青藏高原东南缘的地壳结构与动力学模式研究综述

青藏高原东南缘的川滇地区壳幔变形特征及地球动力学模式一直是研究的热点问题之一,多年来一直受到各国地球科学家的高度关注.青藏高原演变的"下地壳流模型"模拟得到的地表速度和变形场与GPS观测具有很好的一致性,该模型在当前国际地学界很流行,因而寻找下地壳流存在与否的证据,是深部地球物理学必须面对的一个科学问题.本文综合了川滇地区GPS观测、震源机制解和Pms相分裂的结果,旨在探讨川滇地区地壳演变模式的合理性;另外,从层析成像、接收函数反演和大地电磁测深结果分析,认为川滇地壳内存在大范围的低速层,但分布的几何形态较复杂.在云南地区,这一壳内低速区似乎被小江断裂和金沙江—红河断裂限制在特定的区域内.

Review on the study of crustal structure and geodynamic models for the southeast margin of the Tibetan Plateau

The convergence between the Indian plate and the Eurasian plates has resulted in the shortening of crust at least 1500 km and thickening of crust to twice of the normal (about 70 km) beneath the collision zone over the last 45 Ma.To model the mechanism of the surface deformation, several models, including lateral extrusion of rigid blocks along large strike-slip faults and lower crust flow have been proposed. The crustal-flow model, in which the middle/lower crust in central Tibet is partially molten and flows from central Tibet through Sichuan into Yunnan, is gradually accepted by seismologists. Therefore, the study of the deformational characteristics in crust and mantle and the geodynamic model beneath Sichuan and Yunnan (southeastern margin of the Tibetan Plateau) remains one of focused issues in the earth sciences of the world for many years. Although the fine crustal structures have been obtained in the region during the past several decades, the mechanisms for plateau deformation and expansion are still enigmatic due to the resolution limitations of methods and/or data. The aim of this paper is to select the previous results, analyze their discrepancies and conclude the reliable evidence for the lower crustal-flow model.#br#The lower crustal flow model on the evolution of the Tibetan plateau yields surface velocity and deformation field consistent with observations of GPS, thus being very popular in the earth science community. A scientific problem in deep geophysics is searching for evidence on the existence of the lower crustal flow. In this paper, GPS observations, focal mechanism solutions, and Pms phase splitting in Sichuan and Yunnan are combined to explore the reasonability of crustal deformation models. On the other hand, based on the analysis of tomography, receiver function inversion, and magnetotelluric profiles, this paper discusses whether there exits the deep evidence on lower crustal flow beneath Sichuan and Yunnan, as well as the distribution scale of lower crustal flow if it is really present.#br#The geodynamic implications are also investigated in relation to surface geological features, GPS velocities, Pms phase splitting, and focal mechanism. We observe a conspicuous sharper clockwise rotation around the eastern Himalayan syntaxis, and suggest that the deformation between the upper and lower crust within Sichuan and Yunnan is coupled. Resistivity imaging by magnetotelluric measurements have mapped two zones of high electrical conductivity at depth 20~40 km beneath southeast Tibet, only along the Xianshuihe-Xiaojiang fault and Jiali-Nujiang fault, respectively. However, the crustal velocity structure obtained by inversion of receiver functions and surface waves shows that intra-crust low-velocity zone (IC-LVZ) is locally distributed in western Sichuan, and not found in the Sichuan basin. Additionally, by analysis of the polarity of the converted phases at intra-crust interfaces, we suggest that a wide-spread IC-LVZ is present in Yunnan. #br#According to the distribution features of the negative polarity at intra-crust interface and crustal structure from joint inversion of receiver functions of surface waves, it is concluded that an IC-LVZ from eastern Tibet on a large scale may exist beneath Sichuan and Yunnan, and has reached the southern Yunnan, but has a complex geometric shape. In Yunnan, this zone appears to be constrained in a certain area by the Xiaojiang and Jinshajiang-Red River faults. This conclusion is not quite consistent with previous suggestion that the lower-crustal flow reaches only the Lijiang-Jinhe fault, and that the lower crustal flow is only bounded within two arc-like channels.