Deformation Structures in the Southern Ailao Shan, Western Yunnan: A Response to India–Eurasia Plate Convergence

ABSTRACT The Ailao Shan–Red River shear zone is a major lithospheric‐scale structure accommodating Cenozoic deformation along the southeastern margin of the Tibetan Plateau. However, the timing and deformation evolution of its southern segment remain debated. This study integrates field structural analysis, microstructural observations, quartz crystallographic preferred orientation (CPO), and zircon U–Pb geochronology to constrain the deformation history of the southern Ailao Shan–Red River shear zone. Three deformation stages (D1–D3) are identified. D1 is characterised by isoclinal to tight folding, axial‐planar foliation, and boudinage, indicating regional shortening dominated by coaxial strain. D2 represents the principal deformation stage and is defined by pervasive sinistral ductile shearing, with well‐developed mylonitic foliation, stretching lineation, and kinematic indicators such as S–C fabrics and asymmetric porphyroclasts. Quartz microstructures and CPO patterns indicate deformation under medium‐ to high‐temperature conditions of ca. 400°C–600°C. Fabric‐constrained zircon U–Pb ages of ca. 31.9–27.9 Ma provide direct constraints on a key interval of D2 activity, whereas D3 records subsequent low‐temperature ductile–brittle overprinting at ca. 300°C during progressive exhumation and cooling. The D1–D3 sequence records a staged tectonic response to India–Eurasia convergence, evolving from collision‐related crustal shortening, through Oligocene sinistral shear‐zone localisation during lateral extrusion of the Indochina Block, to late‐stage exhumation‐related cooling and ductile–brittle overprinting.