Evolution from fore‐arc oceanic crust to island arc crust: A seismic study along the Izu‐Bonin fore arc

Abstract

[1] Petrological studies have suggested that oceanic crust is formed during the initial stage of subduction. However, there is little geophysical evidence for the formation of oceanic crust in the fore-arc region. We conducted an active-source seismic survey in the fore-arc region of the Izu-Bonin intraoceanic arc to examine processes of crustal formation associated with the initiation of subduction. We used seismic refraction tomography to obtain a detailed seismic velocity model and diffraction-stack migration of picked reflection travel times to derive reflectivity images. These data show a remarkably thin crust (<10 km thick) along the northern half of the Bonin ridge and abrupt crustal thickening (to ∼20 km) toward the south of the ridge. Comparison of velocity-depth profiles of the thin fore-arc crust of the Bonin ridge with those of typical oceanic crusts showed them to be seismologically identical. Boninitic magmatism is evident in the area of thin crust and tholeiitic-calcalkaline andesitic volcanism in the area of thick crust. High-precision dating of the volcanic rocks showed that the thin fore-arc oceanic crust was created soon after initiation of subduction (48-45 Ma) and that the nonoceanic thick crust was created with tholeiitic-calcalkaline andesitic magmatism after the boninitic magmatism was ceased. Our seismological image strongly supports the view that fore-arc oceanic crust was formed by fore-arc spreading during the initial stage of subduction along the Izu-Bonin intraoceanic arc.Citation: Kodaira, S., N. Noguchi, N. Takahashi, O. Ishizuka, and Y. Kaneda (2010), Evolution from fore-arc oceanic crust to island arc crust: A seismic study along the Izu-Bonin fore arc,

Structural variation of the Bonin ridge revealed by modeling of seismic and gravity data

“…This observation strongly supports the view that forearc oceanic crust along the IzuBonin intra-oceanic arc was formed by forearc spreading during the initial stage of subduction (Kodaira et al, 2010). However, Kodaira et al (2010) pointed out that the seismic structure modeled to the north of Chichi-jima was not entirely consistent with the structure across the Bonin ridge reported by Takahashi et al (2009). The ridge-parallel profile of Kodaira et al (2010) and the cross-ridge profile of Takahashi et al (2009) intersect between Chichi-jima and Muko-jima, where the ridge-parallel profile shows a crust of about 10 km thickness, and the cross-ridge profile shows a crust of about 18 km thickness.…”

“…Velocity-depth profiles of the thin forearc crust of the Bonin ridge were seismologically identical to those of typical oceanic crust. This observation strongly supports the view that forearc oceanic crust along the IzuBonin intra-oceanic arc was formed by forearc spreading during the initial stage of subduction (Kodaira et al, 2010). However, Kodaira et al (2010) pointed out that the seismic structure modeled to the north of Chichi-jima was not entirely consistent with the structure across the Bonin ridge reported by Takahashi et al (2009).…”

“…However, Kodaira et al (2010) pointed out that the seismic structure modeled to the north of Chichi-jima was not entirely consistent with the structure across the Bonin ridge reported by Takahashi et al (2009). The ridge-parallel profile of Kodaira et al (2010) and the cross-ridge profile of Takahashi et al (2009) intersect between Chichi-jima and Muko-jima, where the ridge-parallel profile shows a crust of about 10 km thickness, and the cross-ridge profile shows a crust of about 18 km thickness. Kodaira et al (2010) gave three possible explanations for this inconsistency: differences in the initial models used, the effect of smoothing during inversion, or edge effects in the modeling.…”

“…A recent seismic study along the Bonin ridge (Kodaira et al, 2010) revealed striking structural variation along the Bonin ridge. The seismic data show a remarkably thin crust (<10 km thick) along the northern half of the Bonin ridge (beneath Chichi-jima and Muko-jima) and abrupt crustal thickening (to ∼20 km) toward the south of the ridge (beneath Haha-jima).…”

“…Since then, a series of high-resolution deep seismic studies in the IBM arc have presented new insights into the formation of arc crust Kodaira et al, 2010) and SPr2 (processed by Takahashi et al, 2009), respectively. Only the eastern half of line SPr2 (red solid line) was re-modeled in this study.…”