Reinitiation of subduction and magmatic responses in SW Japan during Neogene time

Abstract

Southwestern Japan during Paleogene time was affected by subduction of the Kula and Pacifi c plates beneath the eastern margin of Eurasia, followed by a brief episode of transform faulting to allow the Shikoku Basin to open at 27-15 Ma. Subsequent opening of the Japan Sea back-arc basin in middle Miocene time broke Japan away from Eurasia, and SW Japan rotated clockwise ~45º as it drifted south (17-15 Ma). Southward drift required subduction of the Philippine Sea plate beneath SW Japan, leading to the formation of a magmatic arc. Subduction of the hot Shikoku Basin lithosphere and spreading ridge resulted in distinctive volcanism in the SW Japan forearc between 17 and 12 Ma. This volcanism included midoceanic-ridge basalt (MORB)-like intrusions and oceanic-island basalt (OIB)-type alkali basalts, felsic plutons in the accretionary prism, and high-magnesium andesites along the Setouchi forearc basin. Mafi c magmas in the outermost forearc originated from the subducted Shikoku Basin spreading ridge, whereas Setouchi high-magnesium andesites (HMA) may have resulted from the interaction of melts of the subducted Philippine Sea plate with the overlying mantle. Felsic magmas in the forearc resulted from melting of Shimanto Belt sediments caused by intrusion of HMA magmas. Persistent rear-arc volcanism was caused by upwelling asthenosphere associated with opening of the Sea of Japan. Rear-arc volcanism began ca. 25 Ma as rift-fi ll low-alkali tholeiite volca-nism and was replaced gradually after 12 Ma by alkali basalt volcanism. Upwelling-related alkali volcanism continues up to the present, whereas forearc volcanism ceased at 12 Ma.The volcanic arc narrowed with time as the Philippine Sea slab descended and slowly cooled. Adakitic dacites erupted after 1.7 Ma above the 100-km-depth contour of the subducted Philippine Sea plate, suggesting that melting resulted from interaction of the slab with upwelling asthenosphere. Interactions between upwelled rear-arc asthenosphere and subduction of the hot Philippine Sea slab appear to have been the main controls of magmatism for the Neogene SW Japan arc.

Boron systematics of Hisatsu and Kirishima basaltic rocks from southern Kyushu, Japan

“…This low-velocity zone is consistent with mantle upwelling around the northeastern edge of the currently rifting Okinawa Trough (Sadeghi et al, 2000;Sibuet et al, 1995) and is believed to have activated volcanism in the Kyushu backarc and Ryukyu arc (Kimura et al, 2005).…”

“…Eruptive products include lavas with OIB, island-arc-type basalt (IAB), high-magnesian andesite (HMA) and adakite-type affinities (Kimura et al, 2005). These diverse magma types are also observed in the Kyushu subduction zone.…”

“…1). Southern Kyushu is characterized by the subduction of the southern (old) segment of the Philippine Sea plate (Kimura et al, 2005;Zhao et al, 2000;Iwamori, 2007).…”

K-Ar ages of high-magnesian andesite lavas from northern Kyushu, Japan

“…The second phase of extension is related to a main formation of the trough (Sibuet et al, 1995). This backarc rifting activated the volcanism in the Kyushu backarc region and Ryukyu arc (Kimura et al 2005). The OIB -type basaltic volcanisms in NW Kyushu those are younger than the ones in upper Pliocene may also be affected by this tectonic event.…”

“…The OIB -type basalt magmas were active in the backarc region since about 15 Ma. These activities are probably related to the extension of the backarc (Uto et al, 2004;Kimura et al, 2005). The subduction of the Philippine Sea plate commenced at 6 -4 Ma in northern Kyushu (Kamata and Kodama, 1994).…”