Description: Structure and growth of the Izu‐Bonin‐Mariana arc crust: 1. Seismic constraint on crust and mantle structure of the Mariana arc–back‐arc system Narumi Takahashi Institute for Research on Earth Evolution Japan Agency for Marine‐Earth Science and Technology Kanagawa Japan Shuichi Kodaira Institute for Research on Earth Evolution Japan Agency for Marine‐Earth Science and Technology Kanagawa Japan Yoshiyuki Tatsumi Institute for Research on Earth Evolution Japan Agency for Marine‐Earth Science and Technology Kanagawa Japan Yoshiyuki Kaneda Institute for Research on Earth Evolution Japan Agency for Marine‐Earth Science and Technology Kanagawa Japan Kiyoshi Suyehiro Institute for Research on Earth Evolution Japan Agency for Marine‐Earth Science and Technology Kanagawa Japan A high‐resolution seismic velocity model is presented for the crust and upper mantle of the Mariana arc–back‐arc system (MABS) based on active source seismic profiling. The major characteristics are (1) slow mantle velocity of 〈8 km s −1 in the uppermost mantle, especially, and deep reflectors under the Mariana arc (MA) and the West Mariana Ridge (WMR), (2) a deep reflector in the upper mantle beneath the relative thick crust of the Mariana Trough (MT) axis, (3) distribution of lower‐velocity lower crusts (6.7–6.9 km s −1 ) beneath the volcanic front and adjacent to the MT, and (4) high‐velocity lower crust (7.2–7.4 km s −1 ) beneath the boundary regions between the MA and MT, and between the WMR and the Parece Vela Basin (PVB), adding to structural characteristics of crust and upper mantle beneath the MABS. Of the characteristics described above, characteristic 1 suggests that the origins of the slow mantle velocity and the deep reflectors be explained by transfer of the lower crustal residues to the upper mantle across the Moho, considering that the WMR is extinct arc currently. On the other hand, characteristic 2 suggests that the origin of deep reflectors beneath the MT axis might be lower velocity materials due to the diffractive signals with strong amplitudes, characteristic 3 suggests that the lower‐velocity lower crust advanced crustal growth and characteristic 4 suggests that the high‐velocity lower crust beneath arc–back‐arc transition zone is composed of mafic/ultramafic materials created by extensive partial melting of mantle peridotites or last stage of the arc magmatism rather than serpentinized peridotite.