Abstract:
[Objective] The kinematic history of the South China Block (SCB) following the breakup of Gondwana is of critical significance for constraining the timing of the initial opening of the Ailao Shan–Song Ma Paleo-Tethys Ocean basin. However, the understanding of this process has been limited by the scarcity of reliable paleomagnetic data. [Methods] In this study, systematic paleomagnetic, rock magnetic, and petrographic analyses were conducted on red beds and limestones of the Upper Silurian Kuanti Formation from the Qujing area, Yunnan Province. [Results] The samples were, however, very likely remagnetized during the Cenozoic. Rock magnetic analyses indicate that the red beds are predominated by hematite as the primary carrier of magnetization, with minor contributions possibly from magnetite/titanomagnetite. Anisotropy of magnetic susceptibility (AMS) results reveal combined features of sedimentary, incipient deformation and strong cleavage fabrics, suggesting that the sediments may have undergone significant syn-depositional or post-depositional tectonic deformation. Lithological observations indicate that the magnetic minerals within both the red beds and limestones are primarily authigenic. From 14 sites (85 red bed and limestone specimens), a stable, high-temperature or high-field characteristic remanent magnetization (ChRM) component, converging towards the origin, was isolated, yielding a mean direction of Declination (Dg) = 332.2°, Inclination (Ig) = 51.6°, precision parameter (kg) = 19.0, confidence cone half-angle of the mean direction (α95) = 9.4° before and of Declination (Ds) = 342.5°, Inclination (Is) = 25.7°, precision parameter (ks) = 15.8, confidence cone half-angle of the mean direction (α95) = 10.3° after the tilt-adjustment. Several fold tests indicate a negative result. The corresponding paleomagnetic pole calculated from the in-situ ChRM direction is located at 64.9°N, 35.2°E (A95=7.9°). This paleomagnetic pole is consistent with reference poles for the SCB between 20~5 Ma, suggesting that the Kuanti Formation underwent remagnetization at approximately 20 Ma. [Conclusion] Integrating these results with previously reported reliable paleomagnetic data from small blocks within the southeastern Tibetan Plateau and the SCB since 50 Ma, as well as prior studies on the tectonic evolution of the southeastern Tibetan Plateau, a two-stage model of co-evolution between the SCB and the southeastern Tibetan Plateau is proposed. During 50~20 Ma, collision between the Indian and Eurasian plates resulted in the uplift and crustal shortening of the southeastern Tibetan Plateau. The fault systems along the southeastern Tibetan Plateau exhibited left-lateral strike-slip motion, which induced clockwise rotation of the SCB relative to stable Eurasia. The amount of clockwise rotation varied across different locations of the SCB, with sites closer to the southeastern Tibetan Plateau fault systems experiencing larger rotations. Since 20 Ma, continuous northward subduction of the Indian Plate beneath Eurasia, combined with multiple dynamic processes, including lower to middle crustal flow, gravitational spreading, mantle convection inducing by the tearing of the Indian Plate, and Pacific–Indian Ocean subduction, has driven clockwise rotation and extrusion of the southeastern Tibetan Plateau. During this stage, the fault systems exhibit right-lateral strike-slip motion, while the SCB underwent counterclockwise rotation relative to stable Eurasia. The magnitude of rotation is generally consistent across different locations. [Significance] Therefore, the transition of the SCB from a “clockwise rotation” to a “counterclockwise rotation” behavior essentially represents a direct manifestation of the shift in the geodynamic regime along the southeastern Tibetan Plateau from vertical uplift to tectonic extrusion. The ~20 Ma paleomagnetic data in this study provide robust evidence for a regional reversal of the tectonic framework along the southeastern Tibetan Plateau.
