“Grand” petit-spot volcanism off the Japan Trench

Asuka Yamaguchi, Hiroko Kitajima , Norikatsu Akizawa , Olivier Fabbri, Matt J. Ikari, Kenji M. Matsuzaki, Takashi Miyazaki, Yasuyuki Nakamura, Andrew P. Roberts, Paola Vannucchi, Hirano Naoto, Gou Fujie, Hanaya Okuda , Ayumu Miyakawa, Natsumi Okutsu, Yuya Akamatsu , Lottie Atton, Takanori Kagoshima, Taro Kido, Rachel Lauer, Jennifer M. Lington, Kazuto Mikuni, Francesco Narduzzi, Katie Preece , Louisa H. Stokes, Fuka Takuwa, Taizo Uchida, Hayato Ueda, Christopher Yen, Erwan Le Ber, Adam Wspanialy, Edoardo Barbero, Chin-Yeh Chen, Jose-Abel Flores, Mila Huebsch, Yuji Kato, Eun Young Lee, Jason P. Morgan, Sebastien Pilet

Petit-spot volcanoes are a recently discovered type of small intraplate volcanoes that erupt on the outer rise of subducting plates. At petit-spot regions on the outer rise of the Japan Trench, an anomalously shallow acoustic basement reflector is widely distributed, leading to the hypothesis that it corresponds to sills or sheet-like lava flows and is associated with petit-spot magmatism. Here we report discoveries from the first drilling into the shallow acoustic basement off the Japan Trench. The basement is composed of petit-spot basalt, which is less altered with fewer vesicles than those exposed on the seafloor, resulting in higher bulk density and P-wave velocity. The lack of baked margins at the sediment-basalt boundary suggests that the basalt erupted as a lava onto the seafloor and was covered subsequently by 60 metres of sediment deposited over the past 3.6 Ma. Similar basalt ages and compositions at the drill site and an adjacent petit-spot knoll exposed on the seafloor, together with regional distribution of acoustic basement, suggest that the petit-spot volcanic base extends horizontally for at least 18 kilometres and potentially for hundreds of kilometres. Consequently, petit-spot volcanoes may have a greater impact on geochemical cycling and seismogenesis than previously thought.