Crustal structure and variation along the southern part of the Kyushu-Palau Ridge

As an interoceanic arc, the Kyushu-Palau Ridge (KPR) is an exceptional place to study the subduction process and related magmatism through its interior velocity structure. However, the crustal structure and its nature of the KPR, especially the southern part with limited seismic data, are still in mystery. In order to unveil the crustal structure of the southern part of the KPR, this study uses deep reflection/refraction seismic data recorded by 24 ocean bottom seismometers to reconstruct a detailed P-wave velocity model along the ridge. Results show strong along-ridge variations either on the crustal velocity or the thickness of the KPR. P-wave velocity model is featured with (1) a crustal thickness between 6–12 km, with velocity increases from 4.0 km/s to 7.0 km/s from top to bottom; (2) high gradient (~1 s?1) in the upper crust but low one (<0.2 s?1) in the lower crust; (3) a slow mantle velocity between 7.2 km/s and 7.6 km/s in the uppermost mantle; and (4) inhomogenous velocity anomalies in the lower crust beneath seamounts. By comparing with the mature arc in the Izu-Bonin-Mariana arc in the east, this study suggests the southern part of KPR is a thicken oceanic crust rather than a typical arc crust. The origin of low velocities in the lower crust and upper mantle may be related with crustal differentiation, which implies advanced crustal evolution from normal oceanic crust to partly thicken oceanic crust. High velocities in the lower crust are related to the difference in magmatism.