Tectonic characteristics and numerical simulation analysis of an arcuate structural belt：A case study of the middle and southern segments of the Red River fault

Objective The southeast Yunnan Arcuate Structural Belt, with the central 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 is still controversy as to whether its current motion is primarily characterized by transpression or transtension. This debate is strongly correlated with the complex stress-strain patterns surrounding the southeastern margin of the Qinghai-Tibet 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 interpretation of remote sensing images and geological field surveys to identify evidence for late Quaternary tectonic activity in the central and southern segments of the Red River fault. A three-dimensional (3D) geological model tailored to the actual characteristics of the region was established. It considers the influence of lower crustal flow and includes finite-difference numerical simulations with different velocity boundary conditions set at 26.5°N. Results The study reveals: (1) Numerous geomorphic features and fault profiles in the central and southern segments of the Red River fault indicate that most of the faults along the line are predominantly high-angle, northwest-striking with a complex fault rock development. The presence of structural wedges and infill of overlying material has been observed in several typical outcrops. Additionally, there are significant undulations in the lower part of the overlying strata on both sides of the fault. (2) The numerical simulations show that the influence of the far-field stress on the horizontal and vertical deformation of the southeastern margin of the Qinghai-Tibet Plateau is quite different on both sides of 26.5°N, and the deformation is further enhanced by the presence of lower crustal flow. (3) Numerical simulations of velocity fields, maximum shear strain rates, and maximum principal stresses demonstrate that surface motion trajectories, velocity distributions, stress conditions, and deformation accumulation differ among different models for the arcuate structural belt area; the presence of lower crustal flow promotes deformation accumulation and makes the magnitude of the velocity field closer to that of the current GPS horizontal velocity field. Conclusion (1) The numerous geological profiles along the central and southern segments of the Red River fault reveal 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 Qinghai-Tibet Plateau are mainly controlled by two different force sources: one is the southeastward movement of material, and the other is the arc-parallel extension and slab rollback beneath the Sunda-Java subduction zone. The presence of lower crustal flow influencess the scale of tectonic deformation in the southeastern margin of the Qinghai-Tibet Plateau. 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. Under the constraints of pre-existing structural fabrics and the control of arc-parallel extension and slab retreat, it has led to continuous deformation. Significance The research contributes to the understanding of the current activity of the southeast Yunnan Arcuate Structural Belt and the causes of this activity. It provides a quantitative analysis reference and theoretical basis for the study of the tectonic evolution of the southeastern margin of the Qinghai-Tibet Plateau.