiaAsiacollision:anexperimentalapproach.J.Geophys.Res.,1988,93:15095—15117.
[25] 刘经南,许才军,宋成骅等.青藏高原东部地壳运动的GPS测量分析.地球物理学报,1998,41(4):518—524.LIUJingNan,XUCaiJun,SONGChengHua,etal.StudyofthecrustalmovementinthemiddleeastregionofQinghaiXizangPlateauwithGPSmeasurements.ChineseJ .Geophys.(ActaGeophysicaSinica),1998,41(4):518—524.
[26] Mo
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