基于P波三重震相研究青藏高原上地幔速度结构及其动力学意义
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摘要
青藏高原因其复杂的结构和演化历史,一直都是研究大陆碰撞、构造运动及其动力学的热点区域。本文采用三重震相波形拟合技术,基于中国地震观测台网和大型流动台阵记录到的某地震P波垂向记录,获得了包括拉萨、南羌塘和松潘甘孜地块在内的青藏高原上地幔P波速度结构。结果表明:①拉萨和南羌塘地块下方地幔过渡带存在高速异常,推测是俯冲的印度板片滞留体,过渡带底部的板片残余温度较低,使得660-km相变滞后约3～8 km。而松潘甘孜地块下方过渡带同样存在高速异常,可能是欧亚岩石圈发生拆沉进入地幔过渡带所致。这说明印度板块俯冲作用的影响已经到达地幔过渡带,其俯冲前缘位于班公怒江缝合带附近。②从拉萨、南羌塘到松潘甘孜地块,200 km之上的地幔岩石圈高速盖层速度由南向北逐渐减小,松潘甘孜地块则出现盖层缺失。推测受小规模地幔对流或者热不稳定性的影响,在南羌塘和松潘甘孜地块,增厚的欧亚岩石圈发生拆沉作用,岩石圈被减薄和弱化,造成羌塘地块上地幔低速和松潘甘孜地块上地幔高速盖层缺失。拆沉的冷的欧亚岩石圈可能部分停留在410-km上方,使得410-km抬升约10 km,部分沉入地幔过渡带,表现为松潘甘孜地块地幔过渡带中存在高速异常。低温造成660-km下沉约8 km,导致地幔过渡带增厚。
Due to its complex structure and evolution history, Tibetan plateau has always been a focused region for study continental collision, tectonic movement and its geodynamics. Based on the the vertical P wave records from a selected earthquake recorded by CDSN(Chinese Digital Seismic Network) and temporary seismic stations,the high resolution upper mantle structures beneath the Tibetan Plateau including the Lhasa Terrane,south Qiangtang terrane and Songpan-Ganzi terrane are obtained by triplication waveform fitting technique. Our results show that:(1) The high velocity anomaly observed at the bottom of MTZ(mantle transition zone) beneath Lhasa and south Qiangtang terranes are interpreted as the remnants of the subducted Indian slab,and these cold material depressed the post-spinel phase transition of 660 km discontinuity by 3 ～ 8 km. The high velocity anomaly can also be observed beneath Songpan-Ganzi terrane, which may be caused by the delamination of thickened Eurasian lithosphere. It suggests that the influence of Indian slab subduction has reached the MTZ, and the subduction front beneath the Tibetan Plateau is located near the Bangong-Nujiang suture.(2) From Lhasa, south Qiangtang to Songpan-Ganzi terranes, the upper-mantle velocities over 200 km decreases gradually along the N-S profile, and the high-velocity lid even disappears in Songpan-Ganzi terrane. Influenced by the small-scale mantle convection or thermal instability, the thickened Eurasian lithosphere beneath south Qiangtang and Songpan-Ganzi terranes was delaminated, and the lithosphere was thinned and weakened, resulting in the low velocity of upper mantle in Qiangtang and the absence of high-velocity lid in Songpan-Ganzi terrane. Some of the delaminated lithosphere maybe now sitting atop of the 410 km discontinuity, causing the uplift of 410 km discontinuity by about 10 km and others may have penetrated into the MTZ, characterized by the high velocity in the lower MTZ beneath Songpan-Ganzi Terrane. Low temperature causes the depression of 660 km discontinuity by about 8 km,resulting in the thickening of the MTZ.
Due to its complex structure and evolution history, Tibetan plateau has always been a focused region for study continental collision, tectonic movement and its geodynamics. Based on the the vertical P wave records from a selected earthquake recorded by CDSN(Chinese Digital Seismic Network) and temporary seismic stations,the high resolution upper mantle structures beneath the Tibetan Plateau including the Lhasa Terrane,south Qiangtang terrane and Songpan-Ganzi terrane are obtained by triplication waveform fitting technique. Our results show that:(1) The high velocity anomaly observed at the bottom of MTZ(mantle transition zone) beneath Lhasa and south Qiangtang terranes are interpreted as the remnants of the subducted Indian slab,and these cold material depressed the post-spinel phase transition of 660 km discontinuity by 3 ～ 8 km. The high velocity anomaly can also be observed beneath Songpan-Ganzi terrane, which may be caused by the delamination of thickened Eurasian lithosphere. It suggests that the influence of Indian slab subduction has reached the MTZ, and the subduction front beneath the Tibetan Plateau is located near the Bangong-Nujiang suture.(2) From Lhasa, south Qiangtang to Songpan-Ganzi terranes, the upper-mantle velocities over 200 km decreases gradually along the N-S profile, and the high-velocity lid even disappears in Songpan-Ganzi terrane. Influenced by the small-scale mantle convection or thermal instability, the thickened Eurasian lithosphere beneath south Qiangtang and Songpan-Ganzi terranes was delaminated, and the lithosphere was thinned and weakened, resulting in the low velocity of upper mantle in Qiangtang and the absence of high-velocity lid in Songpan-Ganzi terrane. Some of the delaminated lithosphere maybe now sitting atop of the 410 km discontinuity, causing the uplift of 410 km discontinuity by about 10 km and others may have penetrated into the MTZ, characterized by the high velocity in the lower MTZ beneath Songpan-Ganzi Terrane. Low temperature causes the depression of 660 km discontinuity by about 8 km,resulting in the thickening of the MTZ.
引文
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[7]MCNAMARA D E, WALTER W R, OWENS T J, et al. Upper mantle velocity structure beneath the Tibetan Plateau from Pn travel time tomography[J]. Journal of Geophysical Research:Solid Earth,1997, 102(B1):493-505.
[8]CHEN W P, TSENG T L. Small 660-km seismic discontinuity beneath Tibet implies resting ground for detached lithosphere[J]. Journal of Geophysical Research:Solid Earth, 2007, 112:B05309.
[9]TSENGTL,CHENWP.DiscordantcontrastofP-andS-wavespeedsacrossthe660-km discontinuity beneath Tibet:A case for hydrous remnant of sub-continental lithosphere[J].Earth and Planetary Science Letters, 2008, 268(3):450-462.
[10] LIANG X F, CHEN Y, TIAN X B, et al. 3D imaging of subducting and fragmenting Indian continental lithospherebeneathsouthernandcentralTibetusingbody-wavefinite-frequency tomography[J]. Earth and Planetary Science Letters, 2016, 443:162-175.
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[14] HUNG S H, CHEN W P, CHIAO L Y. A data-adaptive, multiscale approach of finite-frequency,traveltime tomography with special reference to P and S wave data from central Tibet[J].Journal of Geophysical Research:Solid Earth, 2011, 116:B06307.
[15] CHEN Y, LI W, YUAN X H, et al. Tearing of the Indian lithospheric slab beneath southern Tibet revealed by SKS-wave splitting measurements[J]. Earth and Planetary Science Letters, 2015,413:13-24.
[16] BAO X W, SONG X D, LI J T. High-resolution lithospheric structure beneath Mainland China from ambient noise and earthquake surface-wave tomography[J]. Earth and Planetary Science Letters, 2015, 417:132-141.
[17] GRAND S P, HELMBERGER D V. Upper mantle shear structure of North America[J]. Geophysical Journal International, 1984, 76(2):399-438.
[18] ZHANG R Q, WU Q J, LI Y H, et al. Lateral variations in SH velocity structure of the transition zone beneath Korea and adjacent regions[J]. Journal of Geophysical Research:Solid Earth,2012, 117:B09315.
[19] CHU R S, LENG W, HELMBERGER D V, et al. Hidden hotspot track beneath the eastern United States[J]. Nature Geoscience, 2013, 6(11):963-966.
[20] LI G H, BAI L, ZHOU Y Z, et al. Velocity structure of the mantle transition zone beneath the southeastern margin of the Tibetan Plateau[J]. Tectonophysics, 2017, 721:349-360.
[21] CHU R S, SCHMANDT B, HELMBERGER V D. Upper mantle P velocity structure beneath the Midwestern United States derived from triplicated waveforms[J]. Geochemistry Geophysics Geosystems,2012, 13(1):Q0AK04. doi:10.1029/2011GC003818.
[22]郑秀芬,欧阳飚,张东宁,等.“国家数字测震台网数据备份中心”技术系统建设及其对汶川大地震研究的数据支撑[J].地球物理学报, 2009, 52(5):1412-1417.ZHENG X F, OUYANG B, ZHANG D N, et al. Technical system construction of Data Backup Centre forChinaSeismographNetworkandthedatasupporttoresearchesontheWenchuan earthquake[J]. Chinese Journal Geophysics, 2009, 52(5):1412-1417(in Chinese).
[23]中国地震科学台阵.中国地震科学探测台阵波形数据[DB/CD].北京:中国地震局,2006.doi:10.12001/ChinArray.Data.
[24] KENNETTBLN,ENGDAHLER.Traveltimesforglobalearthquakelocationandphase identification[J]. Geophysical Journal International, 1991, 105(2):429-465.
[25] WANG B S, NIU F L. A broad 660 km discontinuity beneath northeast China revealed by dense regional seismic networks in China[J]. Journal of Geophysical Research, 2010, 115:B06308.
[26] LI J, WANG X, WANG X J, et al. P and SH velocity structure in the upper mantle beneath Northeast China:Evidence for a stagnant slab in hydrous mantle transition zone[J]. Earth and Planetary Science Letters, 2013, 367:71-81.
[27]高锐,熊小松,李秋生,等.由地震探测揭示的青藏高原莫霍面深度[J].地球学报, 2009, 30(6):761-773.GAO R, XIONG X S, LI Q S, et al. The Moho depth of Qinghai-Tibet plateau revealed by seismic detection[J]. Acta Geoscientica Sinica, 2009, 30(6):761-773.(in Chinese).
[28] BOYD O S, JONES C H, Sheehan A F. Foundering lithosphere imaged beneath the Southern Sierra Nevada, California, USA[J]. Science, 2004, 305(5684):660-662.
[29] DUAN Y H, TIAN X B, LIANG X F, et al. Subduction of the Indian slab into the mantle transition zone revealed by receiver functions[J]. Tectonophysics, 2017, 702:61-69.
[30] CHU R S. Upper mantle velocity structure beneath the Tibetan plateau from triplicated seismic P waveforms[D]. Saint Louis:Saint Louis University, 2008:56-69.
[31] CHEN M, NIU F L, TROMP J, et al. Lithospheric foundering and underthrusting imaged beneath Tibet[J]. Nature Communications, 2017, 8:15659.
[32]潘桂棠,李兴振, WANG LQ,等.青藏高原及邻区大地构造单元初步划分[J].地质通报, 2002,21(11):701-707.PAN G T, LI X Z, WANG L Q, et al. Preliminary division of tectonic units of the Qinghai-Tibet Plateau and its adjacent regions[J]. Geological Bulletin of China, 2002, 21(11):701-707.(in Chinese).
[33] ZHAO J M, YUAN X H, LIU H B, et al. The boundary between the Indian and Asian tectonic plates below Tibet[J]. Proceedings of the National Academy of Sciences, 2010, 107(25):11229-11233.
[34] XUQ,ZHAOJM,YUANXH,etal.DetailedconfigurationoftheunderthrustingIndian lithospherebeneathwesternTibetrevealedbyreceiverfunctionimages[J].Journalof Geophysical Research:Solid Earth, 2017, 122(10):8257-8269.
[2]吴珍汉,吴中海,江万,等.中国大陆及邻区新生代构造-地貌演化过程与机理[M].北京,地质出版社, 2001:37-71.WU Z H, WU Z H, JIANG W, et al. The Cenozoic evolution and mechanism of tectonic-landforms of China continent and its adjacent areas[M]. Beijing:Geological Publishing House, 2001:37-71.(in Chinese).
[3]MOLNAR P, TAPPONNIER P. Cenozoic tectonics of Asia:Effects of a continental collision[J].Science, 1975, 189(4201):419-426.
[4]KUMAR P, YUAN X H, KIND R, et al. Imaging the colliding Indian and Asian lithospheric plates beneath Tibet[J]. Journal of Geophysical Research:Solid Earth, 2006, 111:B06308.
[5]ZHANG X M, TENG J W, SUN R M, et al. Structural model of the lithosphere-asthenosphere system beneath the Qinghai-Tibet Plateau and its adjacent areas[J]. Tectonophysics, 2014, 634:208-226.
[6]WITTLINGER G, MASSON F, POUPINET G, et al. Seismic tomography of northern Tibet and Kunlun:Evidence for crustal blocks and mantle velocity contrasts[J]. Earth and Planetary Science Letters, 1996, 139(1/2):263-279.
[7]MCNAMARA D E, WALTER W R, OWENS T J, et al. Upper mantle velocity structure beneath the Tibetan Plateau from Pn travel time tomography[J]. Journal of Geophysical Research:Solid Earth,1997, 102(B1):493-505.
[8]CHEN W P, TSENG T L. Small 660-km seismic discontinuity beneath Tibet implies resting ground for detached lithosphere[J]. Journal of Geophysical Research:Solid Earth, 2007, 112:B05309.
[9]TSENGTL,CHENWP.DiscordantcontrastofP-andS-wavespeedsacrossthe660-km discontinuity beneath Tibet:A case for hydrous remnant of sub-continental lithosphere[J].Earth and Planetary Science Letters, 2008, 268(3):450-462.
[10] LIANG X F, CHEN Y, TIAN X B, et al. 3D imaging of subducting and fragmenting Indian continental lithospherebeneathsouthernandcentralTibetusingbody-wavefinite-frequency tomography[J]. Earth and Planetary Science Letters, 2016, 443:162-175.
[11] OWENS T J, ZANDT G. Implications of crustal property variations for models of Tibetan plateau evolution[J]. Nature, 1997, 387:37-43.
[12]吕庆田,姜枚,许志琴,等.印度板块俯冲仅到特提斯喜马拉雅之下的地震层析证据[J].科学通报, 1998, 43(12):1308-1311.LV Q T, JIANG M, XU Z Q, et al. Seismic tomographic evidence of the Indian plate thrusted just below the Tethys Himalayas[J]. Chinese Science Bulletin, 1998, 43(12):1308-1311.(in Chinese).
[13]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[J].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[J]. Chinese Journal Geophysics, 1998, 41(5):669-679.(in Chinese).
[14] HUNG S H, CHEN W P, CHIAO L Y. A data-adaptive, multiscale approach of finite-frequency,traveltime tomography with special reference to P and S wave data from central Tibet[J].Journal of Geophysical Research:Solid Earth, 2011, 116:B06307.
[15] CHEN Y, LI W, YUAN X H, et al. Tearing of the Indian lithospheric slab beneath southern Tibet revealed by SKS-wave splitting measurements[J]. Earth and Planetary Science Letters, 2015,413:13-24.
[16] BAO X W, SONG X D, LI J T. High-resolution lithospheric structure beneath Mainland China from ambient noise and earthquake surface-wave tomography[J]. Earth and Planetary Science Letters, 2015, 417:132-141.
[17] GRAND S P, HELMBERGER D V. Upper mantle shear structure of North America[J]. Geophysical Journal International, 1984, 76(2):399-438.
[18] ZHANG R Q, WU Q J, LI Y H, et al. Lateral variations in SH velocity structure of the transition zone beneath Korea and adjacent regions[J]. Journal of Geophysical Research:Solid Earth,2012, 117:B09315.
[19] CHU R S, LENG W, HELMBERGER D V, et al. Hidden hotspot track beneath the eastern United States[J]. Nature Geoscience, 2013, 6(11):963-966.
[20] LI G H, BAI L, ZHOU Y Z, et al. Velocity structure of the mantle transition zone beneath the southeastern margin of the Tibetan Plateau[J]. Tectonophysics, 2017, 721:349-360.
[21] CHU R S, SCHMANDT B, HELMBERGER V D. Upper mantle P velocity structure beneath the Midwestern United States derived from triplicated waveforms[J]. Geochemistry Geophysics Geosystems,2012, 13(1):Q0AK04. doi:10.1029/2011GC003818.
[22]郑秀芬,欧阳飚,张东宁,等.“国家数字测震台网数据备份中心”技术系统建设及其对汶川大地震研究的数据支撑[J].地球物理学报, 2009, 52(5):1412-1417.ZHENG X F, OUYANG B, ZHANG D N, et al. Technical system construction of Data Backup Centre forChinaSeismographNetworkandthedatasupporttoresearchesontheWenchuan earthquake[J]. Chinese Journal Geophysics, 2009, 52(5):1412-1417(in Chinese).
[23]中国地震科学台阵.中国地震科学探测台阵波形数据[DB/CD].北京:中国地震局,2006.doi:10.12001/ChinArray.Data.
[24] KENNETTBLN,ENGDAHLER.Traveltimesforglobalearthquakelocationandphase identification[J]. Geophysical Journal International, 1991, 105(2):429-465.
[25] WANG B S, NIU F L. A broad 660 km discontinuity beneath northeast China revealed by dense regional seismic networks in China[J]. Journal of Geophysical Research, 2010, 115:B06308.
[26] LI J, WANG X, WANG X J, et al. P and SH velocity structure in the upper mantle beneath Northeast China:Evidence for a stagnant slab in hydrous mantle transition zone[J]. Earth and Planetary Science Letters, 2013, 367:71-81.
[27]高锐,熊小松,李秋生,等.由地震探测揭示的青藏高原莫霍面深度[J].地球学报, 2009, 30(6):761-773.GAO R, XIONG X S, LI Q S, et al. The Moho depth of Qinghai-Tibet plateau revealed by seismic detection[J]. Acta Geoscientica Sinica, 2009, 30(6):761-773.(in Chinese).
[28] BOYD O S, JONES C H, Sheehan A F. Foundering lithosphere imaged beneath the Southern Sierra Nevada, California, USA[J]. Science, 2004, 305(5684):660-662.
[29] DUAN Y H, TIAN X B, LIANG X F, et al. Subduction of the Indian slab into the mantle transition zone revealed by receiver functions[J]. Tectonophysics, 2017, 702:61-69.
[30] CHU R S. Upper mantle velocity structure beneath the Tibetan plateau from triplicated seismic P waveforms[D]. Saint Louis:Saint Louis University, 2008:56-69.
[31] CHEN M, NIU F L, TROMP J, et al. Lithospheric foundering and underthrusting imaged beneath Tibet[J]. Nature Communications, 2017, 8:15659.
[32]潘桂棠,李兴振, WANG LQ,等.青藏高原及邻区大地构造单元初步划分[J].地质通报, 2002,21(11):701-707.PAN G T, LI X Z, WANG L Q, et al. Preliminary division of tectonic units of the Qinghai-Tibet Plateau and its adjacent regions[J]. Geological Bulletin of China, 2002, 21(11):701-707.(in Chinese).
[33] ZHAO J M, YUAN X H, LIU H B, et al. The boundary between the Indian and Asian tectonic plates below Tibet[J]. Proceedings of the National Academy of Sciences, 2010, 107(25):11229-11233.
[34] XUQ,ZHAOJM,YUANXH,etal.DetailedconfigurationoftheunderthrustingIndian lithospherebeneathwesternTibetrevealedbyreceiverfunctionimages[J].Journalof Geophysical Research:Solid Earth, 2017, 122(10):8257-8269.
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