Evidence of an active rift zone in the northern Okinawa Trough

Abstract

The character of continental rifting processes in the northern part of the Okinawa Trough remains largely unknown due to the absence of an obvious axial rift zone or recent rift-related volcanism in the region. This paper describes the discovery of a tectonically and magmatically active rift zone in the northern trough, and discusses the nature of the rift. Ship-based bathymetry identified a group of NE–SW-trending surface lineaments on the western margin of the northern trough that form a small rift structure. The newly discovered rift, named the Uji Rift, is ~30 km long, ~10 km wide and ~180 m deep. A seismic reflection profile near the rift confirmed the existence of well-developed normal faults with large displacements, suggesting that the rift was formed by back-arc rifting. A previously unknown volcanic edifice, named West Uji Knoll, which consists of multiple cones with pronounced summits and high acoustic backscatter characteristics, also exists within the rift. High-resolution mapping with an autonomous underwater vehicle at one of the cones confirmed the existence of lava flow and eruptive fissure morphologies. Lava flows cover the pre-existing lineaments that form the Uji Rift. The flows are undissected by any lineaments, suggesting that West Uji Knoll is a young submarine volcano and the most recent eruptions likely occurred after rifting activity in the early or late Pleistocene. Active magmatism is also inferred from evidence of hydrothermal venting, such as the existence of an acoustic plume and sharp temperature increases of up to ~0.06 °C, even at a depth of ~100 m above the seafloor. In addition, three bubble plumes were detected near the cones by the shipboard multibeam sonar, suggesting emission of magmatic gases. Our volcano-tectonic observations show that the Uji Rift developed at an unusual location, ~170 km behind the arc volcanic front. Continental rifting in the northern Okinawa Trough is considered to extend onshore at the Beppu-Shimabara Rift Zone in SW Japan. However, due to the small number of submarine studies in this area, where and how the rifting is developing in the trough and has extended from the ocean to the continent is poorly understood. As an active rift, the Uji Rift provides an important constraint on the continuity of rifting in the Okinawa Trough, as well as new insights into the propagation and migration processes of continental rifting in subduction systems.