摘要
青藏高原东北部是探索青藏高原地球动力学的重点地区之一.自20世纪60年代以来实施的一系列深部地球物理探测和相关研究获得了该地区地壳上地幔结构的基本特征.深部地球物理探测揭示高原的地壳增厚,且具有低P波速度、低电阻率和高热流值的特点;体波走时层析成像、面波频散和远震体波接收函数反演共同显示中下地壳的低剪切波速度.回顾青藏高原东北部已有的深部地球物理研究成果,梳理出当前存在一些与地球动力学相关的问题.它们是:(1)中上地壳的低速-高导层;(2)高原地壳的增厚方式;(3)地壳和地幔各向异性;(4)下地壳通道流;(5)向南俯冲的欧亚大陆岩石圈.这些问题尚未达成一致的认识.不同的深部构造模型和观点使得对例如"下地壳通道流"和"俯冲的欧亚大陆岩石圈"有关议题争论持续不断.未能达成共识的重要原因之一可能是现有台网的数据成像分辨率和精度仍不足以识别在地壳和上地幔深处的细节.在国家和区域地震台网的基础上,正在实施的大规模流动地震台阵观测,以及重点地区开展高分辨的深部地球物理探测,将较大幅度地改善目标模型的分辨率和可靠性.这是增进青藏高原东北部深部结构和地球动力学知识的有效途径.
The northeastern Tibetan Plateau is one of the key regions to explore the geodynamics of the Tibetan Plateau. In 1958, a research team led by Prof. Zeng Rongsheng did low-frequency seismic exploration in the Qaidam Basin, which was a prelude to deep geophysical research in the northeastern Tibetan Plateau. Since 1960 s, a series of scientific projects have been carried out, including 27 deep seismic sounding profiles, which provided good coverage and fundamental constraints on velocity structure of the crust and upper mantle of tectonic units in the region. Among them, a number of deep seismic reflection profilings were used to reveal fine crustal structures of key tectonic areas. Moreover, crustal electrical structure and density structure were inferred from magnetotelluric sounding profiling and Bouguer gravity anomaly data, respectively. 3-D P-wave and S-wave velocity structures were determined using body-wave travel time tomography, surface-wave group velocity and phase velocity tomography based on seismic data. Since 2000, with the dramatically increased number of broadband seismic stations, application of modern seismology methods, such as teleseismic receiver function inversion, ambient noise imaging, as well as shear-wave splitting and seismic anisotropy, led to further understanding of structure and deformation of crust and upper mantle in the northeastern Tibetan Plateau. Among published research papers, most results on the deep structure are compatible. For example, the ambient noise imaging, surface wave tomography and receiver function inversion jointly show a wide distribution of low shear wave velocity in the middle and lower crust. Deep geophysical explorations show coexistence of crustal thickening, low P-wave velocities, low resistivity and high heat flow values. H-? stacking analysis of receiver functions shows low-tomoderate crustal Poisson's ratios in the Qilian fold system, the northern Songpan-Garze block and the west Qinling orogenic belt. The crustal Poisson's ratios in the northeastern Tibetan Plateau are obviously lower than those in the central plateau. These compatibilities are crucial to understanding basic features of the deep structure. However, several issues related to the dynamics of the region remain in debate:(1) low velocity-high conductivity layer in the upper-middle crust;(2) crustal thickening mode;(3) crustal and mantle anisotropy;(4) lower crustal channel flow;(5) southward subduction of the Eurasian lithosphere. No consensus has been reached for these issues yet at present. Although several deep seismic sounding profiles and magnetotelluric sounding profiles collectively displayed evidence of existence of a low velocity-high conductive layer in the upper-middle crust, results of some other profiles did not show such layer. Mechanism of crustal thickening in the northeastern Tibetan Plateau can be summarized in the following three end-member hypotheses:(1) uniform crustal thickening;(2) lower crustal thickening;(3) upper crustal thickening. A prevailing view is that the crustal shortening generates folding and deformation of the upper crust, and fragment stacking is the main mode of crustal thickening. However, this is inconsistent with the crustal model from deep seismic sounding profiles. Different deep tectonic models and interpretations are causes of the on-going debate on issues such as the "lower crustal channel flow" and "southward subduction of Eurasian continent". One of the reasons for lack of consensus might be that the resolution power of the existing seismic data is still not high enough to identify the details in deep crust and upper mantle. The national and regional seismic networks, and large-scale temporary seismic array observation, which is currently being implemented, will greatly improve the reliability and resolution of the target model. This is an effective way to enhance the knowledge of crustal and upper mantle structures and geodynamics in the northeastern Tibetan Plateau.