青藏高原东部壳幔速度结构和地幔变形场的研究

在青藏高原东部地球动力学问题中,笔者在文中主要考虑与地壳上地幔速度结构和地幔变形场有关的问题,它涉及当前流行的下地壳流动模型和壳-幔的耦合-解耦模型。在2000年完成的穿过川西高原和四川盆地的深地震测深剖面,揭示了川西高原的地壳结构具有地壳增厚(主要是下地壳增厚)、地壳平均速度低等特点,显示地壳的缩短与增厚的碰撞变形特征。根据川西高原上设置各爆炸点的记录截面图共同呈现PmP(莫霍界面反射波)弱能量的特点,推断在川西高原的下地壳介质具有强衰减(Qp=100～300)的性质,支持存在下地壳流动的模型。青藏高原东部和川滇西部地区的上地幔各向异性(SKS波快波偏振方向和快慢波延迟时间)的初步结果表明,这两个地区的壳-幔变形特征是不同的,尽管它们在地理位置上属于同一个板块碰撞带。在青藏高原内部的壳幔变形属于垂直连贯变形,它以缩短为主,而高原外部的地壳(或岩石圈)则相对于其下方地幔运动。在高原内部和外部之间存在一个重要的地幔变形过渡带。然而,高原内部的垂直连贯变形与高原内部存在大范围下地壳流动的模型不一致。笔者在该地区开展了近两年的宽频带流动地震观测,试图从地震记录中确定过渡带的位置和探讨它的流变性质。文中扼要回顾已经取得的结果,并介绍正在进行的研究。

Study of crustal and upper mantle's structure and mantle deformation field beneath the eastern Tibetan plateau

This paper focuses on the crustal and upper mantle structure of the eastern Tibetan plateau,and the mantle deformation related to contrasting lower crustal flow and crust-mantle coupling-decoupling models.Deep seismic profiling conducted in 2000,across the western Sichuan plateau and the Sichuan basin reveals that the western Sichuan plateau has a thickened crust(mainly in the lower crust)and a lower average crustal velocity;its collisional deformation is characterized by both crustal shortening and thickening.Based on the weak energy of seismic phase PmP(reflection from the Moho)on most record sections in the western Sichuan plateau,we infer a crustal model with high attenuation(Q_p = 100-300)in the lower crust,which could be indicative of lower crustal flow.The preliminary measurements of teleseismic shear wave splitting parameter,the fast polarization direction and time delay,indicate that crust-mantle deformation beneath the eastern Tibetan plateau is different from that beneath the western Yunnan and Sichuan region,although both are located in the Indian-Eurasian plate collisional zone.The on-plateau crust-mantle deformation is a vertically coherent deformation,which is dominated by shortening.In contrast,the off-plateau crust(or lithosphere)is in motion with respect to underlying mantle.There is,thus,a fundamental transition in mantle deformation between on-plateau and off-plateau regions.Significantly,the on-plateau vertically coherent deformation is inconsistent with proposed models of large-scale,lower crustal flow within the Tibetan plateau.For this reason,we have conducted a two-year project of mobile broadband seismic observations in the eastern Tibet and western Sichuan region to determine the location of a transition zone between them and to explore its rheological behavior from the seismic records.This paper briefly reviews the results obtained already in this region,and describes the studies in process.