Volcanic and tectonic features of Shirahama Bank in the northern Ryukyu Arc: Implications for cross-arc volcanism controlled by arc-parallel extension

Abstract

This paper presents volcanic and tectonic features of Shirahama Bank, an unexplored shallow submarine edifice in the northern Ryukyu Arc which was previously poorly mapped, and consequently the nature of its volcanism and tectonism have been poorly understood. New high-resolution (5 m) bathymetry revealed that Shirahama Bank has small volcanic edifices and depressions on its flat top with water depths of 60–150 m. The edifices have pointy summits with steep slopes greater than 30°. Within the depressions, a total of 59 acoustic bubble plumes were detected by shipboard multibeam sonar, suggesting that active volcanic degassing occurs in this region. The bank is dissected entirely by NW–SE to WNW–ESE trending lineaments while the volcanic edifices and depressions on the flat top are undissected. Shirahama Bank was formed by multiple-stages of volcanism. The youngest volcanic stage probably occurred after surface erosion by sea-level low-stand during the late Pleistocene and after tectonic deformation, providing a constraint on the timing of Quaternary volcanism in the northern Ryukyu Arc. Structural control of arc volcanism in the area is observed. The NW–SE to WNW–ESE lineaments identified on and around Shirahama Bank are interpreted as normal faults and form cross-arc graben structures. The young volcanic structures and active bubble plumes at Shirahama Bank and surrounding areas are concentrated within these cross-arc grabens, suggesting that the most recent volcanism is controlled by ~NE–SW extension (arc-parallel or along-arc extension) in the northern Ryukyu Arc. Our results, combined with previous studies, suggest that arc-parallel extension, which differs from the previously known back-arc extension in the Okinawa Trough, plays an important role in arc volcanism during its initial stages.