High‐resolution acoustic mapping to understand the ore deposit in the Bayonnaise knoll caldera, Izu‐Ogasawara arc

Abstract

We collected deep‐sea multibeam, side scan, and subbottom profiler data using an autonomous underwater vehicle 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 side‐scan 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 postcaldera 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. 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 back‐arc rift basin. The intersections between the margin of a rift basin and the surrounding knolls have a high potential for ore‐forming areas.

Postcaldera volcanism and hydrothermal activity revealed by autonomous underwater vehicle surveys in Myojin Knoll caldera, Izu‐Ogasawara arc

“…The mosaic (Figure ) shows that the mottled pattern, which probably represents hydrothermal precipitates and cemented sediment, appears in a significant part of the caldera floor. Similar mottled patterns have been also found in the caldera floor of Bayonnaise Knoll caldera [ Honsho et al , ].…”

“…The Sunrise deposit formed in a slope below one of these scarps. The situation is similar to that of the Hakurei deposit in Bayonnaise Knoll caldera, which is distributed in a large landslide landform in the caldera wall [ Honsho et al , ].…”

“…The flat floor extends from northwest to east at a depth of ~1400 m (Figures and ). The caldera floor is not clearly associated with low backscattering intensity (Figure ), as is the case of Bayonnaise Knoll caldera [ Honsho et al , ]. Instead, regular or irregular mottled patterns are generally shown on the caldera floor.…”

“…A detailed high‐resolution (1 m grid) bathymetric map was created by merging the AUV MBES data of the five dives (Figure ). The SSS data were processed using a method considering topographic relief [ Honsho et al , ], but a theoretical radiometric correction for incident angle was not applied to prevent false bathymetric gaps caused by positioning errors from producing artificial patterns in the images. The beam pattern was derived and corrected for each dive.…”

A New Radiometric Correction Method for Side-Scan Sonar Images in Consideration of Seabed Sediment Variation

“…The above processing does not affect the succeeding steps because the final angle-BS curve of a sediment is determined by averaging all of the angle-BS curves of the sediment. When calculating the incident angles, the sonar attitude and slope of the seabed should be considered [21,22]. The yaw and pitch angles do not influence bottom tracking, whereas the roll angle influences sonar altitude in bottom tracking; this influence can be eliminated by the symmetry of the port and starboard water column widths [21].…”