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. Plain Language Summary The growth of the continental crust is an episodic process associated with the supercontinental circle, but a specific mechanism to explain it is still lacking. Here, we combined the seismic images with magnetic and geological data to reveal that two trapped oceanic basins form parts of the lower crust in southern Altaids. These oceanic basins are trapped in the gaps between continents and arcs because their boundaries are rarely having a jigsaw puzzle‐like fit. During this process, the continental crust grows significantly due to the addition of oceanic basins and the associated decrease in subduction erosion. We proposed that numerous oceanic basins could be captured during the formation of supercontinents, leading to peaks in continental crust growth. Key Points We present a new high‐resolution image of the crustal structures of the Eastern Junggar using the receiver function method Combing geomagnetic and geological data to identify trapped oceanic basins and reconstruct their trapping process Investigating the contribution of trapped ocean basins to continental growth and the implications for global continental episodic growth