The enhanced preservation potential of continental material during continental convergence is thought to be responsible for the episodic continental growth process. However, the mechanism of preservation potential variation is unclear. In this study, we use a novel high-density passive-source seismic approach to image the whole-crust architecture of the juvenile continent in southern Altaids. Two arcuate crust fragments are found between Paleozoic island-arc belts, which indicate the relicts of inter-arc oceanic basins. The results show that the trapped oceanic basins make up a large proportion of the juvenile continental crust and that the reduction in subduction erosion due to its incomplete subduction can explain a period of rapid continental growth revealed by previous zircon studies. We suggest that a large number of ocean basins may be trapped during supercontinent formation, and they play a critical role in continental material preservation and continental episodic growth.