HR: 0800h
AN: T41E-1265    [Abstracts]
TI: Rift Basin Architecture Near the Collision Zone in the Northern end of Back-arc Rift of the Izu-Bonin Arc, Based on Airgun Profiles
AU: * TSUJINO, T
EM: taphonomy@ni.aist.go.jp
AF: Geological Survey of Japan/AIST, Higashi, Tsukuba, 3058567 Japan
AU: Ishizuka, O
EM: o-ishizuka@aist.go.jp
AF: Geological Survey of Japan/AIST, Higashi, Tsukuba, 3058567 Japan
AB: Two concatenated basins, Hachijo and Mikura basins in northward order, are distributed in the northern part of the Izu-Bonin Arc, in collision with the NE Japan Arc. The Izu-Bonin Arc has long but punctuated narrow depression in back-arc area; the `back-arc rift'. The Hachijo basin is accepted as `Hachijo Rift'(e.g. Tamaki et al., 1981), whereas the Mikura basin remains uncertain, because the Hachijo basin is similar to the Sumisu Rift, well-investigated typical rift in the Izu-Bonin Arc, while the Mikura basin shows unclear boundary fault. The Hachijo basin is accordant to bathymetric trends in the Izu-Bonin Arc (Murakami, 1996): Rift basins tend to shallow northward. However, the Mikura basin is deeper (1600m) than the Hachijo basin (1100m). The Hachijo basin is 40$\times$60 km$^2$ NS-elongated basin, bounded the eastern and the northern sides by cliff to 500m deep bank and cliff to the Mikura basin respectively. The Mikura basin is 50km wide square basin, bounded the northern and the eastern sides by steep slope to 300m deep bank and slope to the 500m deep Kitakurose Bank respectively. Profiles show the Hachijo basin is thickly-deposited($\geq$1.2sec in two-way travel time) half-graben, bounded the eastern cliff by NS-strike normal fault. The Mikura basin is also filled with thick deposits($\geq$1.5sec) but bounded the northern slope by EW-strike normal faults partially covered with slump deposits. The reflectors in eastern slope of the basin continue to east of the basin, Kitakurose Bank. In this bank, the NS-strike W-dip normal fault is developed with downthrow offset more than 1.2sec. The deposits on the hanging wall are estimated at Pliocene (Yuasa, 1984). The Mikura basin might have been originally NS-elongated (half-)graben, bounded by this fault, and changed its spreading axis. Concludingly the Mikura basin shows tectonic margin, influenced with the collision to the NE Japan Arc as well as repeated rifting, whereas the Hachijo basin represents as the northern end of typical back-arc rift. The northern slope and EW normal fault of Mikura basin is situated at the boundary between northern shallow area rich in volcanogenic islands, and southern deep area rich in rifts, and might indicate the structural boundary.
DE: 9355 Pacific Ocean
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8010 Fractures and faults
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting