[Objective] The Southeast Yunnan Arcuate Structural Belt, with the middle and southern segments of the Red River fault as its main body, serves as the southwestern boundary of the Sichuan-Yunnan block and the forefront of its south-southeastward movement. However, there remains controversy regarding whether its current motion is primarily characterized by thrust-strike-slip due to compression or normal-strike-slip due to tension. This debate is strongly correlated with the complex stress-strain patterns surrounding the southeastern margin of the Tibetan Plateau. Clarification of the kinematic characteristics and genesis of the Southeast Yunnan Arcuate Structural Belt will help to understand the regional tectonic evolution.
[Methods] This study utilized the remote sensing image interpretation and field geological surveys to identify late Quaternary tectonic activity evidence in the middle and southern segments of the Red River fault. A three-dimensional geological model tailored to the actual characteristics of the region was established, considering the influence of lower crustal flow, and finite-difference numerical simulations were conducted with different velocity boundary conditions set at 26.5°N.
[Results] The study reveals: 1) Numerous geomorphic features and fault profiles in the middle and southern segments of the Red River fault indicate that most of the faults along the line are predominantly high-angle, with a northwest orientation and complex fault rock development. The presence of structural wedges and infilling of overlying materials has been observed in multiple typical outcrops. Additionally, there are significant undulations in the lower part of the overlying strata on both sides of the fault; 2) The numerical simulation results show that the influence of the far-field stress on both sides of 26.5°N on the horizontal and vertical deformation of the southeastern margin of the Tibetan Plateau is quite different, and the deformation is further expanded by the presence of lower crustal flow; 3) Results of numerical simulations of velocity fields, maximum shear strain rates, and maximum principal stresses demonstrate differences in surface movement trajectories, velocity distribution, stress conditions, and deformation accumulation among different models in the arcuate structural belt area, with the presence of lower crustal flow promoting deformation accumulation and making the magnitude of the velocity field closer to that of the current GPS horizontal velocity field.
[Conclusion] 1) The numerous geological profiles along the middle and southern segments of the Red River fault reveal a predominantly normal-strike-slip movement, indicating that the region is currently dominated by the effects of transtension; 2) Current tectonic deformation and landscape evolution in the southeastern margin of the Tibetan Plateau are mainly controlled by two different force sources: one is the southeastward movement of materials, and the other is the arc-parallel extension and slab rollback occurring beneath the Sunda-Java subduction zone. The presence of lower crustal flow influencing the scale of tectonic deformation in this region. In the Southeast Yunnan Arcuate Structural Belt, arc-parallel extension and slab retreat play a more significant controlling role; 3) The initial curved shape of the Southeast Yunnan Arcuate Structural Belt is mainly attributed to the influence of material migration towards the southeast and the Xiaojiang sinistral strike-slip fault, resulting in continuous deformation under the constraints of pre-existing structural fabrics and the controls of arc-parallel extension and slab retreat.
[Significance] The research results contribute to understanding the current activity and causes of the Southeast Yunnan Arcuate Structural Belt, and provide a quantitative analysis reference and theoretical basis for the study of tectonic evolution in the southeastern margin of the Tibetan Plateau.
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