Tectonic, volcanic and hydrothermal features of a nascent rift graben in the southern Okinawa Trough

High-resolution geophysical mapping using an autonomous underwater vehicle (AUV) and surface vessel revealed tectonic, volcanic and hydrothermal features of a nascent rift graben in the southern Okinawa Trough. Ship-based bathymetry identified ENE–WSW trending lineaments at a water depth of 1900–2000 m that form a rift graben, the Ishigaki Rift, which is ~50 km long, ~7 km wide and has relief of ~100 m. Shipboard EM122 multibeam sonar detected acoustic bubble plumes and high-backscatter intensity on the seafloor along the rift. The AUV high-resolution bathymetry (2 m) and sidescan imagery revealed the presence of a ropey texture with high acoustic reflectivity, interpreted to be relatively young lava flows. Evidence of hydrothermal venting was observed at two previously unknown sites along the rift. Steep-sided spires up to 23 m in height and a mound discharging smoke-like acoustic plumes were identified. The AUV temperature sensor observed sharp temperature increases of up to ~0.13 °C around the plumes, even at the vehicle's altitude of ~100 m above the seafloor. The presence of relatively young lava flows and indicators of high-temperature hydrothermal venting and gas bubbles suggest the Ishigaki Rift is magmatically active. Compared with adjacent active back-arc rifts, the Ishigaki Rift has a shorter length, narrower width and lower relief, and occurs at a shallower water depth, suggesting that the rift is relatively young and immature. Considering that the Ishigaki Rift is located near the arc volcanic front and that other mature rifts are typically located farther from the front, it is suggested that a new axial rift system is evolving within the Okinawa Trough and that back-arc rifting processes may be gradually migrating arcward in the trough during the Quaternary.