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Seismic imaging of a possible paleoarc in the Izu‐Bonin intraoceanic arc and its implications for arc evolution processes

Crustal evolution processes in intraoceanic arcs, including crustal accretion and rifting, have been long discussed. To examine crustal evolution in the Izu‐Bonin intraoceanic island arc, we conducted an active‐source wide‐angle seismic study along a north‐south profile (500 km long) within a possible paleoarc in the rear arc (i.e., the Nishi‐shichito ridge) about 150 km west of the present‐day volcanic front. In this study, the seismic velocity and reflectivity images are obtained using the wide‐angle seismic data. For the seismic velocity imaging, we applied refraction tomography in which 93,535 picks were used. The overall root‐mean‐square (rms) misfit calculated from the initial model of the refraction tomography was 483.1 ms, and those calculated from the final model were reduced to 66.7 ms. The resultant seismic image shows marked variations of crustal thickness along the seismic profile: thin crust (10–15 km thick) in the northern part, three discrete thick crustal segments (20–25 km thick) in the central part, and a moderately thick crust (∼15 km thick) in the southern part. These variations are mainly attributed to thickness variations of the middle crust having seismic velocity of 6.0–6.8 km/s. This variation of crustal thickness does not correlate with seafloor topography, which is characterized by post‐Miocene across‐arc seamount chains. It does correlate well with crustal variations observed along the present‐day volcanic front of the Izu‐Bonin arc. These findings suggest that the magmatic activity that created the across‐arc seamount chains had little effect on the rear‐arc crust and that the main part of the rear‐arc crust was created before the rear arc separated from the volcanic front. By correlating the structural variations along the rear arc (i.e., the variation of the average seismic velocity as well as the thickness of the middle crust) and those along the present‐day volcanic front, we found that the direction of rifting to separate the rear arc (paleoarc) from the present‐day volcanic front was north‐northeast.