The Yunkai Massif is one of the Precambrian metamorphic crystalline basements within the South China Block. To better understand the Indosinian deformation characteristics in the Yunkai area, the Shili ductile shear zone at the northwestern margin of the Yunkai Massif was selected for this study.

Through detailed field investigations, finite strain measurements, kinematic vorticity analyses, and ⁴⁰Ar/³⁹Ar geochronology, the structural characteristics and evolutionary process of the shear zone were constrained.

The Shili ductile shear zone generally strikes WNW–ESE with NNE-directed thrusting. It has undergone four phases of deformation. Among which, the D2 to D4 deformations all indicate movement from SSW to NNE. D2 to D4 formed under a unified tectonic stress field, reflecting an evolution from ductile shear through brittle-ductile deformation to folding during the Late Triassic. Mylonites formed synchronously with the ductile shear deformation yield a muscovite ⁴⁰Ar/³⁹Ar plateau age of 232 ± 2.3 Ma (MSWD = 3.65). The Flinn index (K) is 0.06–0.77, the Lode parameter (ν) is 0.13–0.89, the Effective strain (E_s) is 0.41–0.51, and the kinematic vorticity value (W_k) is 0.70–0.78. These parameters indicate a flattening-type strain regime dominated by general shear, with comparable contributions of pure shear and simple shear. The dynamic recrystallization of minerals suggests that the deformation environment reached the upper greenschist facies.

This study indicates that the mylonitization, crenulation cleavage development, and folding within the Shili ductile shear zone were dynamically driven by the propagation of far-field stress resulting from the NE–SW collision between the Indochina and South China blocks during the Indosinian period.

These findings provide critical constraints on the Indosinian tectono-thermal event and its geodynamic framework in South China.