贝加尔裂谷区地壳上地幔复杂的各向异性及其动力学意义
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摘要
位于西伯利亚板块东南缘的贝加尔裂谷是最典型的大陆裂谷之一,其形成的动力机制与演化过程一直是地学界争论的焦点.本研究使用一种改进的横波分裂测量方法———全局最小切向能量法,对研究区宽频带固定台站ULN和TLY记录的SKS震相和接收函数PmS震相进行分裂测量,得到了裂谷地区地壳和上地幔的各向异性属性.ULN台的SKS分裂测量结果表明,台站下方存在双层各向异性结构,其中,上层的快波偏振方向为N74°E,快、慢波分裂时差为0.80s,下层的快波偏振方向为N128°E,快、慢波分裂时差为0.80s;PmS震相分裂测量结果表明,台站下方地壳内存在单层各向异性结构,其快波偏振方向为N77°E,与SKS分裂测量的上层各向异性的快波偏振方向相近,快、慢波分裂时差为0.26s,这说明SKS分裂测量的上层各向异性同时包含了地壳和地幔岩石圈.对TLY台进行SKS分裂测量时发现,台站下方上地幔结构表现出横向非均匀性:当反方位角<90°时,快波偏振方向在N60°E左右,快、慢波分裂时差为1.27s;当反方位角>90°时,快波偏振方向约为N120°E,快、慢波分裂时差为1.40s;PmS震相分裂测量没有获得有效的结果,并且不同方位的PmS震相到时基本一致,说明TLY台下方地壳结构接近各向同性.根据分裂测量结果,结合贝加尔裂谷区的构造演化过程,得到以下结论:(1)ULN台双层各向异性的上层主要是岩石圈原始结构的反映,并且存在地壳与地幔岩石圈的一致性形变,而下层指示着现今软流圈地幔的流动;(2)由于刚性的西伯利亚克拉通的阻挡,地幔流动方向在克拉通南缘发生了偏转,在深部绕克拉通边缘流动,因此形成了TLY台下方上地幔结构的横向变化.
The Baikal Rift lies at the southeast edge of Siberian Craton,and is one of the most representative continental rifts.The dynamic mechanism of its formation and evolution has always been the focus of argument.In this study,one improved analysis method of shear-wave splitting is used,which is called 'global minimum transverse energy'(GM),to process the seismic data from long-term-station ULN and TLY,and finally obtain the anisotropy attribute of crust and upper mantle.The SKS measurement at station ULN shows that,there are two anisotropic layers below station ULN: upper( means fast direction) =N74°E,δtupper(δt means delay time)=0.80 s,lower=N128°E,δtlower=0.80 s;while the measurement of phase PmS splitting reveals one anisotropic layer situated in crust: aver=N77°E,δtaver=0.26 s,with the value near upper of the two-layer model,perhaps indicates that the upper layer includes both the crust and mantle lithosphere.On the other hand,the SKS measurement at station TLY shows that,the upper mantle structure displays strong lateral nonuniformity: when BAZ(back azimuth) <90°, is close to N60°E,δtaver=1.27 s;when BAZ>90°, is close to N120°E,δtaver=1.40 s.Besides,no effective PmS splitting measurement is obtained,and the arrival times of phase PmS from different azimuths are consistent with each other,both sustain an isotropic crust.After synthetic analysis we arrive at the following conclusions: 1) Below station ULN,the upper layer may be the frozen structure in lithosphere,and crust and mantle lithosphere have undergone conformable deformation,while the lower layer was formed due to the present flowing of asthenosphere mantle;2) Due to the block of the rigid Siberian Craton,the flow direction of mantle is deflected by the south edge of craton,that is,the deep mantle flows around the edge of craton,which further formed the lateral variation of upper mantle structure below station TLY.
The Baikal Rift lies at the southeast edge of Siberian Craton,and is one of the most representative continental rifts.The dynamic mechanism of its formation and evolution has always been the focus of argument.In this study,one improved analysis method of shear-wave splitting is used,which is called 'global minimum transverse energy'(GM),to process the seismic data from long-term-station ULN and TLY,and finally obtain the anisotropy attribute of crust and upper mantle.The SKS measurement at station ULN shows that,there are two anisotropic layers below station ULN: upper( means fast direction) =N74°E,δtupper(δt means delay time)=0.80 s,lower=N128°E,δtlower=0.80 s;while the measurement of phase PmS splitting reveals one anisotropic layer situated in crust: aver=N77°E,δtaver=0.26 s,with the value near upper of the two-layer model,perhaps indicates that the upper layer includes both the crust and mantle lithosphere.On the other hand,the SKS measurement at station TLY shows that,the upper mantle structure displays strong lateral nonuniformity: when BAZ(back azimuth) <90°, is close to N60°E,δtaver=1.27 s;when BAZ>90°, is close to N120°E,δtaver=1.40 s.Besides,no effective PmS splitting measurement is obtained,and the arrival times of phase PmS from different azimuths are consistent with each other,both sustain an isotropic crust.After synthetic analysis we arrive at the following conclusions: 1) Below station ULN,the upper layer may be the frozen structure in lithosphere,and crust and mantle lithosphere have undergone conformable deformation,while the lower layer was formed due to the present flowing of asthenosphere mantle;2) Due to the block of the rigid Siberian Craton,the flow direction of mantle is deflected by the south edge of craton,that is,the deep mantle flows around the edge of craton,which further formed the lateral variation of upper mantle structure below station TLY.
引文
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[2]Delvaux D,Moeys R,Stapel G,et al.Paleostressreconstructions and geodynamics of the Baikal region,CentralAsia:Part I.Palaeozoic and Mesozoic prerift evolution.Tectonophysics,1995,252:61-101.
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[4]Zorin Y A,Rogozhina V A.Mechanism of rifting and somefeatures of the deep-seated structure of the Baikal rift zone.Tectonophysics,1978,45:23-30.
[5]Zorin Y A.The Baikal rift:an example of the intrusion ofasthenospheric material into the lithosphere as the cause ofdisruption of lithospheric plates.Tectonophysics,1981,73:91-104.
[6]Logatchev N A,Zorin Y A,Rogozhina V A.Baikal rift:active or passive?Comparison of the Baikal and Kenya riftzones.Tectonophysics,1983,94:223-240.
[7]Gao S S,Davis P M,Liu H,et al.Asymmetric upwarp ofthe asthenosphere beneath the Baikal rift zone,Siberia.J.Geophys.Res.,1994,99:15319-15330.
[8]Gao S S,Davis P M,Liu H,et al.Seismic anisotropy andmantle flow beneath the Baikal rift zone.Nature,1994,371:149-151.
[9]Gao S S,Davis P M,Liu H,et al.SKS splitting beneathcontinental rift zone.J.Geophys.Res.,1997,102:22781-22797.
[10]Zorin Y A,Turutanov E K,Mordvinova V V,et al.TheBaikal rift zone:the effect of mantle plumes on olderstructure.Tectonophysics,2003,371:153-173.
[11]Zhao D P,Lei J S,Inoue T,et al.Deep structure and originof the Baikal rift zone.Earth planet Sci.Lett.,2006,243:681-691.
[12]Molnar P,Tapponnier P.Cenozoic tectonics of Asia:effectsof a continental collision.Science,1975,189:419-426.
[13]Ruppel C.Extensional processes in continental lithosphere.J.Geophys.Res.,1995,100:24187-24215.
[14]Petit C,Déverchère J,Houdry F,et al.Present-day stressfield changes along the Baikal rift and tectonic implications.Tectonics,1996,15:1171-1191.
[15]Petit C,Déverchère J.Structure and evolution of the Baikalrift:A synthesis.Geochem.Geophys.Geosyst.,2006,7,Q11016.
[16]Barruol G,Deschamps A,Déverchère J,et al.Upper mantleflow beneath and around the Hangay dome,Central Mongolia.Earth planet.Sci.Lett.,2008,274:221-233.
[17]Wilson J T.Did the Atlantic close and then re-open?Nature,1966,211:676-681.
[18]Silver P G.Seismic anisotropy beneath the continents:probing the depths of geology.Annu.Rev.Earth Planet.Sci.,1996,24:385-432.
[19]张中杰.地震各向异性研究进展.地球物理学进展,2002,l7(2):281-293.Zhang Z J.A review of seismic anisotropy.Progress inGeophys.(in Chinese),2002,17(2):281-293.
[20]Zhang Z J,Wang Y H,Chen Y,et al.Crustal structureacross Longmenshan fault belt from passive source seismicprofiling.Geophys.Res.Lett.,2009,36,L17310.
[21]Zhang Z J,Yuan X H,Chen Y,et al.Seismic signature ofthe collision between the east Tibetan escape flow and theSichuan Basin.Earth planet.Sci.Lett.,2010,292:254-264.
[22]Bowman J R,Ando M.Shear-wave splitting in theuppermantle wedge above the Tonga subduction zone.Geophys.J.R Astron.Soc.,1987,88:25-41.
[23]Silver P,Chan W W.Implications for continental structureand evolution from seismic anisotropy.Nature,1988,335:34-39.
[24]Vinnik L P,Farra V,Romanowicz B.Azimuthal anisotropyin the Earth from observations of SKS at GEOSCOPE andNARS broadband stations.Bull.Seismol.Soc.Am.,1989,79:1542-1558.
[25]Silver P G,Chan W W.Shear wave splitting andsubcontinental mantle deformation.J.Geophys.Res.,1991,96:16429-16454.
[26]Iidaka T,Niu F L.Evidence for an anisotropic lower mantlebeneath eastern Asia:comparison of shear-wave splitting dataof SKS and P660s.Geophys.Res.Lett.,1998,25:675-678.
[27]Levin V,Menke W,Park J.Shear wave splitting inAppalachians and Urals:a case for multilayer anisotropy.J.Geophys.Res.,1999,104:17975-17994.
[28]Tian X B,Zhang J L,Si S K,et al.SKS splittingmeasurement with horizontal component misalignment.Geophys.J.Int.,2011,185:329-340.
[29]Liu K H,Gao S S,Gao Y,et al.Shear wave splitting andmantle flow associated with the deflected Pacific slab beneathnortheast Asia.J.Geophys.Res.,2008,113,B01305.
[30]吴庆举,田小波,张乃玲等.用Wiener滤波方法提取台站接收函数.中国地震,2003,19(1):41-47.
[31]Silver P G,Savage M K.The interpretation of shear-wavesplitting parameters in the presence of two anisotropic layers.Geophys.J.Int.,1994,119:949-963.
[32]罗艳,黄忠贤,彭艳菊等.中国大陆及邻区SKS波分裂研究.地球物理学报,2004,47(5):812-821.Luo Y,Huang Z X,Peng Y J,et al.SKS wave splittingstudy of China and adjacent regions.Chin.J.Geophys.(inChinese),2004,47(5):812-821.
[33]Iidaka T,Niu F L.Mantle and crust anisotropy in theeastern China region inferred from waveform splitting of SKSand PpSms.Earth Planets Space,2001,53:159-168.
[34]田宝峰,李娟,王卫民等.华北太行山区地壳各向异性的接收函数证据.地球物理学报,2008,51(5):1459-1467.Tian B F,Li J,Wang W M,et a1.Crust anisotropy ofTaihangshan mountain range in north China inferred fromreceiver functions.Chin.J.Geophys.(in Chinese),2008,51(5):1459-1467.
[35]Bayasgalan A,Jackson J,McKenzie D.Lithosphere rheologyand active tectonics in Mongolia:relations between earthquakessource parameters,gravity and GPS measurements.Geophys.J.Int.,2005,163:1151-1179.
[36]Tommasi A,Tikoff B,Vauchez A.Upper mantle tectonics:three-dimensional deformation,olivine crystallographic fabrics andseismic properties.Earth Planet.Sci.Lett.,1999,168:173-186.
[37]Ionov D.Mantle structure and rifting processes in the Baikal-Mongolia region:geophysical data and evidence fromxenoliths in volcanic rocks.Tectonophysics,2002,351:41-60.
[38]阮爱国,王椿镛.云南地区上地幔各向异性研究.地震学报,2002,21(3):260-267.Ruan A G,Wang C Y.The upper mantle anisotropy inYunan area,China.Acta Seismologica Sinica(in Chinese),2002,21(3):260-267.
[39]Priestley K,Debayle E,McKenzie D,et al.Upper mantlestructure of eastern Asia from multimode surface waveformtomography.J.Geophys.Res.,2006,111,B10304.
[40]Li Y H,Wu Q J,Jiang L J,et al.Complex seismicanisotropic structure beneath the central Tien Shan revealedby shear wave splitting analyses.Geophys.J.Int.,2010,181:1678-1686.
[41]Li Y H,Wu Q J,Zhang F X,et al.Seismic anisotropy of theNortheastern Tibetan Plateau from shear wave splittinganalysis.Earth planet.Sci.Lett.,2011,304:147-157.
[42]Bai L,Kawakatsu H,Morita Y.Two anisotropic layers incentral orogenic belt of North China Craton.Tectonophysics,2010,494:138-148.
[43]Gao S S,Liu H,Abdelsalam G M.Seismic anisotropybeneath the Afar Depression and adjacent areas:Implicationsfor mantle flow.J.Geophys.Res.,2010,15,B12330.
[44]Lebedev S,Meier T,Hilst R D.Asthenospheric flow andorigin of volcanism in the Baikal Rift area.Earth planet.Sci.Lett.,2006,249:415-424.
[45]司少坤,田小波,张中杰等.贝加尔裂谷区地幔过渡带大范围增厚与局部减薄现象及其动力学意义.中国科学:D辑,待刊Si S K,Tian X B,Zhang Z J,et al.Prevalent thickening andlocal thinning of the mantle transition zone beneath the Baikalrift zone and dynamic implications.Sci.China Earth Sci.,inpress.
[2]Delvaux D,Moeys R,Stapel G,et al.Paleostressreconstructions and geodynamics of the Baikal region,CentralAsia:Part I.Palaeozoic and Mesozoic prerift evolution.Tectonophysics,1995,252:61-101.
[3]Logatchev N A,Zorin Y A.Evidence and causes of the two-stage development of the Baikal rift.Tectonophysics,1987,143:225-234.
[4]Zorin Y A,Rogozhina V A.Mechanism of rifting and somefeatures of the deep-seated structure of the Baikal rift zone.Tectonophysics,1978,45:23-30.
[5]Zorin Y A.The Baikal rift:an example of the intrusion ofasthenospheric material into the lithosphere as the cause ofdisruption of lithospheric plates.Tectonophysics,1981,73:91-104.
[6]Logatchev N A,Zorin Y A,Rogozhina V A.Baikal rift:active or passive?Comparison of the Baikal and Kenya riftzones.Tectonophysics,1983,94:223-240.
[7]Gao S S,Davis P M,Liu H,et al.Asymmetric upwarp ofthe asthenosphere beneath the Baikal rift zone,Siberia.J.Geophys.Res.,1994,99:15319-15330.
[8]Gao S S,Davis P M,Liu H,et al.Seismic anisotropy andmantle flow beneath the Baikal rift zone.Nature,1994,371:149-151.
[9]Gao S S,Davis P M,Liu H,et al.SKS splitting beneathcontinental rift zone.J.Geophys.Res.,1997,102:22781-22797.
[10]Zorin Y A,Turutanov E K,Mordvinova V V,et al.TheBaikal rift zone:the effect of mantle plumes on olderstructure.Tectonophysics,2003,371:153-173.
[11]Zhao D P,Lei J S,Inoue T,et al.Deep structure and originof the Baikal rift zone.Earth planet Sci.Lett.,2006,243:681-691.
[12]Molnar P,Tapponnier P.Cenozoic tectonics of Asia:effectsof a continental collision.Science,1975,189:419-426.
[13]Ruppel C.Extensional processes in continental lithosphere.J.Geophys.Res.,1995,100:24187-24215.
[14]Petit C,Déverchère J,Houdry F,et al.Present-day stressfield changes along the Baikal rift and tectonic implications.Tectonics,1996,15:1171-1191.
[15]Petit C,Déverchère J.Structure and evolution of the Baikalrift:A synthesis.Geochem.Geophys.Geosyst.,2006,7,Q11016.
[16]Barruol G,Deschamps A,Déverchère J,et al.Upper mantleflow beneath and around the Hangay dome,Central Mongolia.Earth planet.Sci.Lett.,2008,274:221-233.
[17]Wilson J T.Did the Atlantic close and then re-open?Nature,1966,211:676-681.
[18]Silver P G.Seismic anisotropy beneath the continents:probing the depths of geology.Annu.Rev.Earth Planet.Sci.,1996,24:385-432.
[19]张中杰.地震各向异性研究进展.地球物理学进展,2002,l7(2):281-293.Zhang Z J.A review of seismic anisotropy.Progress inGeophys.(in Chinese),2002,17(2):281-293.
[20]Zhang Z J,Wang Y H,Chen Y,et al.Crustal structureacross Longmenshan fault belt from passive source seismicprofiling.Geophys.Res.Lett.,2009,36,L17310.
[21]Zhang Z J,Yuan X H,Chen Y,et al.Seismic signature ofthe collision between the east Tibetan escape flow and theSichuan Basin.Earth planet.Sci.Lett.,2010,292:254-264.
[22]Bowman J R,Ando M.Shear-wave splitting in theuppermantle wedge above the Tonga subduction zone.Geophys.J.R Astron.Soc.,1987,88:25-41.
[23]Silver P,Chan W W.Implications for continental structureand evolution from seismic anisotropy.Nature,1988,335:34-39.
[24]Vinnik L P,Farra V,Romanowicz B.Azimuthal anisotropyin the Earth from observations of SKS at GEOSCOPE andNARS broadband stations.Bull.Seismol.Soc.Am.,1989,79:1542-1558.
[25]Silver P G,Chan W W.Shear wave splitting andsubcontinental mantle deformation.J.Geophys.Res.,1991,96:16429-16454.
[26]Iidaka T,Niu F L.Evidence for an anisotropic lower mantlebeneath eastern Asia:comparison of shear-wave splitting dataof SKS and P660s.Geophys.Res.Lett.,1998,25:675-678.
[27]Levin V,Menke W,Park J.Shear wave splitting inAppalachians and Urals:a case for multilayer anisotropy.J.Geophys.Res.,1999,104:17975-17994.
[28]Tian X B,Zhang J L,Si S K,et al.SKS splittingmeasurement with horizontal component misalignment.Geophys.J.Int.,2011,185:329-340.
[29]Liu K H,Gao S S,Gao Y,et al.Shear wave splitting andmantle flow associated with the deflected Pacific slab beneathnortheast Asia.J.Geophys.Res.,2008,113,B01305.
[30]吴庆举,田小波,张乃玲等.用Wiener滤波方法提取台站接收函数.中国地震,2003,19(1):41-47.
[31]Silver P G,Savage M K.The interpretation of shear-wavesplitting parameters in the presence of two anisotropic layers.Geophys.J.Int.,1994,119:949-963.
[32]罗艳,黄忠贤,彭艳菊等.中国大陆及邻区SKS波分裂研究.地球物理学报,2004,47(5):812-821.Luo Y,Huang Z X,Peng Y J,et al.SKS wave splittingstudy of China and adjacent regions.Chin.J.Geophys.(inChinese),2004,47(5):812-821.
[33]Iidaka T,Niu F L.Mantle and crust anisotropy in theeastern China region inferred from waveform splitting of SKSand PpSms.Earth Planets Space,2001,53:159-168.
[34]田宝峰,李娟,王卫民等.华北太行山区地壳各向异性的接收函数证据.地球物理学报,2008,51(5):1459-1467.Tian B F,Li J,Wang W M,et a1.Crust anisotropy ofTaihangshan mountain range in north China inferred fromreceiver functions.Chin.J.Geophys.(in Chinese),2008,51(5):1459-1467.
[35]Bayasgalan A,Jackson J,McKenzie D.Lithosphere rheologyand active tectonics in Mongolia:relations between earthquakessource parameters,gravity and GPS measurements.Geophys.J.Int.,2005,163:1151-1179.
[36]Tommasi A,Tikoff B,Vauchez A.Upper mantle tectonics:three-dimensional deformation,olivine crystallographic fabrics andseismic properties.Earth Planet.Sci.Lett.,1999,168:173-186.
[37]Ionov D.Mantle structure and rifting processes in the Baikal-Mongolia region:geophysical data and evidence fromxenoliths in volcanic rocks.Tectonophysics,2002,351:41-60.
[38]阮爱国,王椿镛.云南地区上地幔各向异性研究.地震学报,2002,21(3):260-267.Ruan A G,Wang C Y.The upper mantle anisotropy inYunan area,China.Acta Seismologica Sinica(in Chinese),2002,21(3):260-267.
[39]Priestley K,Debayle E,McKenzie D,et al.Upper mantlestructure of eastern Asia from multimode surface waveformtomography.J.Geophys.Res.,2006,111,B10304.
[40]Li Y H,Wu Q J,Jiang L J,et al.Complex seismicanisotropic structure beneath the central Tien Shan revealedby shear wave splitting analyses.Geophys.J.Int.,2010,181:1678-1686.
[41]Li Y H,Wu Q J,Zhang F X,et al.Seismic anisotropy of theNortheastern Tibetan Plateau from shear wave splittinganalysis.Earth planet.Sci.Lett.,2011,304:147-157.
[42]Bai L,Kawakatsu H,Morita Y.Two anisotropic layers incentral orogenic belt of North China Craton.Tectonophysics,2010,494:138-148.
[43]Gao S S,Liu H,Abdelsalam G M.Seismic anisotropybeneath the Afar Depression and adjacent areas:Implicationsfor mantle flow.J.Geophys.Res.,2010,15,B12330.
[44]Lebedev S,Meier T,Hilst R D.Asthenospheric flow andorigin of volcanism in the Baikal Rift area.Earth planet.Sci.Lett.,2006,249:415-424.
[45]司少坤,田小波,张中杰等.贝加尔裂谷区地幔过渡带大范围增厚与局部减薄现象及其动力学意义.中国科学:D辑,待刊Si S K,Tian X B,Zhang Z J,et al.Prevalent thickening andlocal thinning of the mantle transition zone beneath the Baikalrift zone and dynamic implications.Sci.China Earth Sci.,inpress.
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