李建有^1,2,,
石宝文^1,2,
徐晓雅^1,2,
胡家富^1,,
1. 云南大学地球物理系, 昆明 650091
2. 昆明南方地球物理技术开发有限公司, 昆明 650091

基金项目: 国家自然科学基金项目（41374106，41774110）资助


详细信息

作者简介: 李建有, 男, 1983年生, 工程师, 本科, 从事地球物理勘探工作.E-mail:ynwilbur@163.com

通讯作者: 胡家富, 男, 1965年生, 教授, 长期从事地壳与上地幔结构研究工作.E-mail:jfhu@ynu.edu.cn

中图分类号: P315

收稿日期:2017-12-12
修回日期:2018-03-15
上线日期:2018-07-05

Crustal structure beneath the Sichuan basin and adjacent regions revealed by teleseismic receiver functions

LI JianYou^1,2,,
SHI BaoWen^1,2,
XU XiaoYa^1,2,
HU JiaFu^1,,
1. Department of Geophysics, YunnanUniversity, Kunming 650091, China
2. Kunming Southern Geophysical Technology Development Inc., Kunming 650091, China


More Information

Corresponding author: HU JiaFu,E-mail:jfhu@ynu.edu.cn

MSC: P315

--> Received Date: 12 December 2017
Revised Date: 15 March 2018
Available Online: 05 July 2018


摘要
摘要:青藏高原东南缘的龙门山断裂两侧具有陡峭的地形特征，在约50~100 km的水平距离内，地形高程从2000 m增加到4000 m，该区强烈的壳幔变形特征及地球动力学模式一直是研究的热点问题.本文从四川地区49个固定台站记录的远震资料提取了P波接收函数，获得了四川盆地及周边的地壳厚度和泊松比，并以此构建反演的初始模型.在线性反演的基础上，引入了分别拟合低频和高频接收函数的两步反演技术，用以反演台站下方的地壳S波速度结构.数字试验表明，该方法可以有效抑制接收函数反演的不唯一性，为了得到最优解，最后用Bootstrap重采样技术估计解的不确定性.结果表明，四川盆地的地壳厚度在40~46 km，松潘-甘孜块体北部的地壳厚度为46~52 km，而南部增厚到50~60 km.从四川盆地向西跨过龙门山断裂，地壳厚度增加了10~15 km.在四川盆地及周边地区，地壳泊松比在0.26~0.32之间，呈块体分布特征，高泊松比（0.28~0.32）主要沿龙门山断裂以及安宁河-小江断裂分布.地壳S波速度结构表明，来自青藏高原中部的中下地壳低速层可能受到了坚硬的四川盆地阻挡，改变原来的运动方向并沿龙门山断裂展布，由于低速层的囤积导致该区地形陡峭和下地壳增厚.
关键词: 四川地区/
P波接收函数/
两步反演技术/
S波速度/
下地壳流

Abstract:The India-Eurasia collision over the past 50~65 million years is undoubtedly the most spectacular and youngest tectonic event on Earth, which made the Tibetan plateau uplift by >4 km and its crust thickened to twice normal thickness (~70 km).Field observations and satellite geodesy indicate that the uplift of the Tibetan plateau has been accompanied of a small crustal shortening from the center to the southeastern margin since about 4 million years ago. The Sichuan region located at the eastern margin of Tibet is characterized by steep topographic relief, where the elevation increases from 1000 m to 4000 m over only 50~100 km distances across the Longmenshan fault. The mechanism of intense deformation in the crust and upper mantle and the dynamic model of this region remain one of debate issues.
The lower-crust flow model is one of several competing models to interpret the growth and expansion of the southeastern Tibetan plateau, in which the strength of the lower crust is several orders of magnitude less than that of the upper crust, so that the mass of lower crust is flowing from central Tibet to Yunnan.The model seems to be feasible and is generally accepted by many geoscientists because it provides a satisfactory explanation for the topography variations and the lack of substantial shortening of the upper crust in the eastern margin of the Tibet plateau. Nonetheless, this model, despite its potential for a reasonable explanation of the regional geodynamics, is still the subject of lively debate due to the lack of conclusive evidence.
In this study, we used teleseismic data recorded by 49 permanent broadband stations deployed in the Sichuan region to obtain the P receiver functions and determined the crustal thickness and Poisson's ratio, permit to build an initial model for inversion. Based on the linearized inversion algorithm, we introduced a two-step inversion technique, in which the S-velocity structure beneath stations is obtained by fitting the low-frequency and high-frequency receiver functions, respectively. Synthetic experiments indicate that this technique can effectively reduce the dependence of inversion on initial models. In order to obtain optimal solutions, the Bootstrap resamping technique is applied to estimate the uncertainty in solutions. Our results show that the crustal thickness beneath the Sichuan basin is 40~46 km, increasing from 46~52 km beneath the northern Songpan-Garzê fold to 50~60 km beneath the southern Songpang-Garzê fold. In general, the crustal thickness increases by 10~15 km from the Sichuan basin westward across the Longmenshan fault. In the Sichuan basin and adjacent regions, the crustal Poisson's ratio is 0.25~0.32, exhibiting a regional distribution. For example, the high Poisson's ratio (0.28~0.30) appears along the Longmenshan fault and the Anninghe-Xiaojiang fault, respectively. The S-velocity structure shows a low-velocity zone in mid/low crust from the central Tibet that may be resisted by the rigid Sichuan basin, so that it redirects to extend along the Longmenshan fault, and results in the steep topography and the thickening of lower crust beneath the fault due to the accumulation of low-velocity zone.
Key words:Sichuan region/
P receiver functions/
two-step inversion technique/
S-wave velocity/
lower crust flow



PDF全文下载地址:

http://www.geophy.cn/data/article/export-pdf?id=dqwlxb_14585