Crustal structure of the central and southern Kyushu-Palau Ridge: Implications for intra-oceanic arc evolution

As an intra-oceanic arc, the Kyushu–Palau Ridge (KPR) is an exceptional place for studying intra-oceanic arc evolution and related magmatism using its interior velocity structure. Two wide-angle seismic profiles along the KPR provide an opportunity to unveil the crustal structure of its central and southern parts. Strong along-ridge variations are found either in P-wave velocity or crustal thickness of the central and southern KPR. The crustal thickness is between 6 and 12 km, with the velocity increasing from 4.0 km/s to 7.0 km/s from top to bottom. The upper crust has a large velocity gradient of ∼1 s−1 and a thickness between 2 and 4 km, while the lower crust has a lower velocity gradient of <0.2 s−1 and is characterized by inhomogeneous velocity anomalies and a substantial lateral variation in thickness. By comparing with the mature arc in the Izu-Bonin-Mariana arc in the east, we suggest the central and southern KPR is a thickened oceanic crust rather than a mature arc crust. Partly thick crust, velocity anomalies in the lower crust and upper mantle imply the crust is in a transition stage from oceanic crust to mature arc one. Compared with the quantitatively calculated seamount volumes, the lower crustal thickness is generally correlated well with the distribution of the seamount volumes along the central and southern KPR. These observations imply that the central and southern parts of the KPR are at the juvenile stage of arc evolution, with crustal growth primarily attributed to the lower crust thickening.