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We collected deep-sea multibeam, sidescan, and sub-bottom profiler data using an autonomous underwater vehicle (AUV) at the Bayonnaise knoll, a submarine caldera located in the rift zone of the Izu-Ogasawara arc. We aimed to reveal topographic and geological features and the origin of a hydrothermal field called the Hakurei site in the caldera.

We performed seafloor classification by textural analysis using calibrated sidescan sonar data, which provided an effective means to understand the geology and to highlight potential areas of hydrothermal constructions. The high-resolution bathymetric map illustrates that the Hakurei hydrothermal field is distributed over a landslide landform in the caldera wall. The distribution of hydrothermal vents indicates that the slip surface has served as a major route of hydrothermal fluids.

The radial alignment of chimneys and mounds indicates radial routes of hydrothermal fluid and/or belching along fragile lines in the landslide landform. Various post-caldera activities are inferred including the formation of a lava dome, a pyroclastic cone, and subsequent phreatic explosions. A general volcano-tectonic structure extending across the caldera in a NW-SE direction is interpreted as an inferred boundary fault of the North Myojin Rift (NMR).

Analogous to the Hokuroku basin and land kuroko deposits, it is suggested that the main contributing factor in the formation of kuroko deposits was volcano-tectonic activity that dominated the margin of the backarc rift basin. The intersections between the margin of a rift basin and the surrounding knolls have a high potential for ore-forming areas.