| Summary: | The marginal sea and back-arc basins in the Western Pacific Ocean have become the focus of tectonics due to their unique tectonic location. To understand the deep crustal structure in the back-arc region, we present a 545-km-long active-source ocean bottom seismometer (OBS) wide-angle reflection/refraction profile in the East China Sea. The P wave velocity model shows that the Moho depth rises significantly, from approximately 30 km in the East China Sea shelf to approximately 16 km in the axis of the Okinawa Trough. The lower crustal high-velocity zone (HVZ) in the southern Okinawa Trough, with Vp of 6.8–7.3 km/s, is a remarkable manifestation of the mantle material upwelling and accretion to the lower crust. This confirms that the lower crustal high-velocity mantle accretion is developed in the southern Okinawa Trough. During the process of back-arc extension, the crustal structure of the southern Okinawa Trough is completely invaded and penetrated by the upper mantle material in the axis region. In some areas of the southern central graben, the crust may has broken up and entered the initial stage of seafloor spreading. The discontinuous HVZs in the lower crust in the back-arc region also indicate the migration of spreading centers in the back-arc region since the Cenozoic. The asthenosphere material upwelling in the continent-ocean transition zone is constantly driving the lithosphere eastward for episodic extension, and is causing evident tectonic migration in the Western Pacific back-arc region.
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