The southeastern Yunnan arcuate structual belt refers to three arc-shaped faults located on the southeastern margin of the Sichuan-Yunnan block, including Qujiang fault, Shiping-Jianshui fault and the central-southern segment of Honghe fault zone. The three faults extend in an arc （in plan view）, striking generally in the NW–WNW direction and being arranged from north to south. The eastern side of the region is bounded by the south-central segment of the sinistral nearly north-south Xiaojiang fault zone. Under the combined action of the northeastward extrusion of the Indian Plate, the southeastward escape of the plateau material, and the blocking of the South China block, the tectonic deformation is strong and the seismic activity is frequent in the arcuate structural belt. As the frontier zone for the southeastern sliding of the Sichuan-Yunnan block, the special geological structure of the arcuate structural belt may serve as a natural laboratory to constrain and validate the regulatory role of the Honghe fault zone in the evolutionary process of the southeastern margin of Qinghai-Xizang Plateau. GNSS technology enables the quantitative characterization of fault activity, facilitating the understanding of the motion patterns and stress accumulation states of the Honghe fault zone and its surrounding areas. This contributes to elucidating the process of strong earthquake incubation and occurrence in the region, providing reference for future seismicity trend forecasting.
This paper builds upon the work of previous researchers, collecting and organizing multiple GNSS observation data sets from the study area since 1999 onwards. Based on the existing GNSS velocity field in the Yunnan region, the regional strain rate fields for the periods 1999−2007, 2009−2014 and 2015−2020 are estimated using Kriging interpolation, revealing the current characteristics of regional deformation evolution in the southeastern Yunnan arcuate structual belt. By analyzing the GNSS profiles, the fault slip rate of the central-southern segment of Honghe fault zone, the Shiping-Jianshui fault and the Qujiang fault are derived by fitting the elastic dislocation model. Based on the existing GNSS continuous observation data in the study area, five sets of regional blocks crossing the central and southern segments of Honghe fault zone, the Shiping-Jianshui fault, the Qujiang fault, and the central-southern segment of Xiaojiang fault zone were constructed to derive the block strain time series. These analyses yield the following conclusions:
1） Based on the regional strain rate field results, the principal strain rate exhibits near NE-trending extension （tensile） and near NW-trending compression （compressional）. This indicates that the material from the Qinghai-Xizang Plateau is being extruded and escaping toward the southeastern end of the Sichuan-Yunnan block. Blocked by the South China block, this escape leads to NW-trending compression and NE-trending extension, with tension being the dominant regime. It is inferred that the southeastern Yunnan arcuate structual belt may be a currently forming zone of extension-shear tectonics.
2） The profile analysis results show that the central-southern segment of Xiaojiang fault zone exhibit left-lateral strike-slip with extension, with a slip rate of 9.14 mm/a. The Shiping-Jianshui fault, the central-southern segment of Honghe fault zone all exhibit right-lateral strike-slip characteristics, but overall slip rate is relatively low. The central segment of Honghe fault zone has a slip rate close to zero, indicating a state of slow creep. In the wedge-shaped area of southeastern Yunnan, differential movement between the two boundary faults （the central-southern segment of Xiaojiang fault zone and the central segment of Honghe fault zone） is accommodated by secondary faults within the wedge through dextral shear and block rotation. The strike-slip rate of the central-southern segment of Honghe fault zone is smaller than that of other secondary faults in the study area, suggesting that its role as the southwest boundary of the Sichuan-Yunnan active block has been significantly weakened.
3） From the trend of cross-fault block strain, the principal strain of Xiaojiang fault zone is in a distinctly tensile state. The strain of the Qujiang fault is slightly lower than that of the southern segment of Xiaojiang fault zone, indicating weak extensional movement. The southern segments of Shiping-Jianshui fault and the Honghe fault zone show weakened strain, dominated by weak extrusion movement. The central segment of Honghe fault zone represents a low strain area. Within the arcuate structual belt, the maximum shear strain gradually increases from south to north. Analysis of the first shear strain time series reveals that the three NW-trending faults in this area are characterized by right-lateral shear. The shear strain of the Shiping-Jianshui and Qujiang faults is significantly stronger than that of the southern segment of Honghe fault zone, with central segment of Honghe fault zone being the least active. This further confirms that the central-southern segment of Honghe fault zone are not the main stress-bearing faults in this area, and cannot independently constitute a boundary zone of active tectonic units. Deformation of the upper crust on the southeastern margin of the Sichuan-Yunnan block is now distributed among secondary faults within the southeastern Yunnan wedge-shaped tectonic zone, with the Shiping-Jianshui and Qujiang faults being the main stress-bearing faults. This result may provide a preliminary explanation for the difference in seismic potential among the three faults in the southeastern Yunnan arcuate structural belt.
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