The uplift of the Tibetan Plateau is closely related to the collision between the Indian and Eurasian plates, with the southeastern Tibetan margin being the leading edge of material transport (Figure 1). Since the Cenozoic, this area has undergone a multi-stage tectonic evolution process, and the crust-mantle deformation is drastic, which is a typical area for studying the crustal deformation mode and tectonic evolution law of the Qinghai-Tibet Plateau. Previous studies have pointed out that large-scale crustal flow is the main dynamic mechanism in the crust of the southeastern Tibetan margin, but recent studies have shown that the low-velocity zone of the middle and lower crust of the Qinghai-Tibet Plateau presents multiple fragmented bands of low-velocity distribution, which is inconsistent with the large-scale crustal flow model. Whether there is a large-scale crustal flow in the southeastern Tibetan margin, or what is the main mechanism of intracrustal deformation in this area, is an important scientific question.
Fig.1 Tectonic and seismic distribution of the southeastern margin of the Tibetan Plateau. The red circles represent earthquakes of magnitude 5 or higher, and the blue arrows indicate the direction of large-scale crustal flow that has been studied previously. The illustration in the lower left corner shows Chinese mainland, with red boxes indicating the study area. SCB: Sichuan Basin, HNB: South China Block, ANH-ZMHF: Anninghe-Zemuhe Fault, CHF: Chenghai Fault, JSJF: Jinshajiang Fault, LCJF: Lancang River Fault, LXF: Lijiang-Xiaojinhe Fault, PDHF: Pudu River Fault, RRF: Honghe Fault, XJF: Xiaojiang Fault, XSF: Xianshuihe Fault, ZDF: Zhongdian Fault
In order to solve this problem, Huang Peixi, a doctoral student in the Structural Geophysics Discipline Group of the State Key Laboratory of Lithospheric Evolution of the Institute of Geology and Geophysics, Chinese Academy of Sciences, under the guidance of his supervisor Wu Jing, and the plateau researcher of the Institute of Earthquake Prediction of the China Earthquake Administration, used the Pms seismic phase travel time inversion method based on the receiving function to select the waveform data of 30 fixed stations in the southeastern margin of the Qinghai-Tibet Plateau for ~10 years (2011.01~2020.12). A total of 2176 events with magnitude greater than 5.5 were analyzed (Fig. 2), and the crustal anisotropy parameters (fast-wave polarization direction φ and slow-wave delay time ∂t) were finally obtained for 30 stations.
Figure 2 Distribution of stations and events. (a) The blue triangle indicates the fixed station, the black line segment indicates the fault zone, abbreviated as shown in Figure 1;
The results show that in the north-east and south-west sub-zones of the study area (areas B and C, Fig. 3), the crustal anisotropy directions obtained by Pms at the time of production are consistent with the trend of the large fault zones (Anninghe-Zemuhe fault, Lancangjiang fault, Pudu River fault) in the study area, and are also consistent with the crustal flow direction proposed by previous researchers, however, in the northwest and southeast sub-regions of the study area (A and D areas, Fig. 3), the crustal anisotropy direction is inconsistent with the fault trend or crustal flow direction. Therefore, it is pointed out that the large fault zone and local crustal flow in this area have a significant effect on the intracrustal deformation, but the large-scale crustal flow mechanism is unlikely. In addition, there are two dominant anisotropy directions in the southeast of the study area (D area), which are speculated to be related to the convergence of two mantle currents from different directions, resulting in the disturbance characteristics of middle and lower crustal deformation in the study area.
Fig.3 Spatial distribution of seismic anisotropy directions. (a) Spatial sub-regional distribution of fast wave direction and slow wave delay time in the crust of the study area. The gray arrows indicate the direction of large-scale crustal flow as previously studied. The white double arrows indicate the principal compressive stress. The four white dotted boxes represent the four sub-regions of A-D, corresponding to the stations represented by the blue, purple, orange and pink triangles. The roses of equal area represent the distribution of fastwave directions in the region, the red lines represent the direction of fastwave polarization dominance in each region, and the color of the roses corresponds to the color of the stations in the subregion. (b) Summary plot of S, Pms, and XKS anisotropy results. The dark grey line indicates the main fault zone in the area, the green line indicates the result of the direct S wave, the red line indicates the result of Pms, and the blue line indicates the result of XKS. The pink arrows depict the direction of mantle flow, while the yellow line segments with arrows indicate the local crustal flow inferred in this study. The blue dotted line represents the boundaries where the areas are grouped
The research results were published in the international academic journal Terra Nova (Huang P, Gao Y, Wu J*. Crustal anisotropy beneath southeast Tibet revealed by Pms arrival times[J]. Terra Nova, 2024. DOI: 10.1111/ter.12714.)。 This research was supported by the National Natural Science Foundation of China (42330311, 42274131, 41774111), the Special Fund of the Key Laboratory of Earthquake Prediction of the China Earthquake Administration (2021IEF0103), and the Independent Research Project of the State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences (SKL-Z202204, SKL-Z202305).
Editor: Fu Shixu (East China Normal University)
Proofreading: Wan Peng