Crustal structure of the southern Okinawa Trough: Symmetrical rifting, submarine volcano, and potential mantle accretion in the continental back-arc basin

Back-arc basins are a primary target to understand lithospheric evolution in extension associated with plate subduction. Most of the currently active back-arc basins formed in intraoceanic settings and host well-developed spreading centers where seafloor spreading has occurred. However, rift structure at its initial stage, a key to understand how the continental lithosphere starts to break in a magma-rich back-arc setting, is poorly documented. Here we present seismological evidence for structure of the southern Okinawa Trough, an active rift zone behind the Ryukyu subduction zone. We find that the southern Okinawa Trough exhibits an almost symmetric rift system across the rift axis (Yaeyama Rift) and that the sedimentary layers are highly cut by inward dipping normal faults. The rift structure also accompanies a narrow (2-7km wide) on-axis intrusion resulted from passive upwelling of magma. On the other hand, an active submarine volcano is located similar to 10km away from the rift axis. The P wave velocity (V-p) model derived from seismic refraction data suggests that the crust has been significantly thinned from the original similar to 25km thick arc crust and the thinnest part with 12km thickness occurs directly beneath the rift axis. The velocity model also reveals that there exists a thick layer with V-p of 6.5-7.2km/s at lower crustal levels and may indicate that mantle materials accreted at the bottom of the crust during the crustal stretching. The abrupt crustal thinning and the velocity-depth profile suggest that the southern Okinawa Trough is at a transitional stage from continental rifting to seafloor spreading.