THE FOREARC FELSIC MAGMATISM ALONG SOUTHWEST JAPAN CAUSED BY RUPTURING OF THE SUBDUCTING SLAB

The genesis of felsic magma is one of the most important issues of earth science since the continental crust is characterized by the granitic upper crust. Felsic magmatism occurred along the forearc SW Japan at 15 Ma, the latest stage of the Japan Sea opening. The Philippine Sea plate (PSP) subducting along SW Japan is composed of the Shikoku Basin (SB) formed between 27 and 15 Ma. Seismic observations indicate that the mantle wedge would be very thin or absent beneath the localities of the felsic rocks at the present day. A seismic exploration at Muroto, where the youngest part of the PSP meets to the Eurasian plate, indicates that sediments on the slab would not subduct but accrete to the overriding plate because of the strong plate coupling at the present day. Since the SB was younger and hence hotter than that at the present day, sediments on the slab could not have subduct at 15 Ma. These lines of evidence indicate that conventional models, such as partial melting of sediments on the subducting slab, could not explain the genesis of forearc magmatism along SW Japan.

The slab gently subducts and shows a strike-parallel folded configuration along Tokai – Shikoku, whereas it steeply subducts along Kyushu. Deep earthquakes are rare beneath localities of the felsic rocks, which indicates that the slab would be ruptured there. The slab should suffer from strike-parallel compressive or strike-parallel extensive stress depending on its descending angle when the slab descends from the spherical earth surface to the mantle. These strike-parallel stresses would rupture the subducting slab along its pre-existing weaknesses such as fracture zones. Such a slab rupturing would have enabled partial melts of the sub-slab asthenosphere and/or the sub-slab asthenosphere itself to intrude to the base of the crust composed of accreted oceanic complexes. Anatexis of accreted oceanic sediments would have formed felsic magmas along SW Japan.