TOPOGRAPHICAL AND GEOMORPHOLOGICAL FEATURES OF THE HYDROTHERMAL FIELDS AND TYPICAL HYDROTHERMAL VENTS IN THE MID-OKINAWA TROUGH

The characteristics of topography and geomorphology of the hydrothermal fields in the Mid-Okinawa Trough are studied with the latest high-precision deep-water multibeam bathymetric data. Results show that the topography is complicated in the study area. There is a rift extending along SW-NE direction,of which the depth of rift ranges approximately from 1 500 m to 1 800 m. A large linear seamount chain extends along SW-NE direction, with some depressions within it. The formation of topography is most likely associated with the back-arc spreading of the Okinawa Trough. In addition, the characteristics of topography and geomorphology of two typical hydrothermal vents are studied with the images obtained by ROV (Remotely Operated Vehicle), combined with the shallow sediment and rock samples. Results show that the topography of hydrothermal vent in the Iheya North area varies from place to place, with many chimneys and mounds surrounded by sulfide sand sediments, while the topography of hydrothermal vent in the Iheya Ridge area is rather smooth. Hydrothermal fluid coming from fissures spread on the hard seafloor. There are little hydrothermal sediments.