Magnetic inversion in the East China Sea and Okinawa Trough: Tectonic implications

In order to better understand continental rifting along the eastern margin of the Eurasian plate, we have performed a magnetic inversion to reveal the distribution and characteristics of the belts and basins in the East China Sea and Okinawa Trough. Equivalent magnetizations are obtained by assuming a constant thickness of 5 km of the magnetized layer whose top surface is assigned to the sea floor and taking into account the present-day geomagnetic field. Because of absence of magnetic reversals in the study area, a high-magnetization zone can reflect a shallow and/or highly magnetized basement. In contrast, a low-magnetization zone generally corresponds to a rifted basin with thick sediments. The belts in the East China Sea shelf basin are in general below sea level and correspond to shallow basement with thin sediments. The inversion results show that the belts are parallel and trending about N075°. This regular trend suggests a simple, southeastward rifting pattern of the East China Sea shelf basin. The belt trend changes from N075° to N095° at the eastern side of the Taiwan-Sinzi belt. Southwest of the Taiwan-Sinzi belt, the belts in the Ryukyu subduction system are parallel to the Ryukyu trench. It implies that rifting of the middle and northern Okinawa Trough was initiated along the eastern side of the Taiwan-Sinzi belt; then the rifting has been mainly controlled by the oblique subduction of the Philippine Sea plate beneath the Ryukyu arc. The southern end of the Taiwan-Sinzi belt terminates at the NW-SE-trending Miyako-Yandang high-magnetization zone. The southern Okinawa Trough, rifted since early Pleistocene, is distinct from the middle and northern Okinawa Trough by the Miyako-Yandang high-magnetization zone. The southern Okinawa Trough consists of two rifting patterns: east of 123.5°E, the backarc rifting is quasi-symmetric as evidenced by symmetrically high magnetization zones on both sides of the Yaeyama Depression. In contrast, west of 123.5°E, the volcanism or high magnetization zones, developed along existing NW-SE-trending strike-slip faults, have generally occurred at the northern margin of the southern Okinawa Trough. The Ryukyu arc, west of 123.5°E, exhibits low magnetization reflecting its non-volcanic nature of the upper crust. East of 123.5°E, the present-day Ryukyu arc volcanism is expressed by relatively high magnetization.