Issues on crustal and upper-mantle structures associated with geodynamics in the northeastern Tibetan Plateau

Abstract

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 1960s, 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- κ stacking analysis of receiver functions shows low-to- moderate 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.