• ISSN 2096-8957
  • CN 10-1702/P

青藏高原东南缘地壳结构与变形机制研究进展

黄周传 吉聪 吴寒婷 石宇通 耿嘉琪 徐弥坚 韩存瑞 徐鸣洁 王良书

引用本文:
Citation:

青藏高原东南缘地壳结构与变形机制研究进展

Review on the crustal structures and deformations in the southeastern margin of the Tibetan Plateau

    Corresponding author: Huang Zhouchuan, huangz@nju.edu.cn ;
  • CLC number: P315

  • 摘要: 新生代青藏高原的隆升改变了整个亚洲的构造格局,对气候、环境均产生了重要的影响,但高原的隆升扩展机制众说纷纭. 青藏高原东南缘作为扩展前缘,其构造演化对了解整个高原的扩展机制具有重要的意义. 本文总结了近年来对青藏高原东南缘地壳结构研究的最新进展,特别是2011年中国地震科学探测台阵计划开展以来,利用密集地震台阵取得的新成果,探讨了青藏高原东南缘地壳的结构与变形机制. 这些研究发现青藏高原的地壳由高原向外围减薄,但在高原边界断裂附近存在地壳厚度突变带;下地壳中存在两个独立的低速异常,一个位于松潘—甘孜块体下方,被高原的边界断裂所围限,另一个位于小江断裂带下方,呈NE-SW向展布. 我们认为青藏高原东南缘下地壳物质被边界(丽江—小金河)断裂所围限,并没有继续向边缘流出,但是地壳挤出产生的应力作用继续向东南方向传递,造成了小江断裂带附近的地壳变形.
  • 图 1  青藏高原及周边地区的构造背景. 紫色、红色、蓝色和灰条线条分别代表研究区的缝合带、逆断层、左行走滑和右行走滑断裂(修改自Styron et al., 2010). 蓝色方框给定了青藏高原东南缘的位置

    Figure 1.  Tectonics in and around the Tibetan Plateau. The purple, red, blue and gray lines denote the suture zones, thrust faults, sinistral and dextral strike-slip faults, respectively (modified from Styron et al., 2010). The blue box marks the location of the southeastern margin of the Tibetan Plateau

    图 2  青藏高原东南缘的地质背景.(a)主要块体和断裂(修改自Han et al., 2020). 蓝色实线表示断层,黑色虚线表示省界.(b)二叠纪玄武岩的分布(修改自Hu et al., 2020). 粉红色线条勾画了峨眉山大火成岩省内带和中带的范围

    Figure 2.  Geological structure in the southeastern margin of the Tibetan Plateau. (a) The major blocks and faults (modified from Han et al., 2020). The solid blue lines denote the faults, the dashed black lines denote the plate boundaries. (b) The distribution of the Permian basalt (modified from Hu et al., 2020). The magenta lines mark the location of the inner and middle zones of the Emeishan large igneous province

    图 3  青藏高原东南缘的GPS速度场与地壳应变场(修改自Bao et al., 2015; Huang et al., 2018).(a)绿色箭头代表相对于华南块体的GPS速度场,黑线表示主要断裂,黑色小球表示震源机制解,粉红色虚线表示大地震分布的东南边界.(b)不同颜色表示应变速率大小,色标在底部. 黑线表示主要断裂,紫线表示主要块体边界. L-X表示丽江—小金河断裂带,XJF表示小江断裂带,RRF表示红河断裂带

    Figure 3.  The GPS velocities and crustal strain in the southeastern margin of the Tibetan Plateau (modified from Bao et al., 2015; Huang et al., 2018). (a) The blue arrows denote the crustal motions with respect to the South China Block. The black lines denote major faults. The beach balls denote the earthquake focal mechanisms. The magenta line denotes a possible frontier of the large earthquakes in the southeastern plateau margin. (b) Different colors denote the strain rate with references shown at the bottom. Black and purple lines denote the faults and major boundaries. The abbreviations are: L-X, Lijiang-Xiaojinhe fault; XJF, Xiaojiang fault; RRF, Red-River fault

    图 4  青藏高原东南缘地壳应力场与大地热流(修改自Huang et al., 2018).(a)蓝色短线表示通过震源机制解反演得到的水平最大主压应力(SH),黄色和红色短线分别表示拉张背景和挤压—走滑背景下的水平拉张方向(Sh). 黑线表示平滑的高程等值线,绿线表示主要块体边界. 黄色阴影区表示下地壳负径向异性(Vsv>Vsh)分布的范围(修改自Xie et al., 2017).(b)不同颜色表示大地热流值,色标在底部. 竖线表示垂直方向的GPS观测值(修改自Pan and Shen, 2017),黑线表示主要断裂,紫色虚线表示主要块体边界,黄线表示下地壳负径向异性(Vsv>Vsh)分布的范围

    Figure 4.  Stress field and heat flow of the southeastern margin of the Tibetan Plateau (modified from Huang et al., 2018). (a) The blue bars denote the maximum horizontal stress orientations (SH) obtained by the inversion of the focal mechanism solutions; the yellow and red bars denote the minimum horizontal stress orientations (Sh) in extensional and compressional-strike-slip environments, respectively. The black lines denote the contour of the topography. The green lines denote the major plate boundaries. The green shades show the region with negative radial anisotropy (Vsv>Vsh) in the lower crust obtained by Xie et al. (2017). (b) Different colors denote different heat flow values with references shown at the bottom. The vertical bars denote the vertical crustal motions (modified from Pan and Shen, 2017). The black lines denote the major faults; The purple lines denote the major plate boundaries. The yellow lines mark the region with negative radial anisotropy (Vsv>Vsh) in the lower crust

    图 5  青藏高原东南缘的地壳厚度和泊松比.(a, b)利用H-k叠加方法获得的平均地壳厚度与泊松比(修改自Wang et al., 2017),黑色虚线表示主要的块体边界.(c, d)利用CCP叠加方向获得的地壳厚度及其水平梯度(修改自Xu et al., 2020). 灰色线条表示主要断裂. 红色和黑色封闭虚线表示莫霍面深度的水平梯度大于15 km/deg的范围,(d)黑色短线表示地壳厚度最大水平梯度的方向

    Figure 5.  Crustal thickness and Poisson's ratio in the southeastern margin of the Tibetan Plateau. (a, b) The crustal thickness and Poisson's ratio obtained by H-k method (modified from Wang et al., 2017). The black lines show the block boundaries. (c, d) The crustal thickness and its horizontal gradient obtained by the CCP method (modified from Xu et al., 2020). Gray lines denote the faults. The closed red and black lines denote the region where the lateral gradient of the Moho depths are larger than 15 km/deg; black bars in (d) show the directions of the maximum horizontal gradients of the crustal thickness

    图 6  青藏高原东南缘下地壳速度与各向异性.(a)接收函数与面波联合反演得到的31 km处的S波速度(修改自Bao et al., 2015),黑线表示主要断裂. (b)面波层析成像获得的32.5 km处的方位各向异性(修改自Bao et al., 2020),短线表示各向异性方向与大小,底图颜色同样表示各向异性强度.(c)面波层析成像获得的25 km的S波速度结构(修改自张智奇等,2020),黑线表示主要断裂,白色虚线表示主要块体边界,圆圈表示5级在上地震分布.(d)P波各向异性层析成像获得的40 km处的P波速度(底图颜色)与方位各向异性(黑色短线)(修改自Huang et al., 2018). 黑线表示主要断裂,紫线表示主要块体边界,红色粗线圈定了结果可靠的区域的范围

    Figure 6.  Lower crustal velocities and anisotropy beneath the southeastern margin of the Tibetan Plateau. (a) The S wave velocities at 31 km depth revealed by joint inversion of receiver function and surface wave (modified from Bao et al., 2015). The black lines denote the faults. (b) The azimuthal anisotropy at 32.5 km depth revealed by surface wave tomography (modified from Bao et al., 2020). The orientations and lengths of the short bars denote the fast velocity directions and strengths of the anisotropy, respectively. The background colors also show the strength of anisotropy with reference shown at the bottom. (c) The S wave velocities at 25 km depth revealed by surface wave tomography (modified from Zhang et al., 2020). The black lines denote the faults. The dashed white lines denote the tectonic boundaries. The circles denote the earthquakes. (d) P wave velocity and anisotropy at 40 km depth (modified from Huang et al., 2018). The orientations and lengths of the short bars denote the fast velocity directions and strengths of the anisotropy, respectively. The black and purple lines denote the faults and block boundaries, respectively. The bold magenta line outlines the region where the results are reliable

    图 7  青藏高原东南缘地壳各向异性.(a)近震S波分裂获得的上地壳各向异性(修改自Shi et al., 2012; Huang et al., 2018). 黑线表示主要断裂,绿线表示主要块体边界.(b)Pms分裂获得的地壳各向异性(修改自Sun et al., 2012; Chen et al., 2013; Cai et al., 2016; Huang et al., 2018).(c, d)利用接收函数马尔科夫链—蒙特卡罗(MCMC)反演获得的上地壳与下地壳的各向异性(修改自Han et al., 2020). 黄线表示断裂分布

    Figure 7.  Crustal anisotropy of the southeastern margin of the Tibetan Plateau. (a) Seismic anisotropy of the upper crust revealed by local S wave splitting (modified from Shi et al., 2012; Huang et al., 2018). The black and green lines denote the faults and major boundaries, respectively. (b) Anisotropy of the whole crust revealed by Pms splitting measurements (modified from Sun et al., 2012; Chen et al., 2013; Cai et al., 2016; Huang et al., 2018). (c, d) The anisotropy in the upper and mid-lower crust obtained by the Markov-Chain-Monte-Carlo inversion of receiver function (modified from Han et al., 2020). The yellow lines denote the faults

  • [1]

    Ando M, Ishikawa Y, Wada H. 1980. S-wave anisotropy in the upper mantle under a volcanic area in Japan[J]. Nature, 286: 43–46. doi: 10.1038/286043a0
    [2]

    Bai D, Unsworth M J, Meju M A, et al. 2010. Crustal deformation of the eastern Tibetan plateau revealed by magnetotelluric imaging[J]. Nature Geoscience, 3: 358-362. doi: 10.1038/ngeo830
    [3]

    Bao X, Song X, Li J. 2015. High-resolution lithospheric structure beneath Mainland China from ambient noise and earthquake surface-wave tomography[J]. Earth and Planetary Science Letters, 417: 132–141. doi: 10.1016/j.jpgl.2015.02.024
    [4]

    Bao X, Song X, Eaton D W, et al. 2020. Episodic lithospheric deformation in eastern Tibet inferred from seismic anisotropy [J]. Geophysical Research Letters, 47(3): e2019GL085721.
    [5]

    Brookfield M. 1993. The Himalayan passive margin from Precambrian to Cretaceous times[J]. Sedimentary Geology, 84 (1-4): 1–35. doi: 10.1016/0037-0738(93)90042-4
    [6]

    Burtman V S, Molnar P H. 1993. Geological and geophysical evidence for deep subduction of continental crust beneath the pamir[M]. Geological Society of America.
    [7]

    Cai Y, Wu J, Fang L, et al. 2016. Crustal anisotropy and deformation of the southeastern margin of the Tibetan Plateau revealed by Pms splitting[J]. Journal of Asian Earth Sciences, 121: 120-126. doi: 10.1016/j.jseaes.2016.02.005
    [8]

    Cao F, Liang C, Zhou L, et al. 2020. Seismic azimuthal anisotropy for the southeastern Tibetan Plateau extracted by Wave Gradiometry analysis[J]. Journal of Geophysical Research: Solid Earth, 125(5): e2019JB018395.
    [9]

    Chen H P, Zhu L B, Su Y J. 2016. Low velocity crustal flow and crust–mantle coupling mechanism in Yunnan, SE Tibet, revealed by 3D S-wave velocity and azimuthal anisotropy[J]. Tectonophysics, 685: 8-20. doi: 10.1016/j.tecto.2016.07.007
    [10]

    Chen M, Huang H, Yao H J, et al. 2014. Low wave speed zones in the crust beneath SE Tibet revealed by ambient noise adjoint tomography[J]. Geophysical Research Letters, 41(2): 334-340. doi: 10.1002/2013GL058476
    [11]

    Chen M, Niu F, Tromp J, et al. 2017. Lithospheric foundering and underthrusting imaged beneath Tibet[J]. Nature Communications, 8(1): 15659. doi: 10.1038/ncomms15659
    [12]

    Chen Y, Zhang Z, Sun C, et al. 2013. Crustal anisotropy from Moho converted Ps wave splitting analysis and geodynamic implications beneath the eastern margin of Tibet and surrounding regions[J]. Gondwana Research, 24(3–4): 946–957. doi: 10.1016/j.gr.2012.04.003
    [13]

    Clark M K, Royden L H. 2000. Topographic ooze: Building the eastern margin of Tibet by lower crustal flow[J]. Geology, 28(8): 703–706. doi: 10.1130/0091-7613(2000)28<703:TOBTEM>2.0.CO;2
    [14]

    Dai A, Tang C C, Liu L, et al. 2020. Seismic attenuation tomography in southwestern China: Insight into the evolution of crustal flow in the Tibetan Plateau[J]. Tectonophysics, 792: 228589. doi: 10.1016/j.tecto.2020.228589
    [15] 邓山泉, 章文波, 于湘伟, 等. 2021. 川滇地区三维P波速度结构研究[J]. 地球物理学进展, 1-17.

    Deng S Q, Zhang W B, Yu X W, et al. 2021. Three-dimensional P-wave velocity structure in Sichuan-Yunnan area[J]. Progress in Geophysics, 1-17 (in Chinese).
    [16] 董蕾, 沈旭章, 钱银苹. 2020. 青藏高原东南缘 Moho面速度密度跃变研究[J]. 地球物理学报, 63(3): 915–927. doi: 10.6038/cjg2020N0168

    Dong L, Shen X Z, Qian Y P. 2020. Study on velocity and density contrasts across the Moho in the southeastern margin of the Tibetan Plateau[J]. Chinese Journal of Geophysics, 63(3): 915-927 (in Chinese). doi: 10.6038/cjg2020N0168
    [17]

    Dueker K G, Sheehan A F. 1997. Mantle discontinuity structure from midpoint stacks of converted P to S waves across the Yellowstone hotspot track[J]. Journal of Geophysical Research: Solid Earth, 102(B4): 8313–8327. doi: 10.1029/96JB03857
    [18]

    England P, Houseman G. 1986. Finite strain calculations of continental deformation: 2. Comparison with the India-Asia collision zone[J]. Journal of Geophysical Research: Solid Earth, 91(B3): 3664–3676. doi: 10.1029/JB091iB03p03664
    [19] 范莉苹, 吴建平, 房立华, 等. 2015. 青藏高原东南缘瑞利波群速度分布特征及其构造意义探讨[J]. 地球物理学报, 58(5): 1555-1567. doi: 10.6038/cjg20150509

    Fan L P, Wu J P, Fang L H, et al. 2015. The characteristic of Rayleigh wave group velocities in the southeastern margin of the Tibetan Plateau and its tectonic implications[J]. Chinese Journal of Geophysics, 58(5): 1555-1567(in Chinese). doi: 10.6038/cjg20150509
    [20]

    Frederick D J, Millner S R, Chang C, et al. 1988. The tectonic evolution of the Tibetan Plateau[J]. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences, 327(1594): 379–413. doi: 10.1098/rsta.1988.0135
    [21]

    French S W, Romanowicz B. 2015. Broad plumes rooted at the base of the Earth's mantle beneath major hotspots[J]. Nature, 525: 95-99. doi: 10.1038/nature14876
    [22]

    Fu Y V, Jia R, Han F, et al. 2018. SH wave structure of the crust and upper mantle in southeastern margin of the Tibetan plateau from teleseismic love wave tomography[J]. Physics of the Earth and Planetary Interiors, 279: 15-20. doi: 10.1016/j.pepi.2018.04.002
    [23] 高原, 石玉涛, 王琼. 2020. 青藏高原东南缘地震各向异性及其深部构造意义[J]. 地球物理学报, 63(3): 802-816. doi: 10.6038/cjg2020O0003

    Gao Y, Shi Y T, Wang Q. 2020. Seismic anisotropy in the southeastern margin of the Tibetan Plateau and its dep tectonic significances[J]. Chinese Journal of Geophysics, 63(3): 802-816 (in Chinese). doi: 10.6038/cjg2020O0003
    [24] 郭希, 陈赞, 李士东, 等. 2017. 峨眉山大火成岩省地壳横波速度结构特征及其动力学意义[J]. 地球物理学报, 60(9): 3338-3351. doi: 10.6038/cjg20170906

    Guo X, Chen Y, Li S D, et al. 2017. Crustal shear-wave velocity structure and its geodynamic implications beneath the Emeishan large igneous province[J]. Chinese Journal of Geophysics, 60(9): 3338-3351 (in Chinese). doi: 10.6038/cjg20170906
    [25]

    Guo Z, Wilson M. 2019. Late Oligocene–early Miocene transformation of post-collisional magmatism in Tibet[J]. Geology, 47(8): 776-780. doi: 10.1130/G46147.1
    [26]

    Han C, Huang Z, Xu M, et al. 2019. Focal mechanism and stress field in the northeastern Tibetan Plateau: insight into layered crustal deformations[J]. Geophysical Journal International, 218, 2066-2078. doi: 10.1093/gji/ggz267
    [27]

    Han C, Xu M, Huang Z, et al. 2020. Layered crustal anisotropy and deformation in the SE Tibetan plateau revealed by Markov-Chain-Monte-Carlo inversion of receiver functions[J]. Physics of the Earth and Planetary Interiors, 306, 106522. doi: 10.1016/j.pepi.2020.106522
    [28]

    Han F Q, Jia R Z, Fu Y V. 2017. Love wave phase velocity models of the southeastern margin of Tibetan Plateau from a dense seismic array[J]. Tectonophysics, 712-713: 125-131. doi: 10.1016/j.tecto.2017.05.013
    [29] 韩明, 李建有, 徐晓雅, 等. 2017. 按方位叠加接收函数分析青藏高原东南缘的地壳各向异性[J]. 地球物理学报, 60(12): 4537-4556. doi: 10.6038/cjg20171202

    Han M, Li J Y, Xu X Y, et al. 2017. Analysis for crustal anisotropy beneath the southeastern margin of Tibet by stacking azimuthal receiver functions[J]. Chinese Journal of Geophysics, 60(12): 4537-4556(in Chinese). doi: 10.6038/cjg20171202
    [30]

    Hu J, Badal J, Yang H, et al. 2018. Comprehensive crustal structure and seismological evidence for lower crustal flow in the southeastern margin of Tibet revealed by receiver functions[J]. Gondwana Research, 55: 42-59. doi: 10.1016/j.gr.2017.11.007
    [31]

    Hu J, Liu J, Song T, et al. 2020. Magma plumbing system of Emeishan large igneous province at the end-Permian: Insights from clinopyroxene compositional zoning and thermobarometry[J]. Minerals, 10: 979. doi: 10.3390/min10110979
    [32]

    Hu S, He L, Wang J. 2000. Heat flow in the continental area of China: A new data set[J]. Earth and Planetary Science Letters, 179(2): 407–419. doi: 10.1016/S0012-821X(00)00126-6
    [33]

    Huang H, Yao H J, Der Hilst R D. 2010. Radial anisotropy in the crust of SE Tibet and SW China from ambient noise interferometry[J]. Journal of Geophysical Research: Solid Earth, 37(21): L21310.
    [34]

    Huang Z, Zhao D, Wang L. 2011. Shear wave anisotropy in the crust, mantle wedge, and subducting Pacific slab under northeast Japan[J]. Geochemistry, Geophysics, Geosystems, 12(1): Q01002.
    [35]

    Huang Z, Wang L, Xu M, et al. 2018. P wave anisotropic tomography of the SE Tibetan Plateau: Evidence for the crustal and upper-mantle deformations[J]. Journal of Geophysical Research: Solid Earth, 123: 8957-8978. doi: 10.1029/2018JB016048
    [36]

    Julia J, Ammon C, Herrmann R, et al. 2000. Joint inversion of receiver function and surface wave dispersion observations[J]. Geophysical Journal International, 143: 99-112. doi: 10.1046/j.1365-246x.2000.00217.x
    [37]

    Kind R, Yuan X, Saul J, et al. 2002. Seismic images of crust and upper mantle beneath Tibet: Evidence for Eurasian plate subduction[J]. Science, 298(5596): 1219–1221. doi: 10.1126/science.1078115
    [38]

    Kong F, Wu J, Liu K H, et al. 2016. Crustal anisotropy and ductile flow beneath the eastern Tibetan Plateau and adjacent areas[J]. Earth and Planetary Science Letters, 442: 72-79. doi: 10.1016/j.jpgl.2016.03.003
    [39]

    Kreemer C, Blewitt G, Klein E C. 2014. A geodetic plate motion and global strain rate model[J]. Geochemistry, Geophysics, Geosystems, 15: 3849–3889. doi: 10.1002/2014GC005407
    [40]

    Lei J S, Li Y, Xie F R, et al. 2014. Pn anisotropic tomography and dynamics under eastern Tibetan plateau[J]. Journal of Geophysical Research: Solid Earth, 119(3): 2174-2198. doi: 10.1002/2013JB010847
    [41]

    Lei J, Zhao D, Xu X, et al. 2019. Is there a big mantle wedge under eastern Tibet?[J]. Physics of the Earth and Planetary Interiors, 292: 100–113. doi: 10.1016/j.pepi.2019.04.005
    [42]

    Li C, Van Der Hilst R D, Engdahl E R, et al. 2008a. A new global model for P wave speed variations in Earth’s mantle [J]. Geochemistry, Geophysics, Geosystems, 9(5): Q05018.
    [43] 李长军, 甘卫军, 秦姗兰, 等. 2019. 青藏高原东南缘南段现今变形特征研究[J]. 地球物理学报, 62(12): 4540–4553. doi: 10.6038/cjg2019M0692

    Li C J, Gan W J, Qin S L, et al. 2019. Present-day deformation characteristics of the southeast borderland of the Tibetan Plateau[J]. Chinese Journal of Geophysics, 62(12): 4540-4553(in Chinese). doi: 10.6038/cjg2019M0692
    [44]

    Li H, Su W, Wang C Y, et al. 2010. Ambient noise Love wave tomography in the eastern margin of the Tibetan plateau[J]. Tectonophysics, 491: 194-204. doi: 10.1016/j.tecto.2009.12.018
    [45]

    Li J T, Song X D, Zhu L P, et al. 2017. Joint inversion of surface wave dispersions and receiver functions with P velocity constraints: application to southeastern Tibet[J]. Journal of Geophysical Research: Solid Earth, 122(9): 7291-7310. doi: 10.1002/2017JB014135
    [46]

    Li M, Zhang S, Wang F, et al. 2016. Crustal and upper-mantle structure of the southeastern Tibetan Plateau from joint analysis of surface wave dispersion and receiver functions[J]. Journal of Asian Earth Sciences, 17: 52-63.
    [47]

    Li X, Bai D H, Ma X B, et al. 2019. Electrical resistivity structure of the Xiaojiang strike-slip fault system (SW China) and its tectonic implications[J]. Journal of Asian Earth Sciences, 176: 57-67. doi: 10.1016/j.jseaes.2019.01.031
    [48]

    Li X, Ma X, Chen Y, et al. 2020a. A plume-modified lithospheric barrier to the southeastward flow of partially molten Tibetan crust inferred from magnetotelluric data[J]. Earth and Planetary Science Letters, 548: 116493. doi: 10.1016/j.jpgl.2020.116493
    [49]

    Li Y, Wu Q, Zhang R, et al. 2008b. The crust and upper mantle structure beneath Yunnan from joint inversion of receiver functions and Rayleigh wave dispersion data[J]. Physics of the Earth and Planetary Interiors, 170(1-2): 134-146. doi: 10.1016/j.pepi.2008.08.006
    [50] 李永华, 徐小明, 张恩会, 等. 2014. 青藏高原东南缘地壳结构及云南鲁甸、景谷地震深部孕震环境[J]. 地震地质, 36(4): 1204-1216. doi: 10.3969/j.issn.0253-4967.2014.04.021

    Li Y H, Xu X M, Zhang E H, et al. 2014. Three-dimensional crust structure beneath SE Tibetan Plateau and its seismotectonic implications for the Ludian and Jinggu earthquakes[J]. Seismology and Geology, 36(4): 1204-1216 (in Chinese). doi: 10.3969/j.issn.0253-4967.2014.04.021
    [51]

    Li Z J, Wang Y, Gan W J, et al. 2020b. Diffuse deformation in the SE Tibetan Plateau: new insights from geodetic observations[J]. Journal of Geophysical Research: Solid Earth, 125(10): e2020JB019383.
    [52]

    Ling Y, Zheng T, He Y, et al. 2020. Response of Yunnan crustal structure to eastward growth of the Tibet Plateau and subduction of the India plate in Cenozoic[J]. Tectonophysics, 797: 228661. doi: 10.1016/j.tecto.2020.228661
    [53]

    Liu H, Niu F. 2011. Estimating crustal seismic anisotropy with a joint analysis of radial and transverse receiver function data[J]. Geophysical Journal International, 188(1): 144–164.
    [54]

    Liu Q Y, Robert D, van der Hilst, et al. 2014. Eastward expansion of the Tibetan Plateau by crustal flow and strain partitioning across faults[J]. Nature Geoscience, 7: 361-365. doi: 10.1038/ngeo2130
    [55] 鲁来玉, 何正勤, 丁志峰, 等. 2014. 基于背景噪声研究云南地区面波速度非均匀性和方位各向异性[J]. 地球物理学报, 57(3): 822-836. doi: 10.6038/cjg20140312

    Lu L Y, He Z Q, Ding Z F, et al. 2014. Azimuth anisotropy and velocity heterogeneity of Yunnan area based on seismic ambient noise[J]. Chinese Journal of Geophysics, 57(3): 822-836(in Chinese). doi: 10.6038/cjg20140312
    [56] 马宏生, 张国民, 刘杰, 等. 2007. 云南及邻区应力应变场分区耦合特性初步研究[J]. 地震学报, 29(2): 130-141. doi: 10.3321/j.issn:0253-3782.2007.02.002

    Ma H S, Zhang G M, Liu J., et al. 2007. Coupling characteristics of stress and strain at different layers of different sub-regions in Yunnan and it adjacent areas[J]. Acta Seismologica Sinica, 29(2): 130-141(in Chinese). doi: 10.3321/j.issn:0253-3782.2007.02.002
    [57]

    Mainprice D. 2015. Seismic Anisotropy of the Deep Earth from a Mineral and Rock Physics Perspective[M]. Elsevier: Treatise on Geophysics, 487–538.
    [58]

    Molnar P, Tapponnier P. 1975. Cenozoic tectonics of Asia: Effects of a continental collision[J]. Science, 189(4201): 419–426. doi: 10.1126/science.189.4201.419
    [59] 潘佳铁, 李永华, 吴庆举, 等. 2015. 青藏高原东南部地区瑞雷波相速度层析成像[J]. 地球物理学报, 58(11): 3993-4006. doi: 10.6038/cjg20151109

    Pan J T, Li Y H, Wu Q J, et al. 2015. Phase velocity maps of Rayleigh waves in the southeast Tibetan plateau[J]. Chinese Journal of Geophysics, 58(11): 3993-4006 (in Chinese). doi: 10.6038/cjg20151109
    [60]

    Pan Y, Shen W B. 2017. Contemporary crustal movement of southeastern Tibet: Constraints from dense GPS measurements[J]. Scientific Reports, 7: 45348. doi: 10.1038/srep45348
    [61]

    Qian H, Mechie J, Li H, et al. 2018. Structure of the crust and mantle down to 700 km depth beneath the Longmenshan from P receiver functions[J]. Tectonics, 37(6): 1688–1708. doi: 10.1029/2017TC004726
    [62]

    Rawlinson N, Pozgay S, Fishwick S. 2010. Seismic tomography: A window into deep Earth[J]. Physics of the Earth and Planetary Interiors, 178 (3–4): 101–135.
    [63]

    Replumaz A, Guillot S, Villaseñor A, et al. 2013. Amount of Asian lithospheric mantle subducted during the India/Asia collision[J]. Gondwana Research, 24(3-4): 936–945. doi: 10.1016/j.gr.2012.07.019
    [64]

    Replumaz A, Funiciello F, Reitano R, et al. 2016. Asian collisional subduction: A key process driving formation of the Tibetan Plateau[J]. Geology, 44(11): 943–946. doi: 10.1130/G38276.1
    [65]

    Romanowicz B. 2003. Global mantle tomography: progress status in the past 10 years[J]. Annual Review of Earth and Planetary Sciences, 31: 303-328. doi: 10.1146/annurev.earth.31.091602.113555
    [66]

    Royden L H, Burchfiel B C, Van Der Hilst R D. 2008. The geological evolution of the Tibetan Plateau[J]. Science, 321(5892): 1054–1058. doi: 10.1126/science.1155371
    [67]

    Shapiro N M, Ritzwoller M H, Molnar P. 2004. Thinning and flow of Tibetan crust constrained by seismic anisotropy[J]. Science, 305(5681): 233–236. doi: 10.1126/science.1098276
    [68]

    Shapiro N, Campillo M, Stehly L. et al. 2005. High-resolution surface-wave tomography from ambient seismic noise[J]. Science, 307: 1615-1618. doi: 10.1126/science.1108339
    [69]

    Shen Z, Lv J, Burgmann R. 2005. Contemporary crustal deformation around the southeast borderland of the Tibetan Plateau[J]. Journal of Geophysical Research: Solid Earth, 110: B11409.
    [70] 石玉涛, 高原, 吴晶, 等. 2006. 云南地区地壳介质各向异性——快剪切波偏振特性. 地震地质, 28: 574-585.

    Shi Y T, Gao Y, Wu J, et al. 2006. Seismic anisotropy of the crust in Yunnan, China: Polarizations of fast shear-waves[J]. Acta Seismologica Sinica, 28(6): 574-585 (in Chinese).
    [71]

    Shi Y, Gao Y, Su Y, et al. 2012. Shear-wave splitting beneath Yunnan area of Southwest China[J]. Earthquake Science, 25(1): 25–34. doi: 10.1007/s11589-012-0828-4
    [72]

    Silver P G, Chan W W. 1991. Shear wave splitting and subcontinental mantle deformation[J]. Journal of Geophysical Research: Solid Earth, 96: 16429–16454. doi: 10.1029/91JB00899
    [73]

    Styron R, Taylor M, Okoronkwo K. 2010. Database of active structures from the Indo-Asian collision[J]. Eos, 91(20): 181–182.
    [74]

    Sun Y, Niu F, Liu H, et al. 2012. Crustal structure and deformation of the SE Tibetan plateau revealed by receiver function data[J]. Earth and Planetary Science Letters, 349-350: 186-197. doi: 10.1016/j.jpgl.2012.07.007
    [75] 太龄雪, 高原, 刘庚, 等. 2015. 利用中国地震科学台阵研究青藏高原东南缘地壳各向异性: 第一期观测资料的剪切波分裂特征[J]. 地球物理学报, 58(11): 4079-4091. doi: 10.6038/cjg20151116

    Tai L X, Gao Y, Liu G, et al. 2015. Crustal seismic anisotropy in the southeastern margin of Tibetan Plateau by ChinAray data: Shear-wave splitting from temporary observations of the first phase[J]. Chinese Journal of Geophysics, 58(11): 4079-4091(in Chinese). doi: 10.6038/cjg20151116
    [76] 唐晗晗, 郭良辉, 方圆. 2020. 青藏高原东南缘热流估算及与地震活动相关性分析[J]. 地球物理学报, 63(3): 1056-1069. doi: 10.6038/cjg2019N0045

    Tang H H, Guo L H, Fang Y. 2020. Estimation of heat flow in southeastern margin of Tibetan Plateau and its analysis of the correlation with earthquake activity[J]. Chinese Journal of Geophysics, 63(3): 1056-1069 (in Chinese). doi: 10.6038/cjg2019N0045
    [77]

    Tapponnier P, Xu Z, Roger F, et al. 2001. Oblique stepwise rise and growth of the Tibet plateau[J]. Science, 294 (5547): 1671–1677. doi: 10.1126/science.105978
    [78]

    Wang C Y, Zhu L P, Lou H, et al. 2010. Crustal thicknesses and Poisson's ratios in the eastern Tibetan Plateau and their tectonic implications[J]. Journal of Geophysical Research: Solid Earth, 115(B11): B11301. doi: 10.1029/2010JB007527
    [79]

    Wang H, Huang Z. 2020. Seismic tomography in the southern margin of the Sichuan Basin: Insight into the plateau-craton interaction and seismotectonics in the SE Tibetan Plateau[J]. Journal of Asian Earth Sciences, 199, 104464. doi: 10.1016/j.jseaes.2020.104464
    [80] 王怀富, 吴建平, 周仕勇, 等. 2020. 青藏高原东南缘基于程函方程的面波方位各向异性研究[J]. 地球物理学报, 63(3): 1070–1084. doi: 10.6038/cjg2020N0104

    Wang H F, Wu J P, Zhou S Y, et al. 2020. Rayleigh wave azimuthal anisotropy in the Southeastern Tibetan Plateau from Eikonal tomography[J]. Chinese Journal of Geophysics, 63(3): 1070-1084(in Chinese). doi: 10.6038/cjg2020N0104
    [81]

    Wang J, Zhao D. 2008. P-wave anisotropic tomography beneath Northeast Japan[J]. Physics of the Earth and Planetary Interiors, 170(1-2): 115–133. doi: 10.1016/j.pepi.2008.07.042
    [82]

    Wang M, Shen Z K. 2020. Present-day crustal deformation of continental China derivated from GPS and its tectonic implications[J]. Journal of Geophysical Research: Solid Earth, 125(2): e2019JB018774.
    [83] 王琼, 高原. 2014. 青藏东南缘背景噪声的瑞利波相速度层析成像及强震活动[J]. 中国科学: 地球科学, 44(11): 2440-2450.

    Wang Q, Gao Y. 2014. Rayleigh wave phase velocity tomography and strong earthquake activity on the southeastern front of the Tibetan Plateau[J]. Science China: Earth Sciences, 57: 2532–2542(in Chinese).
    [84]

    Wang W L, Wu J P, Fang L H, et al. 2017. Crustal thickness and Poisson's ratio in southwest China based on data from dense seismic arrays[J]. Journal of Geophysical Research: Solid Earth, 122(9): 7219-7235. doi: 10.1002/2017JB013978
    [85]

    Wang W, Wu J, Fang L, et al. 2014. S wave velocity structure in southwest China from surface wave tomography and receiver functions[J]. Journal of Geophysical Research: Solid Earth, 119(2): 1061-1078. doi: 10.1002/2013JB010317
    [86]

    Wang Z, Zhao D, Gao R, et al. 2019. Complex subduction beneath the Tibetan plateau: A slab warping model[J]. Physics of the Earth and Planetary Interiors, 292: 42–54. doi: 10.1016/j.pepi.2019.04.007
    [87] 韦伟, 孙若昧, 石耀霖. 2010. 青藏高原东南缘地震层析成像及汶川地震成因探讨[J]. 中国科学: 地球科学, 40(7): 831-839.

    Wei W, Sun R M, Shi Y L. 2010. P-wave tomographic images beneath southeastern Tibet: Investigating the mechanism of the 2008 Wenchuan earthquake[J]. Science China: Earth Sciences, 40(7): 831-839.
    [88]

    Wen L, Badal J, Hu J. 2019. Anisotropic H-k stacking and (revisited) crustal structure in the southeastern margin of Tibet[J]. Journal of Asian Earth Sciences, 169: 93-104. doi: 10.1016/j.jseaes.2018.07.028
    [89]

    Wu J, Zhang Z, 2012. Spatial distribution of seismic layer, crustal thickness, and VP/VS ratio in the Permian Emeishan Mantle Plume region[J]. Gondwana Research, 22(1): 127-139. doi: 10.1016/j.gr.2011.10.007
    [90] 吴建平, 杨婷, 王未来, 等. 2013. 小江断裂带周边地区三维P波速度结构及其构造意义[J]. 地球物理学报, 56(7): 2257-2267. doi: 10.6038/cjg20130713

    Wu J P, Yang T, Wang W L, et al. 2013. The three-dimensional P-wave velocity structure around Xiaojiang fault system and its tectonic implications[J]. Chinese Journal of Geophysics, 56(7): 2257-2267(in Chinese). doi: 10.6038/cjg20130713
    [91]

    Wu T, Zhang S, Li M, et al. 2016. Two crustal flowing channels and volcanic magma migration underneath the SE margin of the Tibetan Plateau as revealed by surface wave tomography[J]. Journal of Asian Earth Sciences, 132: 25-39. doi: 10.1016/j.jseaes.2016.09.017
    [92]

    Wu T, Zhang S, Li M, et al. 2019. Complex deformation within the crust and upper mantle beneath SE Tibet revealed by anisotropic Rayleigh wave tomography[J]. Physics of the Earth and Planetary Interiors, 286: 165-178. doi: 10.1016/j.pepi.2018.12.002
    [93]

    Xie J, Ritzwoller M H, Shen W, et al. 2013. Crustal radial anisotropy across Eastern Tibet and the Western Yangtze Craton[J]. Journal of Geophysical Research: Solid Earth, 118: 4226-4252. doi: 10.1002/jgrb.50296
    [94]

    Xie J, Ritzwoller M H, Shen W, et al. 2017. Crustal anisotropy across eastern Tibet and surroundings modeled as a depth-dependent tilted hexagonally symmetric medium[J]. Geophysical Journal International, 209(1): 466–491.
    [95]

    Xu M, Huang Z, Wang L, et al. 2020. Sharp lateral Moho variations across the SE Tibetan margin and their implications for plateau growth[J]. Journal of Geophysical Research: Solid Earth, 125(5): e2019JB018117.
    [96]

    Xu X M, Ding Z F, Shi D N, et al. 2013. Receiver function analysis of crustal structure beneath the eastern Tibetan plateau[J]. Journal of Asian Earth Sciences, 73: 121-127. doi: 10.1016/j.jseaes.2013.04.018
    [97]

    Xu Y, Liu J, Liu F, et al. 2005. Crust and upper mantle structure of the Ailao Shan-Red River fault zone and adjacent regions[J]. Science in China: Earth Sciences, 48: 156-164.
    [98] 徐义刚, 钟孙霖. 2001. 峨眉山大火成岩省: 地幔柱活动的证据及其熔融条件. 地球化学, 30: 1-9. doi: 10.3321/j.issn:0379-1726.2001.01.002

    Xu Y G, Zhong S L, 2001. The Emeishan large igneous province: Evidence for mantle plume activity and melting conditions[J]. Geochimica, 30(1): 1-9 (in Chinese). doi: 10.3321/j.issn:0379-1726.2001.01.002
    [99]

    Xu Z, Huang Z, Wang L, et al. 2016. Crustal stress field in Yunnan: implication for crust-mantle coupling[J]. Earthquake Science, 29, 105-115. doi: 10.1007/s11589-016-0146-3
    [100] 许志琴, 杨经绥, 侯增谦, 等. 2016. 青藏高原大陆动力学研究若干进展[J]. 中国地质, 43: 1-42. doi: 10.3969/j.issn.1000-3657.2016.01.001

    Xu Z Q, Yang J S, Hou Z Q, et al. 2016. The progress in the study of continental dynamics of the Tibetan Plateau[J]. Geology in China 43(1): 1-42(in Chinese). doi: 10.3969/j.issn.1000-3657.2016.01.001
    [101]

    Yang H, Peng H, Hu J, 2017. The lithospheric structure beneath southeast Tibet revealed by P and S receiver functions[J]. Journal of Asian Earth Sciences, 138: 62-71. doi: 10.1016/j.jseaes.2017.02.001
    [102]

    Yang Y, Ritzwoller M H, Zheng Y, et al. 2012. A synoptic view of the distribution and connectivity of the mid-crustal low velocity zone beneath Tibet[J]. Journal of Geophysical Research: Solid Earth, 117: B04303.
    [103]

    Yao H, Beghein C, Van Der Hilst R D. 2008. Surface wave array tomography in SE Tibet from ambient seismic noise and two-station analysis-II. Crustal and upper-mantle structure[J]. Geophysical Journal International, 173(1): 205–219. doi: 10.1111/j.1365-246X.2007.03696.x
    [104]

    Yao H J, Robert D, van der Hilst, et al. 2010. Heterogeneity and anisotropy of the lithosphere of SE Tibet from surface wave array tomography[J]. Journal of Geophysical Research: Solid Earth, 115(B12): B12307. doi: 10.1029/2009JB007142
    [105]

    Yin A. 2010. Cenozoic tectonic evolution of Asia: A preliminary synthesis[J]. Tectonophysics, 488(1-4): 293–325. doi: 10.1016/j.tecto.2009.06.002
    [106]

    Yin A, Harrison T M. 2000. Geologic Evolution of the Himalayan-Tibetan Orogen[J]. Annual Review of Earth and Planetary Sciences, 28(1): 211–280 doi: 10.1146/annurev.earth.28.1.211
    [107]

    Yu N, Unsworth M, Wang X B, et al. 2020. New insights into crustal and mantle flow beneath the Red River Fault Zone and adjacent areas on the southern margin of the Tibetan Plateau revealed by a 3-D magnetotelluric study[J]. Journal of Geophysical Research: Solid Earth, 125(10): e2020JB019396.
    [108]

    Zhang B, Zhang S, Wu T, et al. 2018. Upper crustal anisotropy from local shear-wave splitting and crust-mantle coupling of Yunnan, SE margin of Tibetan Plateau[J]. Geodesy and Geodynamics, 9(4): 302-311. doi: 10.1016/j.geog.2018.01.004
    [109] 张洪双, 田小波, 滕吉文. 2009. 接收函数方法估计Moho倾斜地区的地壳速度比[J]. 地球物理学报, 52(5): 1243-1252. doi: 10.3969/j.issn.0001-5733.2009.05.013

    Zhang H X, Tian X B, Teng J W, 2009. Estimation of crustal VP/VS with dipping Moho from receiver functions. Chinese Journal of Geophysics, 52(5): 1243-1252 (in Chinese). doi: 10.3969/j.issn.0001-5733.2009.05.013
    [110]

    Zhang X, Wang Y. 2009. Crustal and upper mantle velocity structure in Yunnan, Southwest China[J]. Tectonophysics, 471: 171-185. doi: 10.1016/j.tecto.2009.02.009
    [111]

    Zhang Z, Wang Y, Chen Y, et al. 2009. Crustal structure across Longmenshan fault belt from passive source seismic profiling[J]. Geophysical Research Letters, 36(17): L17310. doi: 10.1029/2009GL039580
    [112] 张智奇, 姚华建, 杨妍. 2020. 青藏高原东南缘地壳上地幔三维S波速度结构及动力学意义[J]. 中国科学: 地球科学, 50(9): 1242-1258.

    Zhang Z Q, Yao H J, Yang Y. 2020. Shear wave velocity structure of the crust and upper mantle in Southeastern Tibet and its geodynamic implications[J]. Science China: Earth Sciences, 50(9): 1242-1258 (in Chinese).
    [113]

    Zhao J, Yuan X, Liu H, et al. 2010. The boundary between the Indian and Asian tectonic plates below Tibet[J]. Proceedings of the National Academy of Sciences of the United States of America, 107(25): 11229–11233. doi: 10.1073/pnas.1001921107
    [114]

    Zhao L, Luo Y, Liu T, et al. 2013a. Earthquake focal mechanisms in Yunnan and their inference on the regional stress field[J]. Bullutin of Seismological Society of America, 103: 2498-2507. doi: 10.1785/0120120309
    [115]

    Zhao L F, Xie X B, He J K, et al. 2013b. Crustal flow pattern beneath the Tibetan Plateau constrained by regional Lg-wave Q tomography[J]. Earth and Planetary Science Letters, 383: 113-122. doi: 10.1016/j.jpgl.2013.09.038
    [116]

    Zhao Y, Guo L H, Guo Z, et al. 2020. High resolution crustal model of SE Tibet from joint inversion of seismic P wave travel-times and Bouguer gravity anomalies and its implication for the crustal channel flow[J]. Tectonophysics, 792: 228580. doi: 10.1016/j.tecto.2020.228580
    [117] 郑晨, 丁志峰, 宋晓东. 2016. 利用面波频散与接收函数联合反演青藏高原东南缘地壳上地幔速度结构[J]. 地球物理学报, 59(9): 3223-3236. doi: 10.6038/cjg20160908

    Zheng C, Ding Z F, Song X D. 2016. Joint inversion of surface wave dispersion and receiver functions for crustal and uppermost mantle structure in Southeast Tibetan Plateau[J]. Chinese Journal of Geophysics, 59(9): 3223-3236 (in Chinese). doi: 10.6038/cjg20160908
    [118]

    Zheng T, Ding Z, Ning J, et al. 2018. Crustal azimuthal anisotropy beneath the southeastern Tibetan Plateau and its geodynamic implications[J]. Journal of Geophysical Research: Solid Earth, 123: 9733-9749. doi: 10.1029/2018JB015995
    [119]

    Zhu L, Kanamori H. 2000. Moho depth variation in southern California from teleseismic receiver functions[J]. Journal of Geophysical Research: Solid Earth, 105(B2): 2969. doi: 10.1029/1999JB900322
  • 加载中
图(7)
计量
  • 文章访问数:  250
  • HTML全文浏览量:  166
  • PDF下载量:  51
  • 被引次数: 0
出版历程
  • 收稿日期:  2021-02-02
  • 网络出版日期:  2021-03-09
  • 刊出日期:  2021-05-01

青藏高原东南缘地壳结构与变形机制研究进展

摘要: 新生代青藏高原的隆升改变了整个亚洲的构造格局,对气候、环境均产生了重要的影响,但高原的隆升扩展机制众说纷纭. 青藏高原东南缘作为扩展前缘,其构造演化对了解整个高原的扩展机制具有重要的意义. 本文总结了近年来对青藏高原东南缘地壳结构研究的最新进展,特别是2011年中国地震科学探测台阵计划开展以来,利用密集地震台阵取得的新成果,探讨了青藏高原东南缘地壳的结构与变形机制. 这些研究发现青藏高原的地壳由高原向外围减薄,但在高原边界断裂附近存在地壳厚度突变带;下地壳中存在两个独立的低速异常,一个位于松潘—甘孜块体下方,被高原的边界断裂所围限,另一个位于小江断裂带下方,呈NE-SW向展布. 我们认为青藏高原东南缘下地壳物质被边界(丽江—小金河)断裂所围限,并没有继续向边缘流出,但是地壳挤出产生的应力作用继续向东南方向传递,造成了小江断裂带附近的地壳变形.

English Abstract

    • 青藏高原主体是由多个块体于三叠纪至侏罗纪碰撞拼贴形成的(Frederick et al., 1988; Burtman et al., 1993; Yin and Harrison, 2000; 许志琴等, 2016)(图1). 新生代以来,印度板块一直在向欧亚板块下方俯冲,形成喜马拉雅造山带和青藏高原(Molnar and Tapponnier, 1975; England et al., 1986; Brookfield, 1993; Yin and Harrison, 2000Royden et al., 2008; Yin, 2010; Replumaz et al., 2016; 许志琴等, 2016). 地震层析成像研究表明,印度板块岩石圈的俯冲深度达 200 km(Li et al., 2008b; Replumaz et al., 2013; Chen et al., 2017; Lei et al., 2019; Wang et al., 2019). 接收函数结果表明不仅印度板块向北俯冲,欧亚板块也向南俯冲(Kind et al., 2002; Zhao et al., 2010). 青藏高原地表在印度板块和欧亚板块的对向挤压碰撞下持续抬升,地壳明显增厚(Kind et al., 2002).

      图  1  青藏高原及周边地区的构造背景. 紫色、红色、蓝色和灰条线条分别代表研究区的缝合带、逆断层、左行走滑和右行走滑断裂(修改自Styron et al., 2010). 蓝色方框给定了青藏高原东南缘的位置

      Figure 1.  Tectonics in and around the Tibetan Plateau. The purple, red, blue and gray lines denote the suture zones, thrust faults, sinistral and dextral strike-slip faults, respectively (modified from Styron et al., 2010). The blue box marks the location of the southeastern margin of the Tibetan Plateau

      地质调查、物理模拟、数值模拟等研究为新生代青藏高原的演化提出了不同的模型. 青藏高原发育了多条缝合带(图1),如雅鲁藏布缝合带、班公怒江缝合带、金沙江缝合带和昆仑断裂带(Yin and Harrison, 2000; Yin, 2010). 这些断裂自新生代以来随着印度板块向北持续挤压被再次激活,在高原内部形成多期次俯冲(Tapponnier et al., 2001),发育了大范围的火山岩(Guo and Wilson, 2019). 除俯冲外,组成高原的次级块体沿主要走滑断裂向东挤出. Molnar和Tapponnier(1975)通过物理模拟还原了上述演化过程,他们认为位于青藏东南缘的红河断裂、鲜水河断裂和小江断裂等大型走滑断裂在高原挤出过程中扮演了重要角色,东南方向的挤出沿着这些走滑断裂进行. GPS观测显示青藏高原东南缘向东南方向存在顺时针旋转,鲜水河—安宁河—小江断裂带也展现出较大的应变速率(Wang and Shen, 2020).

      Clark和Royden(2000)通过对地表高程的分析,结合重力均衡理论提出了中下地壳流模型,认为在高原隆生后产生的横向重力差异使中下地壳产生塑性流动,并向高原周缘膨胀形成较高的地形,这与地球物理研究在青藏高原的中下地壳观测到的低电阻率(Bai et al., 2010)、低地震波波速(Yao et al., 2008; Yang et al., 2012; Liu et al., 2014)和高衰减(Zhao et al., 2013a)所表征的软弱层一致. Royden等(2008)进一步研究认为青藏高原周缘的块体强度对地壳流模型和地表高程有密切的关系. 地壳流向东流动受到刚性的四川盆地阻挡形成了从松潘—甘孜块体到四川盆地陡峭的高程和莫霍面梯度;而向东南方向的流动受到相对软弱的阻挡,地形和莫霍面变化相对平缓.

      青藏高原东南缘在青藏高原构造演化中扮演了重要作用. 两种模型都指出青藏高原的地壳物质存在向东的运移,而四川盆地在其中都起到了阻挡作用. 不同的是,挤出构造模型关注块体之间的相互作用(Tapponnier et al., 2001),对块体内的流变学性质和应力状态缺少解释; 下地壳流模型将青藏高原看作一个整体,以一种理想的流变学模型解释了青藏高原东南缘在地壳缩短量很小的情况下的隆升方式(Royden et al., 2008),然而它忽略了新生代以前的构造活动以及大型断层对高原隆升的影响.

      青藏高原东南缘的地质单元包括属于青藏高原的松潘—甘孜块体、扬子块体西南缘和印支块体北缘(图2a),分别以丽江—小金河断裂和红河断裂带为构造边界,也即扬子块体的西南边界,这三个地质体在岩石组成上具有较大的差异,尤其是在扬子块体西缘出露了大范围的二叠纪玄武岩(图2b)(徐义刚和钟孙霖,2001),可能代表了古地幔柱的存在. 但是现今青藏高原的东南缘并不以先存的断裂带为边界,扬子块体的西南缘向东一直延伸到小江断裂带,被卷入到青藏高原的活动构造中(图3). GPS观测结果显示整个青藏高原东南缘都绕着东构造结做顺时针旋转(图3a)(Shen et al., 2005; Wang and Shen, 2020),但由GPS获得的地壳应变结果显示,主要的地壳应变集中在边界断裂带,特别是鲜水河—安宁河—泽木河—小江断裂带,丽江—小金河的地壳应变也比较大,但在红河断裂带以及块体内部的地壳应变都比较弱(图3b)(Kreemer et al., 2014; Pan and Shen, 2017; Wang and Shen, 2020). 通过震源机制解反演的地壳应力场表明青藏高原东南缘的应力场分区特征明显,在高原内部的松潘—甘孜块体以近南北向的拉张为主,而在其边缘则以与高程梯度一致的挤压为主,即呈扇形展布(图4a)(Zhao et al., 2013a; Xu et al., 2016; Han et al., 2019). 大地热流也存在强烈的横向差异,高地热流区与局部的地壳抬升相关(图4b)(Hu et al., 2000; 唐晗晗等, 2020).

      图  2  青藏高原东南缘的地质背景.(a)主要块体和断裂(修改自Han et al., 2020). 蓝色实线表示断层,黑色虚线表示省界.(b)二叠纪玄武岩的分布(修改自Hu et al., 2020). 粉红色线条勾画了峨眉山大火成岩省内带和中带的范围

      Figure 2.  Geological structure in the southeastern margin of the Tibetan Plateau. (a) The major blocks and faults (modified from Han et al., 2020). The solid blue lines denote the faults, the dashed black lines denote the plate boundaries. (b) The distribution of the Permian basalt (modified from Hu et al., 2020). The magenta lines mark the location of the inner and middle zones of the Emeishan large igneous province

      图  3  青藏高原东南缘的GPS速度场与地壳应变场(修改自Bao et al., 2015; Huang et al., 2018).(a)绿色箭头代表相对于华南块体的GPS速度场,黑线表示主要断裂,黑色小球表示震源机制解,粉红色虚线表示大地震分布的东南边界.(b)不同颜色表示应变速率大小,色标在底部. 黑线表示主要断裂,紫线表示主要块体边界. L-X表示丽江—小金河断裂带,XJF表示小江断裂带,RRF表示红河断裂带

      Figure 3.  The GPS velocities and crustal strain in the southeastern margin of the Tibetan Plateau (modified from Bao et al., 2015; Huang et al., 2018). (a) The blue arrows denote the crustal motions with respect to the South China Block. The black lines denote major faults. The beach balls denote the earthquake focal mechanisms. The magenta line denotes a possible frontier of the large earthquakes in the southeastern plateau margin. (b) Different colors denote the strain rate with references shown at the bottom. Black and purple lines denote the faults and major boundaries. The abbreviations are: L-X, Lijiang-Xiaojinhe fault; XJF, Xiaojiang fault; RRF, Red-River fault

      图  4  青藏高原东南缘地壳应力场与大地热流(修改自Huang et al., 2018).(a)蓝色短线表示通过震源机制解反演得到的水平最大主压应力(SH),黄色和红色短线分别表示拉张背景和挤压—走滑背景下的水平拉张方向(Sh). 黑线表示平滑的高程等值线,绿线表示主要块体边界. 黄色阴影区表示下地壳负径向异性(Vsv>Vsh)分布的范围(修改自Xie et al., 2017).(b)不同颜色表示大地热流值,色标在底部. 竖线表示垂直方向的GPS观测值(修改自Pan and Shen, 2017),黑线表示主要断裂,紫色虚线表示主要块体边界,黄线表示下地壳负径向异性(Vsv>Vsh)分布的范围

      Figure 4.  Stress field and heat flow of the southeastern margin of the Tibetan Plateau (modified from Huang et al., 2018). (a) The blue bars denote the maximum horizontal stress orientations (SH) obtained by the inversion of the focal mechanism solutions; the yellow and red bars denote the minimum horizontal stress orientations (Sh) in extensional and compressional-strike-slip environments, respectively. The black lines denote the contour of the topography. The green lines denote the major plate boundaries. The green shades show the region with negative radial anisotropy (Vsv>Vsh) in the lower crust obtained by Xie et al. (2017). (b) Different colors denote different heat flow values with references shown at the bottom. The vertical bars denote the vertical crustal motions (modified from Pan and Shen, 2017). The black lines denote the major faults; The purple lines denote the major plate boundaries. The yellow lines mark the region with negative radial anisotropy (Vsv>Vsh) in the lower crust

      地壳深部结构是探讨青藏高原东南缘不同地质模型的基础. 2011年开始,中国地震局与多方机构合作开展了中国地震科学探测台阵(ChinArray)计划,第一期在青藏高原东南缘布设了350个宽频带地震台站,使区域内台阵平均台站间距约为30 km. 随着近年来地球物理特别是地震学的发展,揭示了越来越清晰的地壳深部图像,包括地壳厚度的变化趋势、地壳平均的波速比/泊松比、地震波速度与衰减因子、电阻率、地壳内的各向异性与构造变形等,这些结果为探讨青藏高原东南缘的隆升和扩展模型提供了重要的证据.

    • 地壳厚度变化是约束地壳结构的重要因素,在下地壳流模型中,通过莫霍面起伏可以推测挤压前缘块体的性质. 地震波接收函数是研究地壳上地幔速度界面的重要手段. 远震P波传播至莫霍面转换为S波,Ps转换波和直达P波的时差与莫霍面深度和地壳速度有关,目前常用的研究方法有H-k叠加和共转换点叠加. H-k叠加方法假定地壳平均速度,网格搜索Ps、PpPs和PsPs+PpSs震相振幅的加权平均的最大值,获得对应的莫霍面深度和地壳平均波速比(Zhu and Kanamori, 2000). Xu等(2020)通过理论接收函数研究发现,当台站下方界面倾斜时,H-k叠加计算的莫霍面深度将小于实际深度,计算的地壳平均波速比偏大. 共转换点(CCP)叠加方法利用假设的P波和S波速度结构将接收函数按理论射线参数和反方位角反投射到射线路径上,再线性叠加每个网格内的接收函数振幅(Dueker and Sheehan, 1997),其横向分辨率优于H-k叠加方法,但同时也需要更多的地震射线.

      Li等(2008a)利用接收函数获得了青藏高原东南缘的地壳结构,发现地壳厚度从云南北部的56 km缓慢减薄至云南南部的32 km,其中下地壳厚度横向变化明显,且变化趋势与莫霍面深度存在正相关,与利用广角地震剖面数据揭示的地壳结构一致(Zhang and Wang, 2009). Wang等(2010)利用接收函数确定了青藏高原东部的地壳厚度与平均泊松比,发现研究区内的地壳厚度差异可达~30 km,主要断裂带附近的泊松比一般大于0.3. 张洪双等(2009)利用考虑莫霍面倾斜的接收函数方法对青藏高原东南缘地壳厚度和速度比结构进行研究,发现研究区内地壳存在明显非均匀性,松潘—甘孜块体平均地壳厚度约为60 km,四川盆地西缘约为47 km,扬子块体约为34 km,松潘—甘孜块体与扬子块体相邻部位地壳平均地震波速度比普遍偏高,且四川盆地西侧发现一个绕盆地边缘的弧形高波速比异常区(>1.8). Xu等(2013)利用接收函数方法发现青藏高原内部地壳厚度可达~70 km,泊松比范围为0.24~0.27,川滇块体泊松比较高(>0.27),扬子克拉通地壳厚度仅为~27 km,泊松比小于0.24. Sun等(2012)计算了青藏高原东南缘的地壳厚度与波速比,发现地壳厚度从青藏高原向其东南缘减薄,平均波速比从青藏高原的1.79减少为云贵高原的1.69. 李永华等(2014)通过联合反演青藏高原东南缘地区瑞利波群速度频散和固定地震台站的远震接收函数,构建了青藏东南缘三维地壳剪切波速度模型,发现地壳厚度变化强烈(30~65 km),总体趋势是东南浅、西北深. Li等(2016)利用面波和接收函数联合反演发现地壳厚度从高原到东南缘逐渐由62 km变为30 km. Wang等(2017)利用中国地震台阵的密集数据计算了青藏高原东南缘的地壳厚度与平均波速比(图5a, b),发现地壳厚度由青藏高原的60 km向西南递减至35 km,青藏高原东南部及附近的安宁河—泽木河断层、北小江断层等走向滑移断层下存在高泊松比(σ>0.28). Yang等(2017)Hu等(2018)结合P波和S波接收函数方法发现云南西南部的地壳厚度为30~39 km,而云南西北部地区的地壳厚度为60~69 km,泊松比的变化范围是0.24~0.32,断裂带附近的泊松比偏高,在龙门山断裂带、丽江—小金河断裂带以及腾冲火山附近平均泊松比为0.28~0.32,Wen等(2019)利用各向异性H-k叠加方法确定该地区的地壳结构,得到了类似的结果. 董蕾等(2020)利用四川、云南固定台站记录到的远震波形资料,利用接收函数一次转换波和多次波幅度信息确定了青藏高原东南缘莫霍面上的S波速度和密度跃变,结果表明研究区由南到北地壳厚度逐渐增加,从33 km左右增至70 km左右,四川盆地和松潘甘孜块体南部具有高泊松比、速度密度跃变较小的特征,腾冲地区、龙门山西侧的汶川地区、四川盆地西南缘以及则木河断裂同属于高泊松比、速度密度跃变较大. Xu等(2020)通过最新的台阵数据和改进的方法,计算了青藏东南缘的莫霍面分布,发现在滇中块体北部平行于扬子克拉通和松潘—甘孜块体边界断裂,存在明显的莫霍面梯度带(图5c, d),这一现象与龙门山下方已经发现的莫霍面陡峭变化一致(Zhang et al., 2009; Qian et al., 2018).

      图  5  青藏高原东南缘的地壳厚度和泊松比.(a, b)利用H-k叠加方法获得的平均地壳厚度与泊松比(修改自Wang et al., 2017),黑色虚线表示主要的块体边界.(c, d)利用CCP叠加方向获得的地壳厚度及其水平梯度(修改自Xu et al., 2020). 灰色线条表示主要断裂. 红色和黑色封闭虚线表示莫霍面深度的水平梯度大于15 km/deg的范围,(d)黑色短线表示地壳厚度最大水平梯度的方向

      Figure 5.  Crustal thickness and Poisson's ratio in the southeastern margin of the Tibetan Plateau. (a, b) The crustal thickness and Poisson's ratio obtained by H-k method (modified from Wang et al., 2017). The black lines show the block boundaries. (c, d) The crustal thickness and its horizontal gradient obtained by the CCP method (modified from Xu et al., 2020). Gray lines denote the faults. The closed red and black lines denote the region where the lateral gradient of the Moho depths are larger than 15 km/deg; black bars in (d) show the directions of the maximum horizontal gradients of the crustal thickness

    • 层析成像是获得地球深部三维结构最重要的方法(Rawlinson et al., 2010). 地震波走时层析成像是最传统的结构成像方法,它利用地震台站观测到的地震波走时残差反演地下结构相对于一维参考模型的速度异常,从而可以很清晰地揭示出地球深部的热异常或物质组成异常. 走时层析成像又以体波成像最为常见,因为体波(P波和S波)可以穿过地球内部,所以它在不同尺度地球内部结构的研究中都是最重要的方法,揭示了从震源区精细结构到全球俯冲带和地幔柱结构的图像,推动了地球科学基本理论的重要进展(Romanowicz, 2003; French and Romanowicz, 2015). 面波因为只能在接近地球表面传播,其探测的深度有限,但近年来,随着密集地震台阵的布设和背景噪声技术的发展,利用面波研究地壳上地幔结构摆脱了对传统地震的依赖,在研究地球内部结构领域发挥了越来越重要的作用(Shapiro et al., 2005). 此外结合面波对绝对速度敏感和接收函数对速度界面敏感的特性,利用面波频散曲线和接收函数联合反演的方法成为研究地壳精细速度结构的重要方法(Julia et al., 2000). 除了最常见的速度结构外,科学家也尝试利用地震波振幅随距离衰减的特征反演地下介质的衰减因子,利用大地电磁方法反演地壳上地幔的电性结构也是最常见的研究方法. 尽管这些研究的分辨率会远低于对地震波速度的约束,它们仍然能够为研究地壳的结构与动力学提供至关重要的信息.

      Xu等(2005)利用走时层析成像揭示了青藏高原东南地壳上地幔的速度结构,发现哀牢山—红河断裂带中上地壳高速明显,而下地壳速度偏低;云南西部上地幔低速异常明显,东部扬子克拉通速度很高. 韦伟等(2010)利用P波层析成像得到了川滇地区的地壳P波速度,发现上地壳川滇块体相对高速,在中下地壳转变成相对低速,与红河断裂带西南侧印支块体中的中下地壳高速差异明显. 吴建平等(2013)利用流动地震台阵及固定台站的走时观测资料,对小江断裂带及周边区域的壳幔三维P波速度结构进行了研究,在中上地壳,小江断裂带内部主要为低速异常,其东侧主要为高速异常;在中下地壳,小江断裂带中部为低速异常,北部和南部主要为高速异常,其中北部的高速异常可延伸到地表附近,南部的高速异常可一直延伸到上地幔. Huang等(2018)手动拾取了中国地震台阵密集台站记录到的大量P波和S波到时资料,发现青藏高原东南缘下地壳低速在高原的边缘聚集(图6d). 邓山泉等(2021)利用双差地震层析成像方法反演了青藏高原东南缘川滇地区三维P波速度结构,发现川滇地区上地壳结构横向非均匀性明显,四川盆地上地壳10 km深度范围内表现为低速异常,而松潘—甘孜地块、滇中地块则表现为明显的高速异常;川滇地区中下地壳低速异常体明显,但它不是广泛分布在川滇地区,而是沿着川滇块体东部有限的通道分布.

      图  6  青藏高原东南缘下地壳速度与各向异性.(a)接收函数与面波联合反演得到的31 km处的S波速度(修改自Bao et al., 2015),黑线表示主要断裂. (b)面波层析成像获得的32.5 km处的方位各向异性(修改自Bao et al., 2020),短线表示各向异性方向与大小,底图颜色同样表示各向异性强度.(c)面波层析成像获得的25 km的S波速度结构(修改自张智奇等,2020),黑线表示主要断裂,白色虚线表示主要块体边界,圆圈表示5级在上地震分布.(d)P波各向异性层析成像获得的40 km处的P波速度(底图颜色)与方位各向异性(黑色短线)(修改自Huang et al., 2018). 黑线表示主要断裂,紫线表示主要块体边界,红色粗线圈定了结果可靠的区域的范围

      Figure 6.  Lower crustal velocities and anisotropy beneath the southeastern margin of the Tibetan Plateau. (a) The S wave velocities at 31 km depth revealed by joint inversion of receiver function and surface wave (modified from Bao et al., 2015). The black lines denote the faults. (b) The azimuthal anisotropy at 32.5 km depth revealed by surface wave tomography (modified from Bao et al., 2020). The orientations and lengths of the short bars denote the fast velocity directions and strengths of the anisotropy, respectively. The background colors also show the strength of anisotropy with reference shown at the bottom. (c) The S wave velocities at 25 km depth revealed by surface wave tomography (modified from Zhang et al., 2020). The black lines denote the faults. The dashed white lines denote the tectonic boundaries. The circles denote the earthquakes. (d) P wave velocity and anisotropy at 40 km depth (modified from Huang et al., 2018). The orientations and lengths of the short bars denote the fast velocity directions and strengths of the anisotropy, respectively. The black and purple lines denote the faults and block boundaries, respectively. The bold magenta line outlines the region where the results are reliable

      Yao等(2008)最早利用面波反演了青藏高原东南缘的地壳上地幔S波速度结构,发现了松潘—甘孜块体与小江断裂带附近的两个低速异常. Li等(2010)收集了川西和青藏高原东缘49个固定台站的连续波形数据,利用噪声成像方法得到了高精度的地壳S波速度模型,揭示了青藏高原东缘与四川盆地地壳结构的巨大差异,在高原的下地壳发现了明显的低速异常. Yang等(2012)王琼和高原(2014)范莉苹等(2015)潘佳铁等(2015)的结果表明短周期的相速度分布主要受地表沉积层厚度的影响,中等周期的相速度分布主要与中下地壳速度结构、地壳厚度密切相关,小江断裂、松潘—甘孜块体呈现最显著的低速,滇西南地区在较长周期的相速度分布上表现为大范围的显著低速. Chen等(2014)利用最新的全波形反演方法发现在青藏高原东南缘壳内存在低速层,但它在滇中块体北部被明显的高速异常分为了两个部分,一部分位于松潘—甘孜块体,另一部分位于滇中块体南部. 李永华等(2014)构建了青藏东南缘三维地壳剪切波速度模型,发现该区存在两个明显的壳内低速异常带,其中中地壳(15~20 km)低速带主要分布在腾冲、川滇菱形块体内部; 而25~40 km深度范围的中、下地壳低速带分布在研究的北部. Wang等(2014)发现安宁河—泽木河—小江断裂带、红河断裂带和小金河断裂带的低速带可以一直延伸到上地幔,表明存在可能的壳幔物质交换. Bao等(2015)郑晨等(2016)利用中国地震科学探测台阵计划的流动台站获得了高分辨率的地壳S波速度结构,发现青藏高原东南缘中下地壳内由北向南呈条带状分布有两条主要的壳内低速体(图6a),其中一条(A)从川西北向南延伸,穿过丽江断裂到达滇中,另一条低速体(B)沿小江断裂分布,向南延伸到24°N左右,两条低速体在中地壳范围被峨眉山大火成岩省内带下方的高速异常所隔开,后续的研究证实了这一结果(图6c)(Chen et al., 2016Li et al., 2016; Wu et al., 2016; Ling et al., 2020; 张智奇等, 2020). Han等(2017)Fu等(2018)利用远震勒夫波调查青藏高原东南缘的地壳结构,发现青藏高原上地壳沿断裂带附近为低速,而下地壳是大范围广布式的低速异常. Wu和Zhang(2012)郭希等(2017)对峨眉山大火成岩省的地壳结构进行了详细的研究,发现研究区地壳平均S波速度沿剖面呈现自西向东先增大后减少的分带性,内带中下地壳速度较高,尤其是下地壳存在明显的高速异常,是二叠纪古地幔柱作用的遗迹,表明大规模岩浆的底侵和内侵,不仅改造了滇中块体的地壳结构和组分,而且也改变了地壳的流变强度. Li等(2017)在约束P波速度的条件下利用面波频散曲线和接收函数反演了地壳速度结构,发现两个低速条带,而且小江断裂带下地壳的波速比偏高.

      此外,Zhao等(2013b)利用Lg尾波衰减层析成像方法反演了青藏高原东南缘地壳中的Q值,发现深部地壳有一条低Q值异常通道从青藏高原东部一直延伸到东南缘,但被丽江—金河断裂阻挡;此外在小江断裂带南端发现了一个孤立的低Q异常区. Dai等(2020)反演了川滇块体地壳的P波和S波衰减品质因子,发现滇中的峨眉山大火成岩省的Q值较高,而滇中块体的周缘断裂带Q值小于300,反映了地壳物质的高衰减特征. Bai等(2010)利用大地电磁测深方法得到了青藏高原东南缘地壳中的电阻率结构,他们在20~40 km深度发现两个高电导通道,从青藏高原一直延伸到其东南缘,总长达800 km,认为这一结果支持中下地壳物质流动挤出. Li等(2019)调查了小江断裂带附近的电性结构,发现上地壳一般电阻率较高,但被一些断裂带附近的低电阻率条带状分割,下地壳则是分布较广的低电阻体. Li等(2020a)利用大地电磁数据反演了青藏高原东南缘的电阻率,发现在峨眉山大火成岩省的中心地壳高阻异常强烈,并且一直延伸到上地幔,可能反映了该地区二叠纪火山喷发阶段地壳物质被改造形成现今的高阻物质,阻挡了青藏高原东南缘下地壳物质流动. Yu等(2020)利用三维大地电磁方法反演了红河断裂带下方的电阻率结构,发现红河断裂带两侧上地幔电阻率差异明显,红河断裂带东北侧上地幔存在较大的低电阻率体,并向上延伸到地壳,表明地壳的结构受到了上地幔的影响.

    • 地壳变形特征是探讨青藏高原东南缘的不同构造模型的关键,传统地质学和地表GPS观测仅能约束近地表和上地壳的变形,对地壳深部变形缺乏必要的约束. 震源机制解可以反映震源区的应力应变状态,但因为青藏高原东南缘的大部分地震都发生在上地壳(<20 km),所以也只能反映上地壳的变形特征(Han et al., 2019). 地震波各向异性几乎是获得地壳深部变形唯一的手段,地球内部的应力应变造成微结构(如裂隙)和矿物晶体发生定向的排列,形成宏观上的各向异性,可以被经过的地震波捕捉到(Mainprice, 2015). 对各向异性的研究最常用的手段有三种. 第一种是根据S波分裂现象获取相应的各向异性的参数,约束台站下方地壳的变形特征(Ando et al., 1980; Silver and Chan, 1991). 但因为S波分裂没有垂向分辨率,不能准确提供各向异性层的深度信息,所以需要利用不同深度或类型的地震区分各向异性层的深度(Huang et al., 2011). 在大陆地壳地区,常用上地壳发生的近震S波分裂研究上地壳的各向异性,而用接收函数获得的Ps震相分裂约束整个地壳的各向异性(Liu and Niu, 2011),通过比较二者的结果,能够有效评估下地壳各向异性. 第二种方法利用地震P或Pn波速度随传播方位角的特点,通过反演P波走时获得地壳三维方位各向异性结构(Wang and Zhao, 2008). 第三种方法是利用面波频散曲线首先获得不同周期瑞利面波的各向异性,再进一步反演不同深度S波的各向异性(Yao et al., 2010);该方法还可以通过比较瑞利波和勒夫波的速度结构获得径向各向异性(Shapiro et al., 2004),对地壳变形提供重要的信息. 这三种常用方法中,第一种方法横向分辨率很高,但垂向分辨率有限,后两种方法能够提供各向异性的分层信息.

      石玉涛等(2006)Shi等(2012)对云南地震台网资料的分析, 使用剪切波分裂方法获得了云南地区的快剪切波偏振结果,发现云南地区大部分台站的快剪切波偏振优势方向主要为近N-S或NNW方向,位于活动断裂上的台站的快剪切波偏振优势方向与活动断裂的走向一致(图7a). Zhang和Wang(2009)利用广角地震剖面数据揭示了下地壳地震波速各向异性可达4%,而上地壳仅有1.6%. 太龄雪等(2015)利用中国地震科学探测台阵记录到的近震S波分裂获得了研究区内67个台站的剪切波分裂参数,该地区快剪切波偏振方向整体上显示出 NNE向和NE向的优势取向,但在空间分布上比较复杂,虽然大部分台站的快波方向与构造应力场方向一致,但部分断裂附近台站受到断裂的影响. Zhang等(2018)利用近震S波分裂调查了青藏高原东南缘的上地壳各向异性,得到的平均快慢波时差仅有0.054 s,各向异性快波方向一般与最大主压力方向一致,在断裂带附近与断裂带走向一致. 高原等(2020)总结了青藏高原东南缘的各向异性研究,认为青藏高原东南缘上地壳各向异性与地表变形测量结果相符,快剪切波偏振方向呈现与地表运动特征一致的发散性,与主压应力方向一致,但受到地质构造的影响.

      图  7  青藏高原东南缘地壳各向异性.(a)近震S波分裂获得的上地壳各向异性(修改自Shi et al., 2012; Huang et al., 2018). 黑线表示主要断裂,绿线表示主要块体边界.(b)Pms分裂获得的地壳各向异性(修改自Sun et al., 2012; Chen et al., 2013; Cai et al., 2016; Huang et al., 2018).(c, d)利用接收函数马尔科夫链—蒙特卡罗(MCMC)反演获得的上地壳与下地壳的各向异性(修改自Han et al., 2020). 黄线表示断裂分布

      Figure 7.  Crustal anisotropy of the southeastern margin of the Tibetan Plateau. (a) Seismic anisotropy of the upper crust revealed by local S wave splitting (modified from Shi et al., 2012; Huang et al., 2018). The black and green lines denote the faults and major boundaries, respectively. (b) Anisotropy of the whole crust revealed by Pms splitting measurements (modified from Sun et al., 2012; Chen et al., 2013; Cai et al., 2016; Huang et al., 2018). (c, d) The anisotropy in the upper and mid-lower crust obtained by the Markov-Chain-Monte-Carlo inversion of receiver function (modified from Han et al., 2020). The yellow lines denote the faults

      Sun等(2012)利用P波接收函数获得青藏高原附近地壳各向异性,Pms分裂快慢波时差可达0.5~0.9 s(图7b). Cai等(2016)利用中国台阵密集的地震台阵记录的Pms分裂获得的快慢波时差范围是0.02~0.88 s,平均值是0.28 s,认为各向异性的主要来源是中下地壳,各向异性快波方法在块体内部与挤压应力平行,在断裂带附近与断裂带走向一致(图7b). Kong等(2016)利用Pms分裂方法获得了青藏高原东部71个宽频带固定台站下方的地壳各向异性,获得的平均快慢波时差是0.39±0.18 s,各向异性快波方向与主要边界断裂带的走向一致. 韩明等(2017)利用按方位叠加接收函数分析青藏高原东南缘的地壳各向异性,获得了96个Pms震相的分裂参数,发现川滇地区地壳各向异性十分强烈,Pms相分裂时间在0.05~1.27 s之间,平均值为0.54±0.12 s. Zheng等(2018)通过拟合川西台阵Ps震相时间延迟随方位角的变化获得的平均快慢波时差是0.48±0.13 s,是四川盆地内部测量结果(0.23±0.10 s)的两倍. Hu等(2018)利用Pms分裂揭示了地壳的各向异性快波方向为NW-SE向,快慢波时差平均值为0.54 s,表明下地壳发育了强烈的各向异性.

      Lei等(2014)利用Pn波层析成像方法反演了青藏高原东部上地幔顶部的P波速度与各向异性,发现从松潘—甘孜块体延伸到川滇菱形块体的低速异常,各向异性快波方向环绕东喜马拉雅构造结旋转. Huang等(2018)利用P波各向异性层析成像方法获得了青藏高原东南缘地壳的分层各向异性,发现上地壳各向异性快波方向与主要断裂走向一致,但下地壳的各向异性与地表构造明显不同,主要与莫霍面和地表高程的等值线一致(图6d). Han等(2020)首次利用马尔科夫链—蒙特卡罗(MCMC)反演方法来研究青藏高原东南缘地壳分层的各向异性及分区特征,得到了松潘—甘孜、扬子以及印支等典型构造区上地壳以及中下地壳的各向异性分布(图7c, d). 研究发现青藏高原东南缘上地壳各向异性主要受到地表先存的断裂带控制,平均的各向异性强度小于2%. 各向异性主要由中下地壳产生,强度平均在4%左右. 在四川盆地边缘各向异性快轴方向主要表现为北东—南西向,在松潘—甘孜块体内部主要变为南—北向,在滇中块体南部则呈现出环形展布的特征.

      Huang等(2010)通过背景噪声提取瑞利波和勒夫波格林函数,计算了青藏高原东南缘地壳中的S波速度与径向各向异性,发现位于青藏高原边缘的丽江—木里断层下地壳低速明显,且水平速度大于垂直速度,可能与深部地壳流动有关. Yao等(2010)反演了青藏高原东南缘地壳和上地幔的S波速度与各向异性,研究发现上地壳各向异性快波方向绕东喜马拉雅构造结旋转,与地表断裂带走向一致,下地壳速度和各向异性指示可能的下地壳流动只会发生在一些断裂带附近. 鲁来玉等(2014)基于中国地震科学探测台阵项目一期在南北地震带南段架设的300多个地震台站,采用基于背景噪声互相关函数的面波层析成像技术研究了青藏高原东南缘的云南地区面波群速度和方位各向异性分布,结果显示地壳的面波快波方向呈现近南北向,整体表现出围绕东喜马拉雅构造结顺时针旋转的趋势;小江断裂东西两侧的快波方位有一定差异,对反映深度大概在下地壳和上地幔顶部的长周期面波,快波方向从近南北向逐渐向北西向过渡. Bao等(2020)利用背景噪声层析成像确定了青藏高原及周缘地区地壳上地幔顶部的S波速度与各向异性,发现上地壳各向异性与主要断裂构造一致,且与上地幔变形方向一致,而下地壳中青藏高原与边缘地区差异明显,高原内部地震波速度很低,各向异性很弱,高原边缘各向异性很强,快波方向与高原边缘平行;小江断裂带下地壳和各向异性快波方向主体为N-S向,但在小江断裂带南端快波方向转为NE-SW向,与断裂带方向斜交. Wu等(2019)利用瑞利波层析成像方法反演了青藏高原东南缘地壳上地幔的S波速度与各向异性,揭示了非常复杂的地壳变形特征. Cao等(2020)利用新的波形梯度测量方法确定了青藏高原东南缘的地震波方位各向异性,发现快波方向与主要断裂系与造山带走向一致,川滇块体中部各向异性较弱,快波方向与边界断裂一致,可能与峨眉山大火成岩省有关. 王怀富等(2020)利用基于程函方程面波层析成像方法获得了青藏高原东南缘周期14~80 s瑞利面波相速度和方位各向异性分布图像,短周期面波方位各向异性分布与断裂带的走向和最大主压应力的方向密切相关,川滇菱形块体的北部次级块体及丽江—小金河断裂带附近随着面波周期的增加快波方向逐渐与断裂带平行,而其以南的攀枝花附近表现为高相速度和弱各向异性的特征,在红河断裂以西地区,长周期面波各向异性快波方向和红河断裂大致平行.

      Xie等(2013, 2017)结合瑞利波和勒夫波噪声成像,获得了青藏高原东缘地壳上地幔的VsvVsh速度结构,并进一步计算了地壳的径向各向异性,研究发现青藏高原东部上地壳径向各向异性比较统一,均为负各向异性(Vsh<Vsv);但中下地壳各向异性存在横向差异,高原内部中下地壳为正的径向各向异性(Vsh>Vsv),而在高原的东南缘及东北缘中下地壳的径向各向异性为负(Vsh<Vsv)(图4),这与Huang等(2010)获得的在下地壳广泛分布的径向各向异性一致.

    • 青藏高原东南缘上地壳与中下地壳之间的关系是理解青藏高原东南缘构造演化的关键,现今在地表观测到的地壳运动和断层分布是否代表整个地壳的性质对回答青藏高原的隆升扩展机制至关重要. 前已提及,青藏高原上地壳绕着东喜马拉雅构造结做顺时针旋转(图3a)(Shen et al., 2005; Pan and Shen, 2017; Li et al., 2020b; Wang and Shen, 2020),变形主要集中在鲜水河—小河断裂带和丽江—小金河断裂带(图3b)(Kreemer et al., 2014; Pan and Shen, 2017; Wang and Shen, 2020),地壳应力场在松潘—甘孜块体以近N-S向拉张为主,在高原的边缘过渡带则以挤压为主,主压应力方向为SE-NW向的放射性分布(图4)(马宏生等, 2007; Zhao et al., 2013a; Xu et al., 2016; Han et al., 2019李长军等, 2019). 地震学研究揭示的上地壳各向异性快波方向在稳定块体内部与主压力方向一致,反映了上地壳稳定块体在应力作用下产生的裂隙定向排列的方向(图7a)(石玉涛等,2006高原等,2020). 但在断裂带附近,各向异性快波方向与断裂带走向一致,反映了这些地区的裂隙排列受到断裂活动的强烈影响(高原等,2020).

      虽然在主要的断裂带附近上下地壳结构存在明显的一致性,但是青藏高原东南缘中下地壳的结构与上地壳存在明显的差异. 前人关于青藏高原东南缘的中下地壳结构达到了一定的共识,但在一些重要的问题上仍然存在较大的分歧. 青藏高原东南缘的地壳厚度在松潘—甘孜块体大于60 km,但在红河断裂带和小江断裂带附近减薄至30~40 km左右(图5)(Wang et al., 2010; Wang et al., 2017; Xu et al., 2020). 大部分研究认为地壳厚度的变化是一个渐变的过程,符合下地壳流模型中塑性下地壳的挤出造成高原的逐步隆升. 但是Xu等(2020)利用改进的CCP叠加方法揭示了更精细的地壳结构,发现青藏高原东南缘的地壳厚度在丽江—小金河断裂带附近存在一个突变带(图5c,d),它与龙门山造山带下方的地壳突变类似,是青藏高原东缘结构向南的延伸. 这就要求扬子块体的西南缘具有和四川盆地类似的下地壳结构,能够阻挡来自青藏高原中部的下地壳物质挤出(Qian et al., 2018),造成丽江—小金河两侧地壳结构的差异(Xu et al., 2020).

      下地壳结构和各向异性为探讨这一问题提供了新的证据,早期的研究认为青藏高原东南缘下地壳存在广泛分布的低速异常,各向异性方向也指示存在由青藏高原向东南方向挤出的下地壳物质,与下地壳流模型一致(Royden et al., 2008; Sun et al., 2012; Yang et al., 2012). 但在中国地震科学探测台阵计划之后,最新的研究结果表明(图6):(1)下地壳低速、低电阻率异常主要分布在两个地区,一个是在松潘—甘孜块体下方,另一个分布在小江断裂带,它们中间被高速、高电阻率异常体分割;(2)松潘—甘孜块体南缘的各向异性快波方向为NE-SW向,与青藏高原东南缘的边界断裂带(丽江—小金河断裂)一致(图6b),下地壳为负的径向各向异性(Vsh<Vsv)(图4),表明整体以水平方向挤压为主(Xie et al., 2013, 2017; Bao et al., 2015, 2020; 郑晨等, 2016; Huang et al., 2018; 王怀富等, 2020; 张智奇等, 2020). 这些结果明确指示了丽江—小金河断裂带作为青藏高原与扬子块体的构造边界,其东侧扬子块体阻挡了高原下地壳物质继续向东南方向流动(Huang et al., 2018; Bao et al., 2020张智奇等, 2020),高原的扩展受其他的构造控制.

    • 小江断裂带的深部结构及其形成机制对了解青藏高原东南缘的扩展至关重要. 现有研究基本确认了小江断裂带的中下地壳为强烈的低速度、低电阻率、强衰减的异常(图6)(Yao et al., 2008; Zhao et al., 2013a; Bao et al., 2015; Huang et al., 2018; Yu et al., 2020; 王怀富等, 2020; 张智奇等, 2020),但是其低速异常又不仅限于小江断裂带下方,而是继续向东扩展,整体呈NE-SW向展布(图6)(Huang et al., 2018; 张智奇等, 2020). GPS结果也发现在小江断裂带附近存在一个NE-SW向展布的地壳隆升区(Pan and Shen, 2017),其大地热流值偏高,分布范围与该区的下地壳低速体一致(Hu et al., 2000)(图3b). 该地区的大地震分布也存在一个NE-SW向的地震前缘区,继续向东南方向很少有大地震发生(图3a)(Huang et al., 2018; Wang and Huang, 2020). 小江断裂带南端低速体中方位各向异性也有NE-SW方向的趋势(图6b, d)(鲁来玉等, 2014; Bao et al., 2020; Han et al., 2020王怀富等, 2020),与小江断裂带近南北展布的方向不同,却与区域内的峨眉山玄武岩体的分布方向一致(图2b).

      我们认为小江断裂带深部结构的形成与峨眉山地幔柱密切相关. 峨眉山地幔柱形成于~250 Ma的二叠纪—三叠纪,其中心位于滇中块体(图2b)(徐义刚和钟孙霖, 2001; Hu et al., 2020),对应了青藏高原东南缘下地壳高速体的位置,前人研究还揭示了内带相对低衰减、低热流、高泊松比、正重力异常的特点(Hu et al., 2000; Zhao et al., 2013a; 郭希等, 2017; Wang et al., 2017; Zhao et al., 2020),这些结果均表明内带的地壳是强度较高的稳定块体,阻挡了青藏高原下地壳物质向东南方向的挤出. 相反,小江断裂带中下地壳低速体的范围则对应了峨眉山大火成岩省的中带区域,具有高衰减、高热流、低泊松比、低电阻率的特点(Bai et al., 2010; Hu et al., 2000; Zhao et al., 2013a; Wang et al., 2017; Yu et al., 2020). 我们认为虽然峨眉山内带的高速体阻挡了青藏高原下地壳物质的挤出,但是它同时也受到后者持续的挤压,它自身强度较大,不会发生内部的变形,而会将大部分来自青藏高原的挤压应力继续向东南方向传递. 小江断裂带位于峨眉山大火成岩省的中带,一方面,在火山喷发的过程中,会有零星的岩浆充填地壳裂隙,降低了地壳的强度;另一方面,它被滇中块体(内带)和右江造山带两个稳定块体挟持,最容易通过变形吸收来自青藏高原的挤压应力. 在此过程中,小江断裂带附近的地壳由于自发生热和垂向增厚,可能发生部分熔融,形成低速度、低电阻率、高衰减的下地壳. 同时,下地壳塑性物质在NW-SE向的挤压作用下,在小江断裂带南段形成NE-SW向的各向异性.

    • 本文总结了青藏高原东南缘近年来的地震学研究成果,总结如下:

      (1)青藏高原东南缘地壳厚度从高原内部的~60 km减少到高原周围块体的~30 km,在高原的边界断裂处存在一个地壳厚度突变带,是龙门山断裂带向南的延伸;

      (2)青藏高原东南缘的中下地壳存在两个低速异常体,中间被峨眉山大火成岩省内带分割. 一个低速体位于松潘—甘孜块体下方,被高原边界的丽江—小金河断裂阻挡,各向异性快波方向(NE-SW)与高原边缘一致;另一个低速体位于小江断裂带下方,呈NE-SW向展布,各向异性快波方向与小江断裂带走向存在差异.

      这些结果为研究青藏高原东南缘的扩展机制提供了重要的约束. 我们认为青藏高原的下地壳物质在其东南边界受到阻挡,并没有继续向云南地区流动. 但是来自高原下地壳挤出的应力通过峨眉山大火成岩省内带的稳定块体继续向东南传递,造成了小江断裂带附近地壳强烈变形和地表的抬升.

参考文献 (119)

目录

    /

    返回文章
    返回