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Chinese Science Bulletin, Volume 63, Issue 19: 1949-1961(2018) https://doi.org/10.1360/N972018-00337

A P-wave velocity study beneath the eastern region of Tibetan Plateau and its implication for plateau growth

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  • ReceivedApr 9, 2018
  • AcceptedMay 21, 2018
  • PublishedJun 13, 2018

Abstract

The eastern region of Tibetan Plateau (ERTP) and surrounding regions have gentle to moderate slope in topography, contrasting to the steep margins in the north and south. Many geodynamic models have been proposed to explain plateau growth in the east part of Tibetan Plateau and no consensus have yet been reached. In part, this is because of lack of detailed tomography images of the deep structure, which is essential for understanding the deep dynamics that govern the evolution of ERTP.

Two stages of ChinArray have been conducted from 2011 to 2015. Nearly 1000 temporary seismic stations, with an average spacing ~40 km, were installed in ERTP and its adjacent regions. In addition, there was 297 temporary stations deployed in eastern margin of Tibetan Plateau during September 2006 to July 2009. Additional data from 271 permanent stations in 2007−2015 were also included. The overall station distribution densely covers the northeastern, eastern and southeastern margins of Tibetan Plateau.

We imaged high-resolution 3D P-wave velocity structures of the crust and upper mantle beneath ERTP and its surrounding regions through teleseismic traveltime tomography. The horizontal and vertical resolutions are 0.8° and 80 km, respectively. The imaged velocity anomalies correlate well with geological blocks. Slow velocity anomalies, extending down to ~200 km, are found beneath the Tibetan Plateau. Fast anomalies associated with cratonic blocks around the plateau are imaged within 200−300 km depth. Below 400 km there are less velocity anomalies, indicating a normal mantle close to the global average. Neither fast subducted slabs nor slow mantle upwellings are obeserved beneath the study region. The plateau growth is mainly caused by the deformation of the weak lithosphere at the shallow depth. Surrounded by the Indian plate and surrounding rigid blocks, the weak lithosphere beneath Tibetan Plateau is horizontally shortened and vertically stretched. The horizontal shortening helps accommodate the ongoing collision between the Indian and Asian plates and the plateau growth is resulted from the vertical stretching.


Funded by

国家公益性地震行业科研专项(201308011)

国家自然科学基金(41674103)


Acknowledgment

感谢中国地震科学台阵探测项目及川西流动地震台阵项目中参与数据收集和处理的所有人员. 感谢中国地震局地球物理研究所中国地震科学探测台阵数据中心[76]和国家测震台网数据备份中心[34]、北京数字遥测地震台网、中国地震台网中心和重庆、甘肃、广西、贵州、宁夏、青海、四川、陕西、云南地震台网为本研究提供地震波形数据. 衷心感谢两位审稿专家提出的宝贵修改意见, 其对本文内容的完善有非常大的帮助.


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  • Figure 1

    The distribution of stations and seismic epicenters, as well as the tectonic sketch map of study region. The blue triangles and red triangles denote the temporary stations deployed by two projects: ChinaArray Phase Ⅰ&Ⅱ and Western Sichuan seismic array, respectively. The purple circles denote the permanent stations. The light blue huge triangle is the area enclosed by Songpan-Ganzi and Yidun terranes. The black lines represent the tectonic boundaries. In inset map at the bottom left corner, green frame enclose the study area and red circles mark the seismic epicenters

  • Figure 2

    The tradeoff curve between traveltime residual and velocity anomaly. The tradeoff curve shows that the optimal damp value ranges from 7 to 15. We chose value of 10 as an optimal value

  • Figure 3

    The statistic of the traveltime residuals before (a) and after (b) inversion. The variance and RMS value are marked at top left corner of each figure

  • Figure 4

    The results of the checkerboard resolution test. These figures show a checkerboard resolution test when the size of inputting anomalies are 0.8° × 0.8° × 80 km. The horizontal slices (a) follow the depths indicated by dash lines in vertical sections (b)–(e); and the vertical sections (b)–(e) follow the profiles indicated by dash lines in horizontal slices (a)

  • Figure 5

    The horizontal slices of the tomography results. The layer depth is labeled in top right corner of each figure. The green lines labeled a, b and so on at the slice of 10 km depth, represent the location of sections shown in Figure 6. The faults and main geological boundaries represented by black curves are the same as those in Figure 1

  • Figure 6

    The vertical sections of the tomography results. The profile positions is shown in a horizontal slice at 10 km depth in Figure 5. In each figure the top row shows surface elevation, and the bottom row shows wave velocity anomalies. The dashed lines are the 410 and 660 km discontinuities

  • Figure 7

    The restoring test. The left panel is for horizontal slice, the right panel for vertical slice. The profile positions are shown in top left figure. In bottom left corner of each figure, strings “input” (“output”) are written down to discern the starting (recovered) structures

  • Figure 8

    A push-deformation model. Red area is for weak lithosphere, and blue area for rigid lithosphere. Note that light blue area denotes motion part while dark blue area denotes obstructed part. At the initial status (a), rigid lithosphere and weak lithosphere have approximately same thickness. As the collision begins to happen (b), the weak lithosphere generate slight deformation. A smooth topographic gradient is appearing at the boundary between rigid and weak lithosphere. After the collision convergence has reached a considerable degree (c), horizontal shortening, also vertical stretching, obviously occurred in the weak lithosphere. Then a steep topographic gradient is formed at the boundary between rigid and weak lithosphere

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