3:45 PM - 4:00 PM
[SVC36-08] Crustal structure of the Izu-Bonin arc revisited
★Invited Papers
Keywords:Izu-Bonin arc, Ogasawara Islands, Seismic velocity structure, Formation of continent
The Izu-Bonin arc, including the Ogasawara Islands, is the only region in the world where the entire process of continental formation—from the initiation of plate subduction to the growth and accumulation of continental crust—can be comprehensively observed. This presentation aims to reevaluate the Ogasawara Islands' value as a World Heritage site from geomorphological and geological perspectives by introducing the scientific evidence established through the project funded by the Environment Research and Technology Development Fund, entitled "Formation Process of Continental Crust at Nishinoshima, Ogasawara Islands: A Scenario from the Initiation of Plate Subduction to Continental Formation in Collision Zones" (Principal Investigator: Yoshihiko Tamura). We will particularly review the crustal structure of the Izu-Bonin Arc from a seismological perspective.
In the 1990s and 2000s, the Japan Agency for Marine-Earth Science and Technology conducted multiple active-source seismic surveys using ocean-bottom seismographs along the volcanic front of the Izu-Bonin arc and the Ogasawara Ridge. These surveys, covering more than 1,000 km in total length, have revealed the seismic velocity (P-wave velocity) structure beneath the seafloor. The results indicate that andesitic middle crust is extensively distributed along the volcanic front, with a tendency for thicker middle crust beneath active volcanoes that erupt basaltic magma, such as Miyakejima and Hachijojima (IBr1 profile: Kodaira et al., 2007a, JGR; IBr3 profile: Kodaira et al., 2007b, Geology). Furthermore, in the Izu Arc, rhyolitic submarine calderas, such as Kurose Hole and Myojin Knoll, exist between basaltic volcanic islands. Beneath these submarine calderas, the middle crust is observed to be thinner, which has led to the conclusion that basaltic magma's latent heat induces partial melting of the middle crust, generating rhyolitic magma (Tamura et al., 2009, J Petrology).
Since 2013, volcanic activity has intensified at the Nishinoshima Island. Tamura et al. (2016, Sci Rep) proposed a hypothesis based on the composition of the erupted magma, suggesting that andesitic middle crust forms when mantle melting occurs at shallow depths. Reanalyzing the seismic velocity structure of Kodaira et al. (2007a, 2007b) in light of this petrological study revealed the following two findings regarding the relationship between the middle and lower crust beneath the volcanic front (Figure):The thickness of the lower crust (with P-wave velocities of 6.8–7.6 km/s) increases with overall crustal thickness, whereas the middle crust (with P-wave velocities of 6.0–6.8 km/s) does not exceed 18 km in thickness.When the lower crust is thin, the thicknesses of the middle and lower crust show a positive correlation. However, when the lower crust exceeds 13 km in thickness, further growth of the lower crust is accompanied by a reduction in middle crust thickness. The first finding supports the hypothesis of Tamura et al. (2016), indicating that basaltic magma forming the lower crust is continuously supplied as the mantle melting can occur at depths, regardless of the overlying crust thickness. In contrast, mantle melting leading to the formation of andesitic middle crust cannot occur at depths shallower than a certain threshold. The second finding implies that the latent heat of basaltic magma contributes to the partial melting and differentiation of the middle crust (Tamura et al., 2009).
This presentation will reevaluate the continental crust growth process in the Izu-Bonin arc based on these findings and discuss the geological significance of the Ogasawara Islands.
In the 1990s and 2000s, the Japan Agency for Marine-Earth Science and Technology conducted multiple active-source seismic surveys using ocean-bottom seismographs along the volcanic front of the Izu-Bonin arc and the Ogasawara Ridge. These surveys, covering more than 1,000 km in total length, have revealed the seismic velocity (P-wave velocity) structure beneath the seafloor. The results indicate that andesitic middle crust is extensively distributed along the volcanic front, with a tendency for thicker middle crust beneath active volcanoes that erupt basaltic magma, such as Miyakejima and Hachijojima (IBr1 profile: Kodaira et al., 2007a, JGR; IBr3 profile: Kodaira et al., 2007b, Geology). Furthermore, in the Izu Arc, rhyolitic submarine calderas, such as Kurose Hole and Myojin Knoll, exist between basaltic volcanic islands. Beneath these submarine calderas, the middle crust is observed to be thinner, which has led to the conclusion that basaltic magma's latent heat induces partial melting of the middle crust, generating rhyolitic magma (Tamura et al., 2009, J Petrology).
Since 2013, volcanic activity has intensified at the Nishinoshima Island. Tamura et al. (2016, Sci Rep) proposed a hypothesis based on the composition of the erupted magma, suggesting that andesitic middle crust forms when mantle melting occurs at shallow depths. Reanalyzing the seismic velocity structure of Kodaira et al. (2007a, 2007b) in light of this petrological study revealed the following two findings regarding the relationship between the middle and lower crust beneath the volcanic front (Figure):The thickness of the lower crust (with P-wave velocities of 6.8–7.6 km/s) increases with overall crustal thickness, whereas the middle crust (with P-wave velocities of 6.0–6.8 km/s) does not exceed 18 km in thickness.When the lower crust is thin, the thicknesses of the middle and lower crust show a positive correlation. However, when the lower crust exceeds 13 km in thickness, further growth of the lower crust is accompanied by a reduction in middle crust thickness. The first finding supports the hypothesis of Tamura et al. (2016), indicating that basaltic magma forming the lower crust is continuously supplied as the mantle melting can occur at depths, regardless of the overlying crust thickness. In contrast, mantle melting leading to the formation of andesitic middle crust cannot occur at depths shallower than a certain threshold. The second finding implies that the latent heat of basaltic magma contributes to the partial melting and differentiation of the middle crust (Tamura et al., 2009).
This presentation will reevaluate the continental crust growth process in the Izu-Bonin arc based on these findings and discuss the geological significance of the Ogasawara Islands.