西藏定日-青海格尔木上地幔各向异性研究
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摘要
43个三分量台站地震记录的各向异性计算结果显示:印度河—雅鲁藏布江缝合带两侧的各向异性特征明显不同,其北面较古老的班公湖—怒江缝合线和金沙江缝合线没有明显反映。各向异性方向与地表构造线方向多存在着明显的差异,因此不能简单地用与造山带走向相一致的上地幔变形来解释。另一方面各向异性在空间上的变化规律,如沿雅鲁藏布江两侧相互垂直的各向异性方向;羌塘地区较强的各向异性与Sn(一种沿莫霍面传播的折射横波)缺失区存在良好的对应关系。这些说明青藏高原还存在更复杂的碰撞变形机制。就现有资料我们认为,青藏高原的抬升是由于Argand式俯冲和Dewey式的均匀缩短联合作用的结果。
During the period of the Sino-French cooperative research programme (1992 - 1993), a total of 108 digital seismic stations were deployed in two phases along the profile from Tingri of southern Tibet to Golmud of northern Qinghai. Four months of field seismic recording resulted in hundreds of teleseismic events. Obvious SKS and PKS splitting has been observed on the seismo-grams of the 43 three-component stations with more than one events. Measurements of the fast polarization and the delay time show a very big difference on the two sides of the Indus-Yarlung Zangbo suture (ITS) but little difference on both sides of the older Banggong-Nujiang River suture (BNS) and the Jinsha River suture (JS). Obvious inconsistency exists between the anisotropy directions and the superficial tectonic-geological trends, which can not be explained directly by the coherent upper mantle deformation occurring usually parallely to the trend of the collisional belt. On the other hand, distinct anisotropy in spatial relationships, such as the perpendicular directions of anisotropy on both sides of ITS and the good correlation between the larger magnitude of anisotropy and the poor Sn zone in the Qiangtang region as well as the systematic rotation of the directions of anisotropy in the northern part of the profile, affirm much more complicated aspects of the continental collision deformation mechanism in some sence. On the basis of the existing data, the authors tend to suppose that the uplift of the Qinghai-Tibet Plateau might result from a combined effect of Argand type underthrust and Dewey type diffuse deformation and that the Qiangtang region might most probably have undergone delamination.
During the period of the Sino-French cooperative research programme (1992 - 1993), a total of 108 digital seismic stations were deployed in two phases along the profile from Tingri of southern Tibet to Golmud of northern Qinghai. Four months of field seismic recording resulted in hundreds of teleseismic events. Obvious SKS and PKS splitting has been observed on the seismo-grams of the 43 three-component stations with more than one events. Measurements of the fast polarization and the delay time show a very big difference on the two sides of the Indus-Yarlung Zangbo suture (ITS) but little difference on both sides of the older Banggong-Nujiang River suture (BNS) and the Jinsha River suture (JS). Obvious inconsistency exists between the anisotropy directions and the superficial tectonic-geological trends, which can not be explained directly by the coherent upper mantle deformation occurring usually parallely to the trend of the collisional belt. On the other hand, distinct anisotropy in spatial relationships, such as the perpendicular directions of anisotropy on both sides of ITS and the good correlation between the larger magnitude of anisotropy and the poor Sn zone in the Qiangtang region as well as the systematic rotation of the directions of anisotropy in the northern part of the profile, affirm much more complicated aspects of the continental collision deformation mechanism in some sence. On the basis of the existing data, the authors tend to suppose that the uplift of the Qinghai-Tibet Plateau might result from a combined effect of Argand type underthrust and Dewey type diffuse deformation and that the Qiangtang region might most probably have undergone delamination.
引文
1 Argand E. La tectonique de I'Asie: International geological congress, 13th, Brussels, 1922, Reports, 1924, 1: 170-372．
2 Barazangi M, Ni J. Propagation characteristics of Pn and Sn beneath the Himalayan arc and Tibetan plateau: Possible evidence forunderthrusting of Indian continental lithosphere beneath Tibet. Geology, 1982, 10: 179-185．
3 Beghoul N, Barazangi M, Isacks B L. Lithospheric structure of Tibet and western north America: mechanisms of uplift and a comparative study. J. Geophys. Res., 1993, 98: 1997-2016．
4 Dewey J, Burke K. Tibetan, Variscan, and Precambrian basement reactivation: Products of continental collision. J. Geol.,1973, 81: 683-692．
5 England P, Houseman G. Finite strain calculations of continental deformation 2． Comparison with the India-Asia collision zones. J. Geophys. Res., 1986, 91: 3664-3676．
6 Monlar P. Structure and tectonics of the Himalaya, constraints and implications of geophysical data. Annual Rev. Earth Planet. Sci., 1984, 12: 489-518．
7 Zhao W L, Yuen, Dave A. Injection of Indian crust into Tibetan lower crust: A temperature-dependent viscous model.Tectonics, 1987, 6 (4) : 505-514．
8 曾融生,朱介寿,周兵等.青藏高原及其东部邻区的三维地震波速度结构与大陆碰撞模型.地震学报,1992(14增刊):523-533。
9 Mattews D, Hirn A. Crust thickening in Himalayas and Caledonides. Nature, 1984, 308: 497-498．
10 Zhao W L, Morgan W J. Uplift of the Tibetan plateau. Tectonics, 1985, 4: 359-369．
11 Zhao W L, Morgan W J. Injection of Indian crust into Tibetan lower crust: A Two-dimensional finite element model study. Tectonics, 1987, 6 (4) : 489-504．
12 England P, Houseman G. Extension during continental convergence,with application to the Tibetan plateau. J. Geophys.Res., 1989, 94: 17561-17579．
13 姜枚,吕庆田,薛光琦等.中、法两国联合进行青藏高原天然地震探测地壳结构的研究.地球物理学报,1994,37(3) :412-413．
14 Vinnik L P,Farra V, Romanowicz B. Azimuthal anisotropy in the Earth from observations of SKS at Geoscope and Nars broad band stations. Bull. Seismol. Soc. Am., 1989, 79: 1542-1558．
15 Silver P G, Chan W W. Shear wave splitting and subcontinental mantle deformation. J. Geophys. Res., 1991, 96: 16429-16454．
16 McNamara D E, Owens T J, Silver P G, Wu F T. Shear wave anisotropy beneath the Tibetan Plateau, J. Geophys. Res.,1994, 99,B7: 13655-13665．
17 丁志峰,曾融生. 青藏高原SKS波各向异性的研究.中国固体地球物理学进展,北京:地震出版社,1994． 162-168页.
18 曾融生,孙为国.青藏高原及其邻区的地震活动性和震源机制以及高原物质东流的讨论.地震学报,1992,(14增刊):534-563．
19 Silver P G, Chan W W. Implications for continental structure and evolution from seismic anisotropy. Nature, 1988, 335:34-39．
20 Makeyeva L I, Vinnik L P, Roecker S W. Shear-wave splitting and small-scale convection in the continental upper mantle.Nature, 1992, 358: 144-147．
21 Monlar P. S-wave residuals from earthquakes in the Tibetan region and lateral variations in the upper mantle. Earth Planet.Sci. Lett., 1990, 101: 68-77．
2 Barazangi M, Ni J. Propagation characteristics of Pn and Sn beneath the Himalayan arc and Tibetan plateau: Possible evidence forunderthrusting of Indian continental lithosphere beneath Tibet. Geology, 1982, 10: 179-185．
3 Beghoul N, Barazangi M, Isacks B L. Lithospheric structure of Tibet and western north America: mechanisms of uplift and a comparative study. J. Geophys. Res., 1993, 98: 1997-2016．
4 Dewey J, Burke K. Tibetan, Variscan, and Precambrian basement reactivation: Products of continental collision. J. Geol.,1973, 81: 683-692．
5 England P, Houseman G. Finite strain calculations of continental deformation 2． Comparison with the India-Asia collision zones. J. Geophys. Res., 1986, 91: 3664-3676．
6 Monlar P. Structure and tectonics of the Himalaya, constraints and implications of geophysical data. Annual Rev. Earth Planet. Sci., 1984, 12: 489-518．
7 Zhao W L, Yuen, Dave A. Injection of Indian crust into Tibetan lower crust: A temperature-dependent viscous model.Tectonics, 1987, 6 (4) : 505-514．
8 曾融生,朱介寿,周兵等.青藏高原及其东部邻区的三维地震波速度结构与大陆碰撞模型.地震学报,1992(14增刊):523-533。
9 Mattews D, Hirn A. Crust thickening in Himalayas and Caledonides. Nature, 1984, 308: 497-498．
10 Zhao W L, Morgan W J. Uplift of the Tibetan plateau. Tectonics, 1985, 4: 359-369．
11 Zhao W L, Morgan W J. Injection of Indian crust into Tibetan lower crust: A Two-dimensional finite element model study. Tectonics, 1987, 6 (4) : 489-504．
12 England P, Houseman G. Extension during continental convergence,with application to the Tibetan plateau. J. Geophys.Res., 1989, 94: 17561-17579．
13 姜枚,吕庆田,薛光琦等.中、法两国联合进行青藏高原天然地震探测地壳结构的研究.地球物理学报,1994,37(3) :412-413．
14 Vinnik L P,Farra V, Romanowicz B. Azimuthal anisotropy in the Earth from observations of SKS at Geoscope and Nars broad band stations. Bull. Seismol. Soc. Am., 1989, 79: 1542-1558．
15 Silver P G, Chan W W. Shear wave splitting and subcontinental mantle deformation. J. Geophys. Res., 1991, 96: 16429-16454．
16 McNamara D E, Owens T J, Silver P G, Wu F T. Shear wave anisotropy beneath the Tibetan Plateau, J. Geophys. Res.,1994, 99,B7: 13655-13665．
17 丁志峰,曾融生. 青藏高原SKS波各向异性的研究.中国固体地球物理学进展,北京:地震出版社,1994． 162-168页.
18 曾融生,孙为国.青藏高原及其邻区的地震活动性和震源机制以及高原物质东流的讨论.地震学报,1992,(14增刊):534-563．
19 Silver P G, Chan W W. Implications for continental structure and evolution from seismic anisotropy. Nature, 1988, 335:34-39．
20 Makeyeva L I, Vinnik L P, Roecker S W. Shear-wave splitting and small-scale convection in the continental upper mantle.Nature, 1992, 358: 144-147．
21 Monlar P. S-wave residuals from earthquakes in the Tibetan region and lateral variations in the upper mantle. Earth Planet.Sci. Lett., 1990, 101: 68-77．
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