Is the East China Sea an exotic microcontinent from the Paleo-Pacific? ——Paleomagnetic Insights from the East China Sea Basin

    The West Pacific-East Asia transition zone is characterized by a remarkable continental mosaic system and a chain of marginal basins. However, the complexity of the continental amalgamation process has led to controversy regarding the origin and migration of many micro-continents. In particular, the origin of the East China Sea (ECS) remains a subject of debate. The question of whether the ECS is "part of the South China Block (SCB)" or an "exotic microcontinent" has yet to be definitively resolved (Niu et al., 2015; Fu et al., 2022). Furthermore, there is divergence in perspectives concerning the evolution of the ECS, exemplified by models such as "back-arc spreading" and "strike-slip pull-apart", which in turn fuel disputes regarding the nature of the ECS basin.

    In this study, we conducted paleomagnetic sampling of Cretaceous-Eocene cores from nine boreholes in the ECS basin. A systematic paleomagnetic study was undertaken, employing rock magnetic experiments, scanning electron microscope (SEM) analysis, and stepwise thermal demagnetization. Utilizing the inclination data of characteristic remanent magnetization (ChRM) obtained from thermal demagnetization experiments, we have, for the first time, derived paleomagnetic records for Early Cretaceous to Eocene cores from the ECS boreholes. The results indicate that the paleolatitudes of the ECS were 18.7° ± 4.5° (134 Ma), 21.4° ± 6.4° (107.2 ± 0.6 Ma), 18.1° ± 4.5° (66-61 Ma), 20.3° ± 4.3° (61-56 Ma), and 26.4° ± 8.2° (49-34 Ma). The investigation and comparison of the paleomagnetic data reveal that the paleolatitudes of the ECS are similar to those of the SCB from the Early Cretaceous to the Eocene. This suggests that the ECS and SCB have been part of the same tectonic block since the Early Cretaceous.

    Further analysis of the spatial relationship between the ECS and SCB confirms that their relative motions can be delineated into three distinct phases: (1) During the Cretaceous period, the ECS and the SCB moved in the same direction, albeit with a disparity in their velocities; (2) During the Late Cretaceous to Early Paleocene period, the ECS migrated northward while the SCB shifted southward; (3) During the Middle Paleocene to Eocene period, the ECS and the SCB moved in concert, with negligible differences in velocity, thereby establishing a stable connection. It is concluded that the kinematic transitions of the ECS and the SCB from the Early Cretaceous to the Eocene were directly governed by changes in the subduction direction of the Izanagi/Pacific Plate.