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

Abstract

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.