Abstract
Abstract Wide-angle refraction experiments were conducted to reveal the crustal structure at the transition between the intra-oceanic island arc crust of the mid Izu-Ogasawara (Bonin) arc and the backarc oceanic crust of the Shikoku Basin. The island arc crust consists of an upper crust about 5 km thick with a P-wave velocity <6.0 km/s, a middle crust 5 km thick with a P-wave velocity of 6.0–6.3 km/s, and a lower crust 10 km thick with a P-wave velocity of 6.8–7.2 km/s. The total crustal thickness is about 20 km. The thickness thins to approximately 6 km over a distance of 30 km at the western margin of the Izu-Ogasawara arc (IOA). These features are very similar to those of the northern IOA, which indicates that the crustal structure is relatively constant within 200 km at the northern and mid IOA. The Kinan Escarpment, a 500-km-long fault with a maximum offset of 800 m, characterizes the transition zone between the IOA and Shikoku Basin. The seismic crustal model indicates that the escarpment is a fault which tears the whole oceanic crust along the western margin of the IOA. However, no significant differences exist in the crustal structure on either side of the escarpment, and the Kinan Escarpment seems to be a zone of the structural weakness from its birth.
Highlights
The Izu-Ogasawara (Bonin) island arc-trench-backarc system is located on the Philippine Sea plate along with the Izu-Ogasawara Trench (Fig. 1(a))
Since the seafloor topography showing the Kinan Escarpment (KE) is clearly identifiable at 30◦N, we were able to examine the crustal structure related to the escarpment beneath the transition zone between the Izu-Ogasawara arc (IOA) and Shikoku Basin (SB)
PmP signals from the Moho of the island arc crust are observed at some ocean bottom seismographs (OBS)
Summary
The Izu-Ogasawara (Bonin) island arc-trench-backarc system is located on the Philippine Sea plate along with the Izu-Ogasawara Trench (Fig. 1(a)). We conducted two wide angle seismic experiments at around 30◦N, about 150 km south of the previous study (Suyehiro et al, 1996; Takahashi et al, 1998), in order to elucidate the variation in the seismic velocity models along the arc axis.
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