Crust and upper mantle velocity structure in SE Tibet and its geodynamic implications

Southeastern Tibet is an important area for studying the deformation and transport of the Tibetan Plateau materials. Previous studies in this area found two significant crustal low-velocity belts and its might be the channels for the southeastward escape of the Tibetan Plateau materials. Recently, a new research reveals the spatial connectivity and formation of these two low-velocity belts and proposes three major geodynamic modes in the crust and upper mantle of this region. The title of this research paper is "Shear wave velocity structure of the crust and upper mantle in Southeastern Tibet and its geodynamic implications" and it published in Science China: Earth Sciences in No.9, 2020. The first author of this paper is Zhiqi Zhang, a doctoral graduate student, and the corresponding author is Professor Huajian Yao from University of Science and Technology of China. As an important corridor for the extrusion of Tibetan Plateau materials, a lot of geophysical studies have been conducted in southeastern Tibet. Many surface wave, body wave, and magnetotelluric tomography studies have observed two low-velocity belts and high-conductivity layers in the middle-lower crust in this area. However, previous studies could not reveal the connectivity between these two low-velocity belts due to the limitations of the imaging methods and the distribution of stations.

This new research is mainly based on the 10-year teleseismic Rayleigh wave data and one year continuous ambient noise data from permanent stations in the southeastern Tibetan Plateau. The high-resolution shear wave velocity (VSV) structure of the crust and upper mantle in this area has been obtained through the 3-D direct surface-wave inversion method. The new model also shows two low-velocity belts in the middle-lower crust in this area. One belt is mainly in the SongPan-GangZi block and northwestern part of the Chuan-Dian diamond block, whereas the other belt is mainly in the Xiaojiang fault zone and its eastern part, the Yunnan-Guizhou Plateau. While an obvious high-velocity body is observed in the crust of the inner zone of the Emishan large igneous province (ELIP), which may represent mafic-ultramafic material that remained in the crust when the ELIP formed. This high velocity zone separates the two low-velocity belts near the Anninghe-Zemuhe fault system (Figure 1). Researchers think the low-velocity belt in the northwestern side may be mainly due to the eastward extrusion of low-viscosity materials from the Tibetan Plateau, but the low-velocity belt in the Xiaojiang fault zone and its eastern side is likely caused by plastic deformation or partial melting of felsic rocks due to crustal thickening. Moreover, the significant positive radial anisotropy (VSH>VSV) around the Xiaojiang fault zone further enhances the amplitude of low velocity anomaly in our VSV model. This crustal low-velocity zone also extends southward across the Red River fault and farther to northern Vietnam, which may be closely related to heat sources in the upper mantle. In the upper mantle, there is a large-scale low-velocity anomaly in the Indochina and South China blocks south of the Red River fault. The low-velocity anomaly gradually extends northward along the Xiaojiang fault zone into the Yangtze Craton as depth increases. Through the tomographic model, the researchers think that southeastern Tibet is undergoing three different tectonic modes at the same time: (1) the upper crust is rigid, and as a result, the tectonic mode is mainly rigid block extrusion controlled by large strike-slip faults; (2) the viscoplastic materials in the middle-lower crust, separated by rigid materials related to the ELIP, migrate plastically southward under the control of the regional stress field and fault systems; and (3) the upper mantle south of the Red River fault is mainly controlled by large-scale asthenospheric upwelling and may be closely related to lithospheric delamination and the eastward subduction and retreat of the Indian plate beneath Burma.

This research provides a new explanation for the connectivity and formation of the crustal low-velocity belts and the dynamic processes of crust and upper mantle in southeastern Tibet, which is very valuable for the future geodynamic research in this area.

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This research was funded by the by the National Key R & D Program of China (Grant No. 2018YFC1503400) and the China Seismic Experiment Site project, China Earthquake Administration (Grant No. 2018CSES0101).

For more details, see the article Zhang Z, Yao H, Yang Y. 2020. Shear wave velocity structure of the crust and upper mantle in Southeastern Tibet and its geodynamic implications. Science China Earth Sciences, 63(9): 1278-1293, https://doi.org/10.1007/s11430-020-9625-3

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