青藏高原东部的Pn波层析成像研究
摘要
利用INDEPTH/ASCENT台阵和其它布设在青藏高原的流动宽频带地震仪数据,反演了青藏高原东部和周边区域的上地幔顶层Pn波速度以及台站延迟.研究区域的平均Pn波速度是8.1km/s,略高于中国大陆的平均Pn波速度.低速区主要分布在羌塘地块的西部和松潘—甘孜地块,高温异常的岩石圈上地幔很可能是导致这一低速区的原因.班公-怒江缝合带东端区域的Pn波速度达到8.35km/s,这一高速区可能与向北俯冲的印度板块(东端)有关.另一Pn波高速区分布在祁连山和昆仑山之间,主要由柴达木盆地和共和盆地及其周边地区,两个并不完全连续的高速异常区组成,它可能对应于特提斯洋闭合时北部增生的克拉通地体;在后来的欧亚板块与印度板块的碰撞中,这一地体有可能阻挡了青藏高原向北的生长.相对密集的台站提供了高分辨率的速度结构横向分布和地壳厚度变化.台站延迟显示青藏高原北部和东部的地壳存在显著的减薄——松潘—甘孜地块东北缘的地壳厚度仅为约50km,而羌塘地块东部唐古拉山地壳最厚,达到75km,这可能是由于印度—欧亚板块碰撞引起的羌塘地块内部变形增厚所致.
Data from recent INDEPTH IV and other broadband deployments in Tibet are used to invert for crustal delays and Pn velocities beneath the eastern Tibetan Plateau and surrounding regions.The average Pn velocity for the region is 8.1 km/s but varies from 7.8 to over 8.3 km/s.Generally low Pn velocities are found in the northeastern Tibet in the Qiangtang and Songpan-Garzê terrains.This includes a zone of very low Pn velocity northwest of the Longmen Shan thrust,along the eastern Kunlun fault,and beneath the eastern Qilian Shan.A region of high Pn velocity underlies the eastern end of the Bangong-Nujiang Suture(bounded by 31—33°N;90—98°E).It could represent part of the underthrusting Indian shield.The region between the Qilian Shan and Kunlun Shan is also characterized by high Pn velocity with several zones of extremely high velocity.This includes two high velocity features beneath Qaidam basin and Gonghe basin.These features may correspond to cratonic fragments that accreted during the closure of the Tethys Ocean and have impeded,but not stopped,the northward growth of the plateau.The station delays show the thickest crust is beneath Tanggula Shan in central Tibet and should be the result of the internal deformation within Qiangtang terrain due to the collision of Indian and Eurasia plates.There is a significant decrease of crustal thickness in the northeast plateau and lateral variation also exists within the region.
Data from recent INDEPTH IV and other broadband deployments in Tibet are used to invert for crustal delays and Pn velocities beneath the eastern Tibetan Plateau and surrounding regions.The average Pn velocity for the region is 8.1 km/s but varies from 7.8 to over 8.3 km/s.Generally low Pn velocities are found in the northeastern Tibet in the Qiangtang and Songpan-Garzê terrains.This includes a zone of very low Pn velocity northwest of the Longmen Shan thrust,along the eastern Kunlun fault,and beneath the eastern Qilian Shan.A region of high Pn velocity underlies the eastern end of the Bangong-Nujiang Suture(bounded by 31—33°N;90—98°E).It could represent part of the underthrusting Indian shield.The region between the Qilian Shan and Kunlun Shan is also characterized by high Pn velocity with several zones of extremely high velocity.This includes two high velocity features beneath Qaidam basin and Gonghe basin.These features may correspond to cratonic fragments that accreted during the closure of the Tethys Ocean and have impeded,but not stopped,the northward growth of the plateau.The station delays show the thickest crust is beneath Tanggula Shan in central Tibet and should be the result of the internal deformation within Qiangtang terrain due to the collision of Indian and Eurasia plates.There is a significant decrease of crustal thickness in the northeast plateau and lateral variation also exists within the region.
引文
[1]Yin A,Harrison T M.Geologic evolution of the Himalayan-Tibetan orogen.Annu.Rev.Earth Planet.Sci.,2000,28(1):211-2800.
[2]Yin A.Cenozoic tectonic evolution of the Himalayan orogenas constrained by along-strike variation of structural geometry,exhumation history,and foreland sedimentation.Earth-Science Reviews,2006,76(1-2):1-131.
[3]Owens T J,Zandt G.Implications of crustal propertyvariations for models of Tibetan plateau evolution.Nature,1997,387(6628):37-43.
[4]Tapponnier P,Xu Z Q,Franooise R,et al.Obliquestepwise rise and growth of the Tibet plateau.Science,2001,294(5547):1671-1677.
[5]Tilmann F,Ni J,INDEPTH III Seismic Team.Seismicimaging of the downwelling Indian Lithosphere beneathcentral Tibet.Science,2003,300(5624):1424-1427.
[6]Huang Z X,Peng Y J,Luo Y,et al.Azimuthal anisotropyof Rayleigh waves in East Asia.Geophys.Res.Lett.,2004,31(15):L15617.
[7]Kosarev G,Kind R,Sobolev S V,et al.Seismic evidence fora detached Indian lithospheric mantle beneath Tibet.Science,1999,283(5406):1306-1309.
[8]Braitenberg C,Zadro M,Fang J,et al.The gravity andisostatic Moho undulations in Qinghai-Tibet plateau.J.Geodyn.,2000,30(5):489-505.
[9]Haines S S,Klemperer S L,Brown L,et al.INDEPTH IIIseismic data:From surface observations to deep crustalprocesses in Tibet.Tectonics,2003,22(1):1001,10.1029/2001TC001305.
[10]汪素云,Hearn T M,许忠淮等.中国大陆上地幔顶部Pn速度结构.中国科学(D辑:地球科学),2001,31(6):449-454.Wang S Y,Hearn T M,Xu Z H,et al.Velocity structure ofuppermost mantle beneath China continent from Pn tomography.Science in China Series D:Earth Sciences,2001,45(2):143-150.
[11]李飞,周仕勇,苏有锦等.川滇及邻区Pn波速度结构和各向异性研究.地球物理学报,2011,54(1):44-54.Li F,Zhou S Y,Su Y J,et al.Study on Pn-wave velocitystructure and anisotropy in the Sichuan-Yunnan and itsadjacent areas.Chinese J.Geophys.(in Chinese),2011,54(1):44-54.
[12]崔仲雄,裴顺平.青藏高原东构造结及周边地区上地幔顶部Pn速度结构和各向异性研究.地球物理学报,2009,52(9):2245-2254.Cui Z X,Pei S P.Study on Pn velocity and anisotropy in theuppermost mantle of the Eastern Himalayan Syntaxis andsurrounding regions.Chinese J.Geophys.(in Chinese),2009,52(9):2245-2254.
[13]Ritzwoller M H,Barmin M P,Villasenor A,et al.Pn andSn tomography across Eurasia to improve regional seismicevent locations.Tectonophysics,2002,358(1-4):39-55.
[14]裴顺平,许忠淮,汪素云等.新疆及邻区Pn速度层析成像.地球物理学报,2002,45(2):218-225.Pei S P,Xu Z H,Wang S Y,et al.Pn velocity tomographyof Xinjiang,China and adjacent region.Chinese J.Geophys.(in Chinese),2002,45(2):218-225.
[15]Sun Y S,Toksz M N.Crustal structure of China andsurrounding regions from P wave traveltime tomography.J.Geophys.Res.,2006,111(B3):B03310.
[16]Hearn T M,Wang S Y,Ni J F,et al.Uppermost mantlevelocities beneath China and surrounding regions.J.Geophys.Res.,2004,109(B11):B11301
[17]Pei S P,Zhao J M,Sun Y S,et al.Upper mantle seismicvelocities and anisotropy in China determined through Pn andSn tomography.J.Geophys.Res.,2007,112(B5):B05312.
[18]Liang C T,Song X D,Huang J L.Tomographic inversion ofPn travel times in China.J.Geophys.Res.,2004,109(B11):B11304.
[19]Hearn T M,Beghoul M N,Barazangi M.Tomography of theWestern United States from regional arrival times.J.Geophys.Res.,1991,96(B10):16369-16381.
[20]Hearn T M.Anisotropic Pn tomography in the westernUnited States.J.Geophys.Res.,1996,101(B4):8403-8414.
[21]Tapponnier P,Meyer B,Avouac J P,et al.Active thrustingand folding in the Qilian Shan,and decoupling between uppercrust and mantle in northeastern Tibet.Earth Planet.Sci.Lett.,1990,97(3-4):382-403.
[22]Meyer B,Tapponnier P,Bourjot L,et al.Crustal thickeningin Gansu-Qinghai,lithospheric mantle subduction,andoblique,strike-slip controlled growth of the Tibet plateau.Geophys.J.Int.,1998,135(1):1-47.
[23]Yin A,Zhang Y Q,Chen M,et al.Cenozoic tectonicevolution of the Qaidam Basin and its surrounding regions(part 3):structural geology,sedimentation,and regionaltectonic reconstruction.Geol.Soc.Am.Bull.,2008,120(7-8):847-876.
[24]Liang C T,Song X D.A low velocity belt beneath northernand eastern Tibetan Plateau from Pn tomography.Geophys.Res.Lett.,2006,33(22):L22306.
[25]Liang X F,Shen Y,Chen Y J,et al.Crustal and mantlevelocity models of southern Tibet from finite frequencytomography.J.Geophys.Res.,2011,116(B2):B02408.
[26]Ingersoll R,Graham S,Dickinson W.Remnant ocean basins.//Busby C,Ingersoll R eds.Tectonics of Sedimentary Basins.Cambridge:Blackwell Sci.,1995:363-392.
[27]Yin A,Harrison T.A Phanerozoic palinspastic reconstruction ofChina and its neighboring region.//Yin A,Harrison T eds.The Tectonic Evolution of Asia.Cambridge:CambridgeUniv.Press,2000:442-485.
[28]Shi D N,Shen Y,Zhao W J,et al.Seismic evidence for aMoho offset and south-directed thrust at the easternmostQaidam-Kunlun boundary in the Northeast Tibetan plateau.Earth.Planet.Sci.Lett.,2009,288(1-2):329-334.
[29]Ni J,Barazangi M.High-frequency seismic wave propagationbeneath the Indian shield,Himalayan arc,Tibetan plateauand surrounding regions:High uppermost mantle velocitiesand efficient Sn propagation beneath Tibet.Geophys.J.R.Astron.Soc.,1983,72(3):665-689.
[30]裴顺平,许忠淮,汪素云.中国及邻区Pn波速度结构成因探讨.地震学报,2004,26(1):1-10.Pei S P,Xu Z H,Wang S Y.Discussion on origin of Pnvelocity variation in China and adjacent region.Acta SeismologicaSinica(in Chinese),2004,26(1):1-10.
[31]傅征祥.中国大陆余震活动深度分布及其流变学机制.中国地震,1996,12(2):147-154.Fu Z X.Distribution of aftershock deepths in China mainlandand its rheological mechanism.Earthquake Research in China(in Chinese),1996,12(2):147-154
[32]汪素云,许忠淮,裴顺平.中国大陆的Pn波速度结构与强震孕育的深度背景.地球物理学报,2003,46(6):779-785.Wang S Y,Xu Z H,Pei S P.Pn velocity variation beneathChina mainland and deep structure background for majorearthquake generation.Chinese J.Geophys.(in Chinese),2003,46(6):779-785.
[33]McNamara D E,Walter W R,Owens T J,et al.Uppermantle velocity structure beneath the Tibetan Plateau fromPn travel time tomography.J.Geophys.Res.,1997,102(B1):493-505.
[34]Xu Q,Zhao J M,Cui Z X,et al.Moho offset beneath thecentral Bangong-Nujiang suture of Tibetan Plateau.Chin.Sci.Bull.,2010,55(7):607-613.
[35]Zhang Z J,Klemperer S L.West-east variation in crustalthickness in northern Lhasa block,central Tibet,from deepseismic sounding data.J.Geophys.Res.,2005,110(B9):B09403.
[36]Zhang P Z,Shen Z,Wang M,et al.Continuous deformationof the Tibetan plateau from global positioning system data.Geology,2004,32(9):809-812.
[37]Huang W C,Ni J F,Tilmann F,et al.Seismic polarizationanisotropy beneath the central Tibetan Plateau.J.Geophys.Res.,2000,105(B12):27979-27989.
[38]Silver P G.Seismic anisotropy beneath the continents:Probing the depths of geology.Annu.Rev.Earth Planet.Sci.,1996,24(1):385-432.
[39]Xu Z J,Song X D.Joint inversion for crustal and Pnvelocities and Moho depth in Eastern Margin of the TibetanPlateau.Tectonophysics,2010,491(1-4):185-193.
[40]Wittlinger G,Masson F,Poupinet G,et al.Seismictomography of northern Tibet and Kunlun:Evidence forcrustal blocks and mantle velocity contrasts.Earth Planet.Sci.Lett.,1996,139(1-2):263-279.
[41]Zhao J M,Yuan X H,Liu H B,et al.The boundarybetween the Indian and Asian tectonic plates below Tibet.Proc.Natl.Acad.Sci.USA,2010,175(25):11229-11233.
[2]Yin A.Cenozoic tectonic evolution of the Himalayan orogenas constrained by along-strike variation of structural geometry,exhumation history,and foreland sedimentation.Earth-Science Reviews,2006,76(1-2):1-131.
[3]Owens T J,Zandt G.Implications of crustal propertyvariations for models of Tibetan plateau evolution.Nature,1997,387(6628):37-43.
[4]Tapponnier P,Xu Z Q,Franooise R,et al.Obliquestepwise rise and growth of the Tibet plateau.Science,2001,294(5547):1671-1677.
[5]Tilmann F,Ni J,INDEPTH III Seismic Team.Seismicimaging of the downwelling Indian Lithosphere beneathcentral Tibet.Science,2003,300(5624):1424-1427.
[6]Huang Z X,Peng Y J,Luo Y,et al.Azimuthal anisotropyof Rayleigh waves in East Asia.Geophys.Res.Lett.,2004,31(15):L15617.
[7]Kosarev G,Kind R,Sobolev S V,et al.Seismic evidence fora detached Indian lithospheric mantle beneath Tibet.Science,1999,283(5406):1306-1309.
[8]Braitenberg C,Zadro M,Fang J,et al.The gravity andisostatic Moho undulations in Qinghai-Tibet plateau.J.Geodyn.,2000,30(5):489-505.
[9]Haines S S,Klemperer S L,Brown L,et al.INDEPTH IIIseismic data:From surface observations to deep crustalprocesses in Tibet.Tectonics,2003,22(1):1001,10.1029/2001TC001305.
[10]汪素云,Hearn T M,许忠淮等.中国大陆上地幔顶部Pn速度结构.中国科学(D辑:地球科学),2001,31(6):449-454.Wang S Y,Hearn T M,Xu Z H,et al.Velocity structure ofuppermost mantle beneath China continent from Pn tomography.Science in China Series D:Earth Sciences,2001,45(2):143-150.
[11]李飞,周仕勇,苏有锦等.川滇及邻区Pn波速度结构和各向异性研究.地球物理学报,2011,54(1):44-54.Li F,Zhou S Y,Su Y J,et al.Study on Pn-wave velocitystructure and anisotropy in the Sichuan-Yunnan and itsadjacent areas.Chinese J.Geophys.(in Chinese),2011,54(1):44-54.
[12]崔仲雄,裴顺平.青藏高原东构造结及周边地区上地幔顶部Pn速度结构和各向异性研究.地球物理学报,2009,52(9):2245-2254.Cui Z X,Pei S P.Study on Pn velocity and anisotropy in theuppermost mantle of the Eastern Himalayan Syntaxis andsurrounding regions.Chinese J.Geophys.(in Chinese),2009,52(9):2245-2254.
[13]Ritzwoller M H,Barmin M P,Villasenor A,et al.Pn andSn tomography across Eurasia to improve regional seismicevent locations.Tectonophysics,2002,358(1-4):39-55.
[14]裴顺平,许忠淮,汪素云等.新疆及邻区Pn速度层析成像.地球物理学报,2002,45(2):218-225.Pei S P,Xu Z H,Wang S Y,et al.Pn velocity tomographyof Xinjiang,China and adjacent region.Chinese J.Geophys.(in Chinese),2002,45(2):218-225.
[15]Sun Y S,Toksz M N.Crustal structure of China andsurrounding regions from P wave traveltime tomography.J.Geophys.Res.,2006,111(B3):B03310.
[16]Hearn T M,Wang S Y,Ni J F,et al.Uppermost mantlevelocities beneath China and surrounding regions.J.Geophys.Res.,2004,109(B11):B11301
[17]Pei S P,Zhao J M,Sun Y S,et al.Upper mantle seismicvelocities and anisotropy in China determined through Pn andSn tomography.J.Geophys.Res.,2007,112(B5):B05312.
[18]Liang C T,Song X D,Huang J L.Tomographic inversion ofPn travel times in China.J.Geophys.Res.,2004,109(B11):B11304.
[19]Hearn T M,Beghoul M N,Barazangi M.Tomography of theWestern United States from regional arrival times.J.Geophys.Res.,1991,96(B10):16369-16381.
[20]Hearn T M.Anisotropic Pn tomography in the westernUnited States.J.Geophys.Res.,1996,101(B4):8403-8414.
[21]Tapponnier P,Meyer B,Avouac J P,et al.Active thrustingand folding in the Qilian Shan,and decoupling between uppercrust and mantle in northeastern Tibet.Earth Planet.Sci.Lett.,1990,97(3-4):382-403.
[22]Meyer B,Tapponnier P,Bourjot L,et al.Crustal thickeningin Gansu-Qinghai,lithospheric mantle subduction,andoblique,strike-slip controlled growth of the Tibet plateau.Geophys.J.Int.,1998,135(1):1-47.
[23]Yin A,Zhang Y Q,Chen M,et al.Cenozoic tectonicevolution of the Qaidam Basin and its surrounding regions(part 3):structural geology,sedimentation,and regionaltectonic reconstruction.Geol.Soc.Am.Bull.,2008,120(7-8):847-876.
[24]Liang C T,Song X D.A low velocity belt beneath northernand eastern Tibetan Plateau from Pn tomography.Geophys.Res.Lett.,2006,33(22):L22306.
[25]Liang X F,Shen Y,Chen Y J,et al.Crustal and mantlevelocity models of southern Tibet from finite frequencytomography.J.Geophys.Res.,2011,116(B2):B02408.
[26]Ingersoll R,Graham S,Dickinson W.Remnant ocean basins.//Busby C,Ingersoll R eds.Tectonics of Sedimentary Basins.Cambridge:Blackwell Sci.,1995:363-392.
[27]Yin A,Harrison T.A Phanerozoic palinspastic reconstruction ofChina and its neighboring region.//Yin A,Harrison T eds.The Tectonic Evolution of Asia.Cambridge:CambridgeUniv.Press,2000:442-485.
[28]Shi D N,Shen Y,Zhao W J,et al.Seismic evidence for aMoho offset and south-directed thrust at the easternmostQaidam-Kunlun boundary in the Northeast Tibetan plateau.Earth.Planet.Sci.Lett.,2009,288(1-2):329-334.
[29]Ni J,Barazangi M.High-frequency seismic wave propagationbeneath the Indian shield,Himalayan arc,Tibetan plateauand surrounding regions:High uppermost mantle velocitiesand efficient Sn propagation beneath Tibet.Geophys.J.R.Astron.Soc.,1983,72(3):665-689.
[30]裴顺平,许忠淮,汪素云.中国及邻区Pn波速度结构成因探讨.地震学报,2004,26(1):1-10.Pei S P,Xu Z H,Wang S Y.Discussion on origin of Pnvelocity variation in China and adjacent region.Acta SeismologicaSinica(in Chinese),2004,26(1):1-10.
[31]傅征祥.中国大陆余震活动深度分布及其流变学机制.中国地震,1996,12(2):147-154.Fu Z X.Distribution of aftershock deepths in China mainlandand its rheological mechanism.Earthquake Research in China(in Chinese),1996,12(2):147-154
[32]汪素云,许忠淮,裴顺平.中国大陆的Pn波速度结构与强震孕育的深度背景.地球物理学报,2003,46(6):779-785.Wang S Y,Xu Z H,Pei S P.Pn velocity variation beneathChina mainland and deep structure background for majorearthquake generation.Chinese J.Geophys.(in Chinese),2003,46(6):779-785.
[33]McNamara D E,Walter W R,Owens T J,et al.Uppermantle velocity structure beneath the Tibetan Plateau fromPn travel time tomography.J.Geophys.Res.,1997,102(B1):493-505.
[34]Xu Q,Zhao J M,Cui Z X,et al.Moho offset beneath thecentral Bangong-Nujiang suture of Tibetan Plateau.Chin.Sci.Bull.,2010,55(7):607-613.
[35]Zhang Z J,Klemperer S L.West-east variation in crustalthickness in northern Lhasa block,central Tibet,from deepseismic sounding data.J.Geophys.Res.,2005,110(B9):B09403.
[36]Zhang P Z,Shen Z,Wang M,et al.Continuous deformationof the Tibetan plateau from global positioning system data.Geology,2004,32(9):809-812.
[37]Huang W C,Ni J F,Tilmann F,et al.Seismic polarizationanisotropy beneath the central Tibetan Plateau.J.Geophys.Res.,2000,105(B12):27979-27989.
[38]Silver P G.Seismic anisotropy beneath the continents:Probing the depths of geology.Annu.Rev.Earth Planet.Sci.,1996,24(1):385-432.
[39]Xu Z J,Song X D.Joint inversion for crustal and Pnvelocities and Moho depth in Eastern Margin of the TibetanPlateau.Tectonophysics,2010,491(1-4):185-193.
[40]Wittlinger G,Masson F,Poupinet G,et al.Seismictomography of northern Tibet and Kunlun:Evidence forcrustal blocks and mantle velocity contrasts.Earth Planet.Sci.Lett.,1996,139(1-2):263-279.
[41]Zhao J M,Yuan X H,Liu H B,et al.The boundarybetween the Indian and Asian tectonic plates below Tibet.Proc.Natl.Acad.Sci.USA,2010,175(25):11229-11233.
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