ARTICLES
Chinese Science Bulletin © 2007
Science in China Press Springer-Verlag
Study on crustal, lithospheric and asthenospheric thickness beneath the Qinghai-Tibet Plateau and its adjacent areas ZHANG XueMei1†, SUN RuoMei1 & TENG JiWen1,2 1 2
Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; College of Geo-exploration Science and Technology, Jilin University, Changchun 130026, China
Based on the results of pure dispersions of Rayleigh wave tomography in the Qinghai-Tibet Plateau and its adjacent areas, taking S wave velocities from previous linear inversion as the initial model, using the simulated annealing algorithm, a nonlinear simultaneous inversion has been carried out for S wave velocity and thickness of different layers, including the crust, the lithosphere and the asthenosphere. The results indicate: The crustal thickness shows strong correlation with geology structures sketched by the sutures and major faults. The crust is very thick in the Qinghai-Tibet Plateau, varying from 60 km to 80 km. The lithospheric thickness in the Qinghai-Tibet Plateau is thinner (130―160 km) than its adjacent areas. And two blocks can be recognized, divided by an NNE strike boundary running between 90°E―92°E inside the plateau. Its asthenosphere is relatively thick, varies from 150 km to 230 km, and the thickest area is located in the western Qiangtang. India has a thinner crust (32―38 km), a thicker lithosphere of 190 km and a rather thin asthenosphere of only 60 km. Sichuan and Tarim basins have the crust thickness less than 50 km. Their lithospheres are thicker than the Qinghai-Tibet Plateau, and their asthenospheres are thinner. A discussion has been made on the character and formation mechanism of the typical crust-mantle transition zone in the western Qiangtang block.
1 Introduction The lithosphere and asthenosphere are most active layers from the ground to 400 km depth of the earth. They are mainly dynamic space, such as crust uplift, seafloor extension, plate movement, earthquake generation and volcano eruption. The structural framework of lithosphere and asthenosphere is one of the important bases of plate tectonics. The confirmation of these boundaries and properties is important to strengthen the theory of plate tectonics, layer coupling in the lithosphere and dynamic process and to research the resources, energy ― sources, disaster and environment[1 3]. Though the tectonic pattern, for example, orogenic belt, rift zone and craton, is in the shallow part of the lithosphere, the force system forming these structures and driving tectonic www.scichina.com
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movements is affected together by the deep part of the lithosphere and the asthenosphere. Collision and compression thicken the lithosphere. Delamination or extension thins the lithosphere. Obviously, it is important to study the structures, material properties and states of the lithosphere and the asthenosphere to gainfor getting knowledge of their formation and evolution, dynamic process, layer coupling and exchange of material and ― energy[4 6]. The Qinghai-Tibet Plateau is the place of the continent-continent collision between Indian and Eurasian Received December 20, 2005; accepted October 13, 2006 Doi: 10.1007/s11434-007-0110-7 † Corresponding author (email:
[email protected]) Supported by the National Natural Science Foundation of China (Grant Nos. 40274009 and 40434009) and Open Fund of Geo-detection Laboratory, Ministry of Education of China, China University of Geosciences (Grant No. GDL0607)
Chinese Science Bulletin | March 2007 | vol. 52 | no. 6 | 797-804
GEOPHYSICS
Qinghai-Tibet Plateau, lithospheric thickness, asthenospheric thickness, nonlinear inversion, seismic tomography
plates. Because of their interaction the shallow and deep structures are very complicated. It is essential to study the three-dimensional velocity structures of the crust and the upper mantle for discussing the tectonic evolution and dynamic character. Nowadays the models of lithospheric and asthenospheric thickness beneath the Qinghai-Tibet Plateau are mainly inversed by using seismic data. Chen et al.[7] studied the difference of lithospheric structures beneath the southern Qinghai- Tibet Plateau using 10―100 s group velocities of the Afghanistan earthquakes at the Kunming (KIM) station and Chengdu (CD2) station. Sun et al.[8] calculated the phase velocities at periods of 30―110 s recorded at the Bangge, Chali, Lozagh and Latze stations for earthquakes in San Salvador, Honshu and Kurils and obtained the velocity distribution in the crust and the upper mantle and the lithospheric thickness beneath the southern Qinghai-Tibet Plateau. Teng et al.[9,10] presented a basic model of thicker crust and thinner lithosphere after studying the 2D and 3D velocities in the Qinghai-Tibet Plateau. Wu et al.[11] investigated the average velocities of the upper mantle by the broadband body waveform inversion. Su et al.[12] gained the shear velocity distributions of the crust and the upper mantle at 0―420 km depths beneath the Qinghai-Tibet Plateau and its adjacent areas, using Rayleigh group velocity dispersions between 7 s and 184 s periods. Zhu et al.[13] showed a three-dimensional lithosphere model in Eurasia and west Pacific regions gathering various models from data of natural and passive shots and other geophysical data, providing a lithospheric structure beneath the Qinghai-Tibet Plateau in a large scale. Griot and Montagner[14] discussed the phase velocity structures in Tibet and its neighboring regions considering anisotropy of Rayleigh and Love waves. Hirn et al.[15] researched the structures of the crust and the upper mantle beneath the Qinghai-Tibet Plateau using wide-angle reflection data. Yanovskaya et al.[16] by using group velocities of Rayleigh wave and Love wave inverted the shear velocities in 0―160 km depths in 10°×10° size beneath the eastern Asia region. These studies represented the velocity structures of the crust and the upper mantle of this area in a large scale or just in some profiles about the lithospheric thickness. However there are few studies on the structures of the lithosphere-asthenosphere system. It is significant to study fine structures of the lithosphere and the asthenosphere for discussing the deep structures 798
and dynamic characters. In general inversion methods, especially the linear inversion method, parameters of layer number and layer thickness are fixed. The shortage of these methods can not provide more exact depths of interfaces. In order to cover the shortage, we carry out a nonlinear inversion (the fast simulated annealing algorithm) to invert the interface positions and velocities of the crust and the upper mantle simultaneously. We collected the seismic data of Chinese Centre Digital Seismic Net (CCDSN) and partial Indian seismic stations surrounding the Qinghai-Tibet Plateau as many as possible besides GSN stations in order to improve the resolution. Based on the above two aspects, after the inversion we get the crustal thickness, lithospheric thickness and asthenospheric thickness.
2 Data and preprocessing In the study of regional deep structures, it is one of the most important methods to invert the crust and mantle structures using the seismic observations. Surface wave can provide the deep structures in long distances. Its ray paths can cover an area well despite seismic stations inside the area are sparse. Hereby, it is useful to study the deep structures of S velocities beneath oceans and some regions difficult to set up stations. In this study, we collect long period (8―150 s) seismic records over the Qinghai-Tibet Plateau and its adjacent areas during the period of 1987―2003. These data are from 11 digital seismic stations of GSN and some seismic stations of CCDSN near the Qinghai-Tibet Plateau, such as Chengdu (CD2), Guilin (GUL), Guiyang (GYA), Panzhihua (PZH), Yinchuan (YCH), Wushi (WUS), etc. Thinking of few seismic events in India, we get some long period seismic records from Bombay (BOM) station with the help of International Centre for Theoretical Physics (ICTP). In addition, some dispersion curves (
