The study revealed the mechanism of crust deformation in the northeast edge of Qinghai-Tibet and adjacent areas

Recently, the team of Dong Dongdong, a researcher at the Institute of Oceanology, Chinese Academy of Sciences, and the marine geophysics team of the Department of Marine Science and Engineering of Southern University of Science and Technology have made important progress in the study of the crustal deformation mechanism in the northeast edge of the Qinghai-Tibet Plateau and adjacent areas by using seismological background noise imaging technology. The findings were published in the Journal of Geophysical Research-Solid Earth, a geoscience nature index journal.

Schematic diagram of the uplift of the Qinghai-Tibet Plateau and the crust deformation mechanism of the northeast margin Courtesy of the Institute of Oceanography

The uplift of the Qinghai-Tibet Plateau is the result of the continuous collision between the Indian plate and the Eurasian plate since the Cenozoic era. Regarding the dynamic mechanism of lithospheric deformation on the Qinghai-Tibet Plateau, there are mainly several end element models such as distributed shortening model, onshore subduction model, and crustal channel flow model. The distributed shortening model is pure shear thickening of the lithosphere; The continental subduction model is that the Asian lithosphere subducts under the Indian lithosphere, and the lithospheric mantle is decoupled from the crust; The crustal channel flow model is the lateral motion of the material in the lower crust. The northeast edge of the Qinghai-Tibet Plateau, as the leading edge of the plateau expansion to North China, is a key area for studying the uplift and outward expansion and growth of the plateau, and there has been great controversy about the deformation mode of the earth’s crust in this area. Therefore, the construction of high-resolution crustal fine structure by seismic imaging can provide key evidence for understanding the mechanism of crust deformation in the northeast margin of Qinghai-Tibet. 

Seismic wave azimuth anisotropy refers to the dependence of seismic wave velocity on propagation azimuth, which is an important means to study the deformation mode of the earth’s crust (lithosphere). The research team used the three-component continuous seismic waveform data recorded by the ChinArray II seismic network of the Chinese earthquake science exploration project to develop a background noise imaging method based on dense arrays, bipolybeam imaging, which can simultaneously extract phase velocity and azimuth anisotropy information. Furthermore, a high-resolution crustal and upper mantle shear wave velocity structure and azimuth anisotropy model of the northeast margin of the Qinghai-Tibet Plateau were constructed. 

The results show that the middle and lower crust of the northeast Songpan-Kardze and the Qilian orogenic belt have obvious low-velocity anomalies, but have different azimuth anisotropic characteristics. Among them, the Vs of the lower and middle crust in northeast Songpan-Kardze was < 3.4 km/s, and it had strong anisotropic characteristics near E-W pointing azimuth. However, the anisotropic intensity under the Qilian orogenic belt is relatively weak, with a Vs of about 3.4-3.6 km/s, which is higher than that of the Songpan-Ganzi middle and lower crust. These observations show that there is a difference in the deformation mechanism of the lower and middle crusts between the two.

Combined with other geophysical data such as radial anisotropy, reception function, thermodynamic simulation, etc., the research team believes that the Qinghai-Tibet Plateau is in a stepped expansion mode, while the northeastern Songpan-Kardze and Qilian orogenic belts represent different stages of plateau development. Among them, the uplift of Songpan-Ganzi coincided with the collision of the Indo-Eurasian plate, representing a relatively mature zone of plateau development, and the deformation of the earth’s crust was mainly controlled by the crustal channel flow. The Qilian orogenic belt is the leading edge of the expansion of the Qinghai-Tibet Plateau, and its uplift represents the early stage of the high native growth, which is mainly dominated by the shear thickening of the earth’s crust, but it cannot be ruled out that it is in the embryonic stage of the development of the earth’s crustal flow.

By calculating the shear wave splitting delay time and fast axis direction in the shell and comparing it with the observed SKS, the research team further found that there was shell mantle decoupling deformation from the eastern part of the Alxa block to the western boundary of the Ordos massif, while the vertical coherent deformation occurred with the orogenic shell mantle at the West Qinling orogeny.

This study provides important seismological observation evidence for understanding the crust deformation mechanism of the northeast margin of the Qinghai-Tibet Plateau and inferring the expansion and growth process of the Qinghai-Tibet Plateau. 

This research was supported by the National Natural Science Foundation of China. (Source: China Science News, Liao Yang, Wang Min)

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