青藏高原中部地壳和上地幔各向异性分析
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摘要
对布设于青藏高原中部INDEPTH-Ⅲ宽频带数字地震台阵的41个台站记录的远震体波资料所提取出的P波接收函数和SKS波形资料做偏振分析,并采用以误差为权的叠加分析方法求得每一个台站的Pms和SKS快波偏振方向和快慢波的时间延迟,获得了从拉萨块体中部,经喀喇昆仑嘉黎断裂系和班公湖—怒江缝合带,到羌塘块体中部的地壳和岩石圈地幔的地震波各向异性图像.从各向异性分析结果可以看到:Pms快慢波的时间延迟为0.3～0.5 s,在拉萨块体,快波方向主要为NE-SW向,在羌塘块体,快波方向为近E-W向.SKS快慢波的时间延迟为1～2 s,主要分布在拉萨块体的北端和羌塘块体,并且向靠近班公湖怒江缝合带和昆仑—嘉黎断裂带的方向时差增大,快波方向基本与Pms快波方向一致.在喀喇昆仑—嘉黎断裂带以南的拉萨块体中部没有测量到明显的SKS分裂,这可能与该区存在双层快轴方向近垂直的各向异性层有关.结合研究区已有的研究成果可以推测:拉萨块体地壳各向异性层的快轴方向与印度—欧亚板块汇聚方向一致,可能与地壳较强刚性有关,其在板块汇聚过程中不易发生流展变形;而羌塘块体地壳和岩石圈中各向异性层的快轴方向与青藏高原物质逃逸方向一致,表明这一块体流变性均较强,在板块汇聚挤压力的作用下发生了侧向流变变形.
This paper analyzes S-wave splitting analysis in terms of P-wave receiver functions and the SKS shear waves from the INDEPTH-Ⅲprofile which consists of 41 seismic stations deployed in central Tibet.Using the weight average technique with a weighting factor of the measuring error,the direction and extent of seismic polarization anisotropy in the crust and upper mantle beneath central Tibet,from the central Lhasa terrane,across the Karakoram-Jiali fault and Bangong-Nujiang suture to the central Qiangtang terrane,were imaged.Substantial splitting of Pms phase with delay time 0.3~0.5 s,and fast directions varying from NE-SW in the Lhasa terrane to EW in the Qiangtang terrane,was observed.The delay time of SKS for stations in the northernmost Lhasa terrane and Qiangtang terrane is 1～2 s which increases near the Bangong- Nujiang suture and Jiali fault,and the fast direction of SKS is similar to the Pms.No detectable SKS splitting was observed for stations located farther south in the central Lhasa terrane,which is related to a model with two-layer anisotropy of perpendicular fast direction.In combination with previous geological and geophysical studies,we infer that the Lhasa crust possesses high rigidity and has not experienced rheological deformation,so that the fast direction of the anisotropy layer in the crust is consistent with the direction of the India-Eurasia convergence. However,the Qiangtang terrane is possibly weaker and more likely has been rheologically deformed during the convergence process,so that the fast direction of the anisotropy layer in the crust and upper mantle is coincident with escaping direction of material of the Tibetan plateau.
This paper analyzes S-wave splitting analysis in terms of P-wave receiver functions and the SKS shear waves from the INDEPTH-Ⅲprofile which consists of 41 seismic stations deployed in central Tibet.Using the weight average technique with a weighting factor of the measuring error,the direction and extent of seismic polarization anisotropy in the crust and upper mantle beneath central Tibet,from the central Lhasa terrane,across the Karakoram-Jiali fault and Bangong-Nujiang suture to the central Qiangtang terrane,were imaged.Substantial splitting of Pms phase with delay time 0.3~0.5 s,and fast directions varying from NE-SW in the Lhasa terrane to EW in the Qiangtang terrane,was observed.The delay time of SKS for stations in the northernmost Lhasa terrane and Qiangtang terrane is 1～2 s which increases near the Bangong- Nujiang suture and Jiali fault,and the fast direction of SKS is similar to the Pms.No detectable SKS splitting was observed for stations located farther south in the central Lhasa terrane,which is related to a model with two-layer anisotropy of perpendicular fast direction.In combination with previous geological and geophysical studies,we infer that the Lhasa crust possesses high rigidity and has not experienced rheological deformation,so that the fast direction of the anisotropy layer in the crust is consistent with the direction of the India-Eurasia convergence. However,the Qiangtang terrane is possibly weaker and more likely has been rheologically deformed during the convergence process,so that the fast direction of the anisotropy layer in the crust and upper mantle is coincident with escaping direction of material of the Tibetan plateau.
引文
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[2]England P.Houseman G.Finite strain calculations of continental deformation 2,comparison with the India-Asia collision zone.Journal of Geophysical Research,1986,91 (B3):3664-3676
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[6]Zhang Z J,Yuan X H,Chen Y,et al.Seismic signature of the collision between the east Tibetan escape flow and the Sichuan Basin.Earth and Planetary Science Letters,2010, 292(3-4):254-264
[7]Zhang Z J,Klemperer S.Crustal structure of the Tethyan Himalaya,southern Tibet:new constraints from old wideangle seismic data.Geophysical Journal International, 2010,181(3):1247-1260
[8]Owens T J,Zandt G.Implications of crustal property variations for models of Tibetan plateau evolution.Nature, 1997,387(6628):37-43
[9]张中杰,李英康,王光杰等.藏北地壳东西向结构与“下凹”莫霍面——来自宽角反射剖面的启示.中国科学(D辑), 2001,31(11):881-888 Zhang Z J,Li Y K,Wang G J,et al.E-W crustal structure under the northern Tibet and "down-bowing" Moho under the northern Tibet revealed by wide-angle seismic profile.Science inChina(Series D)(in Chinese),2001,31(11):881-888
[10]张中杰,滕吉文,李英康等.藏南地壳速度结构与地壳物质东西向“逃逸”——以佩枯错-普莫雍错宽角反射剖面为例.中国科学(D辑),2002,32(10):793-798 Zhang Z J,Teng J W,Li Y K,et al.Crustal structure and eastwards escaping of crustal material from wide-angle seismic profile in the southern Tibet.Science in China (SeriesD)(in Chinese),2002,32(10):793-798
[11]Royden L H,Burchfiel B C,King R W,et al.Surface deformation and lower crustal flow in eastern Tibet.Science, 1997,276(5313):788-790
[12]Wang C Y,Flesch L M,Silver P G,et al.Evidence for mechanically coupled lithosphere in central Asia and resulting implications.Geology,2008,36(5):363-366
[13]Gan W J,Zhang P Z,Shen Z K,et al.Present-day crustal motion within the Tibetan Plateau inferred from GPS measurements.J.Geophys.Res.,112:B08416,doi:10. 1029/2005JB004120
[14]Ozacar A A,Zandt G.Crustal seismic anisotropy in central Tibet:Implications for deformational style and flow in the crust.Geophysical Research Letters,2004,31:L23601, doi:10.1029/2004GL021096
[15]Ni J,Barazangi M.High-frequency seismic wave propagation beneath the Indian Shield,Himalayan arc,Tibetan Plateau, and surrounding regions:high uppermost mantle velocities and efficient S_n propagation beneath Tibet.Geophys.J. Roy.Astronom..1983,72(3):665-689
[16]Holt W E,Wallace T C.Crustal thickness and upper mantle velocities in the Tibetan Plateau region from the inversion of regional Pn/waveforms:Evidence for a thick upper mantle lid beneath southern Tibet.J.Geophys.Res.,1990,95(B8): 12499-12525
[17]Kind R.Ni J,Zhao W J,et al.Evidence from earthquake data for a partially molten crustal layer in southern Tibet. Science,1996,274(5293):1692-1694
[18]Tilmann F,Ni J,IndepthⅢSeismic Team.Seismic imaging of the downwelling Indian lithosphere beneath central Tibet. Science,2003,300(5624):1424-1427
[19]Wittlinger G.Vergne J,Tapponnier P,et al.Teleseismic imaging of subducting lithosphere and Moho offsets beneath western Tibet.Earth Planet.Sci.Lett.,2004,221(1-4): 117-130
[20]McNamaraD E.Owens T J,Walter W R.Observations of regional phase propagation across the Tibetan Plateau.J. Geophys.Res.,1995,100(B11):22215-22229
[21]McNamara D E,Owens T J,Silver P G,et al.Shear wave anisotropy beneath the Tibetan Plateau.J.Geophys.Res., 1994,99(B7):13655-13665
[22]Huang W C,Ni J F,Tilmann F,et al.Seismic polarization anisotropy beneath the central Tibetan Plateau.J.Geophys. Res.,2000,105(B12):27979-27989
[23]Wu J,Gao Y,Chen Y.Shear-wave splitting in the crust beneath the southeast Capital area of North China.J. Seismol.,2009,13(2):277-286
[24]吴庆举,曾融生.用宽频带远震接收函数研究青藏高原的地壳结构.地球物理学报,1998,41(5):669-679 Wu Q J,Zeng R S.The crustal structure of Qinghai-Xizang Plateau inferred from broadband teleseismic waveform. Chinese J.Geophys.(Acta Geophysica Sinica)(in Chinese), 1998,41(5):669-679
[25]Savage M K.Lower crustal anisotropy or dipping boundaries? Effects on receiver functions and a case study in New Zealand.Journal of Geophysical Research,1998,103 (B7):15069-15087
[26]张洪双,田小波,滕吉文.接收函数方法估计Moho倾斜地区的地壳速度比.地球物理学报,2009,52(5):1243-1252 ??Zhang H S,Tian X B,Teng J W.Estimation of crustal V_p/ V_s with dipping Moho from receiver functions.Chinese J. Geophys.(in Chinese),2009,52(5):1243-1252
[27]Silver P G,Chan W W.Shear wave splitting and subcontinental mantle deformation.J.Geophys.Res.,1991,96 (B10):16429-16454
[28]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.A study on SKS wave splitting beneath the China mainland and adjacent regions. Chinese J.Geophys.(in Chinese),2004,47(5):812-821
[29]常利军,王椿镛,丁志峰.云南地区SKS波分裂研究.地球物理学报,2006,49(1):197-204 Chang L J,Wang C Y,Ding Z F.A study on SKS splitting beneath the Yunnan region.Chinese J.Geophys.(in Chinese), 2006,49(1):197-204
[30]王椿镛,常利军,吕智勇等.青藏高原东部上地幔各向异性及相关的壳幔耦合型式.中国科学(D辑):地球科学,2007, 37(4):495-503 Wang C Y,Chang L J,L(u|¨) Z Y,et al.The crustal and mantle coupling model of Anisotropy analysis of upper mantle in eastern Tibetan plateau.Science in China(Series D)(in Chinese):Earth Science,2007,37(4):495-503
[31]Tian X B,Zhang J L,Si S K,et al.SKS splitting measurements with horizontal component misalignment. Geophysical Journal International,2011,185(1):329-340
[32]Crampin S.The fracture criticality of crustal rocks. Geophys.J.Int.,1994,118(2):428-438
[33]Gao S S,Liu K H.Significant seismic anisotropy beneath the southern Lhasa Terrane.Tibetan Plateau.Geochemistry Geophysics Geosystems,2008,10:Q02008,doi:10.1029/ 2008GC002227
[34]Chen Y,Badal J,Hu J F.Love and Rayleigh Wave Tomography of the Qinghai-Tibet Plateau and surrounding Areas.Pure and Applied Geophysics,2010,167(10):1171- 1203
[35]Chen Y,Badal J,Zhang Z J.Radial anisotropy in the crust and upper mantle beneath the Qinghai-Tibet Plateau and surrounding regions.Journal of Asian Earth Sciences, 2009,36(4-5):289-302
[36]DeCelles P G,Robinson D M,Zandt G.Implications of shortening in the Himalayan fold-thrust belt for uplift of the Tibetan Plateau.Tectonics,2002,21,1062,doi:10.1029/ 2001TC001322
[37]Turner S,Hawkesworth C J,Liu J,et al.Timing of Tibetan uplift constrained by analysis of volcanic rocks.Nature, 1993,364(6432):50-53
[38]Turners S,Armaud N,Liu L,et al.Post-collision, shoshonitic volcanism on the Tibetan Plateau:implications for convective thinning of the lithosphere and the source of ocean island basalts.Journal of Petrology,1996,37(1): 45-71
[2]England P.Houseman G.Finite strain calculations of continental deformation 2,comparison with the India-Asia collision zone.Journal of Geophysical Research,1986,91 (B3):3664-3676
[3]李永华,吴庆举,田小波等.青藏高原拉萨及羌塘块体的地壳结构研究.地震学报,2006,28(6):586-595 Li Y H,Wu Q J,Tian X B,et al.The crustal structure beneath Qiangtang and Lhasa terrane from receiver function. Acta Seismologica Sinica(in Chinese),2006,28(6):586- 595
[4]李永华,田小波,吴庆举等.青藏高原INDEPTH-Ⅲ剖面地壳厚度与泊松比:地质与地球物理含义.地球物理学报, 2006,49(4):1037-1044 Li Y H,Tian X B,Wu Q J,et al.The Poisson ratio and crustal structure of the central Qinghai-Xizang inferred from INDEPTH-Ⅲteleseismic waveforms:Geological and geophysical implications.Chinese J.Geophys.(in Chinese), 2006,49(4):1037-1044
[5]Wang C Y,Han W B,Wu J P,et al.Crustal structure beneath the eastern margin of the Tibetan Plateau and its tectonic implications.J.Geophys.Res.,112:B07307,doi: 10.1029/2005JB003873
[6]Zhang Z J,Yuan X H,Chen Y,et al.Seismic signature of the collision between the east Tibetan escape flow and the Sichuan Basin.Earth and Planetary Science Letters,2010, 292(3-4):254-264
[7]Zhang Z J,Klemperer S.Crustal structure of the Tethyan Himalaya,southern Tibet:new constraints from old wideangle seismic data.Geophysical Journal International, 2010,181(3):1247-1260
[8]Owens T J,Zandt G.Implications of crustal property variations for models of Tibetan plateau evolution.Nature, 1997,387(6628):37-43
[9]张中杰,李英康,王光杰等.藏北地壳东西向结构与“下凹”莫霍面——来自宽角反射剖面的启示.中国科学(D辑), 2001,31(11):881-888 Zhang Z J,Li Y K,Wang G J,et al.E-W crustal structure under the northern Tibet and "down-bowing" Moho under the northern Tibet revealed by wide-angle seismic profile.Science inChina(Series D)(in Chinese),2001,31(11):881-888
[10]张中杰,滕吉文,李英康等.藏南地壳速度结构与地壳物质东西向“逃逸”——以佩枯错-普莫雍错宽角反射剖面为例.中国科学(D辑),2002,32(10):793-798 Zhang Z J,Teng J W,Li Y K,et al.Crustal structure and eastwards escaping of crustal material from wide-angle seismic profile in the southern Tibet.Science in China (SeriesD)(in Chinese),2002,32(10):793-798
[11]Royden L H,Burchfiel B C,King R W,et al.Surface deformation and lower crustal flow in eastern Tibet.Science, 1997,276(5313):788-790
[12]Wang C Y,Flesch L M,Silver P G,et al.Evidence for mechanically coupled lithosphere in central Asia and resulting implications.Geology,2008,36(5):363-366
[13]Gan W J,Zhang P Z,Shen Z K,et al.Present-day crustal motion within the Tibetan Plateau inferred from GPS measurements.J.Geophys.Res.,112:B08416,doi:10. 1029/2005JB004120
[14]Ozacar A A,Zandt G.Crustal seismic anisotropy in central Tibet:Implications for deformational style and flow in the crust.Geophysical Research Letters,2004,31:L23601, doi:10.1029/2004GL021096
[15]Ni J,Barazangi M.High-frequency seismic wave propagation beneath the Indian Shield,Himalayan arc,Tibetan Plateau, and surrounding regions:high uppermost mantle velocities and efficient S_n propagation beneath Tibet.Geophys.J. Roy.Astronom..1983,72(3):665-689
[16]Holt W E,Wallace T C.Crustal thickness and upper mantle velocities in the Tibetan Plateau region from the inversion of regional Pn/waveforms:Evidence for a thick upper mantle lid beneath southern Tibet.J.Geophys.Res.,1990,95(B8): 12499-12525
[17]Kind R.Ni J,Zhao W J,et al.Evidence from earthquake data for a partially molten crustal layer in southern Tibet. Science,1996,274(5293):1692-1694
[18]Tilmann F,Ni J,IndepthⅢSeismic Team.Seismic imaging of the downwelling Indian lithosphere beneath central Tibet. Science,2003,300(5624):1424-1427
[19]Wittlinger G.Vergne J,Tapponnier P,et al.Teleseismic imaging of subducting lithosphere and Moho offsets beneath western Tibet.Earth Planet.Sci.Lett.,2004,221(1-4): 117-130
[20]McNamaraD E.Owens T J,Walter W R.Observations of regional phase propagation across the Tibetan Plateau.J. Geophys.Res.,1995,100(B11):22215-22229
[21]McNamara D E,Owens T J,Silver P G,et al.Shear wave anisotropy beneath the Tibetan Plateau.J.Geophys.Res., 1994,99(B7):13655-13665
[22]Huang W C,Ni J F,Tilmann F,et al.Seismic polarization anisotropy beneath the central Tibetan Plateau.J.Geophys. Res.,2000,105(B12):27979-27989
[23]Wu J,Gao Y,Chen Y.Shear-wave splitting in the crust beneath the southeast Capital area of North China.J. Seismol.,2009,13(2):277-286
[24]吴庆举,曾融生.用宽频带远震接收函数研究青藏高原的地壳结构.地球物理学报,1998,41(5):669-679 Wu Q J,Zeng R S.The crustal structure of Qinghai-Xizang Plateau inferred from broadband teleseismic waveform. Chinese J.Geophys.(Acta Geophysica Sinica)(in Chinese), 1998,41(5):669-679
[25]Savage M K.Lower crustal anisotropy or dipping boundaries? Effects on receiver functions and a case study in New Zealand.Journal of Geophysical Research,1998,103 (B7):15069-15087
[26]张洪双,田小波,滕吉文.接收函数方法估计Moho倾斜地区的地壳速度比.地球物理学报,2009,52(5):1243-1252 ??Zhang H S,Tian X B,Teng J W.Estimation of crustal V_p/ V_s with dipping Moho from receiver functions.Chinese J. Geophys.(in Chinese),2009,52(5):1243-1252
[27]Silver P G,Chan W W.Shear wave splitting and subcontinental mantle deformation.J.Geophys.Res.,1991,96 (B10):16429-16454
[28]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.A study on SKS wave splitting beneath the China mainland and adjacent regions. Chinese J.Geophys.(in Chinese),2004,47(5):812-821
[29]常利军,王椿镛,丁志峰.云南地区SKS波分裂研究.地球物理学报,2006,49(1):197-204 Chang L J,Wang C Y,Ding Z F.A study on SKS splitting beneath the Yunnan region.Chinese J.Geophys.(in Chinese), 2006,49(1):197-204
[30]王椿镛,常利军,吕智勇等.青藏高原东部上地幔各向异性及相关的壳幔耦合型式.中国科学(D辑):地球科学,2007, 37(4):495-503 Wang C Y,Chang L J,L(u|¨) Z Y,et al.The crustal and mantle coupling model of Anisotropy analysis of upper mantle in eastern Tibetan plateau.Science in China(Series D)(in Chinese):Earth Science,2007,37(4):495-503
[31]Tian X B,Zhang J L,Si S K,et al.SKS splitting measurements with horizontal component misalignment. Geophysical Journal International,2011,185(1):329-340
[32]Crampin S.The fracture criticality of crustal rocks. Geophys.J.Int.,1994,118(2):428-438
[33]Gao S S,Liu K H.Significant seismic anisotropy beneath the southern Lhasa Terrane.Tibetan Plateau.Geochemistry Geophysics Geosystems,2008,10:Q02008,doi:10.1029/ 2008GC002227
[34]Chen Y,Badal J,Hu J F.Love and Rayleigh Wave Tomography of the Qinghai-Tibet Plateau and surrounding Areas.Pure and Applied Geophysics,2010,167(10):1171- 1203
[35]Chen Y,Badal J,Zhang Z J.Radial anisotropy in the crust and upper mantle beneath the Qinghai-Tibet Plateau and surrounding regions.Journal of Asian Earth Sciences, 2009,36(4-5):289-302
[36]DeCelles P G,Robinson D M,Zandt G.Implications of shortening in the Himalayan fold-thrust belt for uplift of the Tibetan Plateau.Tectonics,2002,21,1062,doi:10.1029/ 2001TC001322
[37]Turner S,Hawkesworth C J,Liu J,et al.Timing of Tibetan uplift constrained by analysis of volcanic rocks.Nature, 1993,364(6432):50-53
[38]Turners S,Armaud N,Liu L,et al.Post-collision, shoshonitic volcanism on the Tibetan Plateau:implications for convective thinning of the lithosphere and the source of ocean island basalts.Journal of Petrology,1996,37(1): 45-71
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