MECHANISM OF BACKARC OPENING IN THE JAPAN SEA - ROLE OF ASTHENOSPHERIC INJECTION

The process of backarc extension in the Japan Sea includes two modes of spreading, the parallel and rotational or fan-shape opening; data from paleomagnetism for rocks on the Northeast Japan arc, radiometric age determination for dredged volcanic rocks and heat flow in the basin have suggested that the parallel opening of the Japan Sea took place earlier than the rotational opening at 21-14 Ma. The mechanism of opening of the Japan Sea has been examined based on the secular change in the geometry of the subducted slab and in the volcanic activity on the Northeast Japan arc. Mechanisms of backarc rifting including mantle diapirism, mantle flow induced by the slab subduction, or interaction of the over- and the underriding plates cannot reasonably explain the process of opening of the Japan Sea. The most plausible mechanism operating in the spreading process of the Japan Sea is injection of the asthenosphere into the mantle wedge, that forcefully pushed the subducted slab to cause both steepening of the slab and the backarc extension. The asthenospheric current towards the Northeast Japan arc-trench system may have been associated with the large-scale upwelling of the asthenosphere beneath the northeastern China region, that caused intra-plate volcanism and produced continental rift systems in the region. The present mechanism can also explain the active backarc rifting in the Okinawa Trough in the western Pacific and the Taupo Depression in North Island, New Zealand. © 1990.