Crustal structure of the ocean-island arc transition at the mid Izu-Ogasawara (Bonin) arc margin

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.

Backarc basin oceanic crust and uppermost mantle seismic velocity structure of the Shikoku Basin, south of Japan

“…In this study, we did not find any crustal thinning beneath the KE on SPr10 and SPr7. This result may be correlated with the smaller bathymetric offsets of the escarpment beneath SPr10 and SPr7 compared with that of Nishizawa et al (2006). Crusts with a V p < 3.5 km/s thicken eastward from around the KE, and this thickening is related to the volcanic activity of the IOA.…”

“…In the SB, P n velocity estimated in this study for the spreading direction is at most 8.1 km/s. On the other hand, P n velocity for the northsouth line of Nishizawa et al (2006) to the south of SPr10 (Fig. 1) was ∼8 km/s.…”

“…In the eastern part of the SB, Nishizawa et al (2006) were barely able to detect any crustal thinning at the Kinan Escarpment (KE), which is a 500-km-long fault with a maximum offset of 800 m that characterizes the transition zone between the IOA and SB (Fig. 1).…”

Variations in seismic velocity distribution along the Ryukyu (Nansei-Shoto) Trench subduction zone at the northwestern end of the Philippine Sea plate

“…The V p model of ECr31, along the arc line, reveals a typical island structure, the same as the Japan arc compiled by Iwasaki and Sato (2009). A high V p of around 7.2 km/s at the bottom of the lower crust beneath intra-oceanic island arcs, such as the Izu-Ogasawara island arc and the Kyushu-Palau Ridge (e.g., Nishizawa et al 2006Nishizawa et al , 2016, was not found in the Ryukyu arc. None of the arc crusts of the Japan Islands have such a high V p at the bottom of the lower crust, which might indicate differences in the evolution tectonics between the intra-oceanic island arcs and the Japan island arcs.…”

Transition from collision to subduction and its relation to slab seismicity and plate coupling

“…Black arrow indicates relative plate motion between the Philippine Sea plate and Eurasian plate (Seno et al 1993). The GMT software (Wessel and Smith 1998) was used to draw the maps arc-like crust (several kilometers thicker than typical oceanic crust) exists just south of the ISTL (Nakanishi et al 1998;Nishizawa et al 2006;Kodaira et al 2008), leaving the open question of the nature of the Philippine Sea plate subducting beneath the ISTL.…”