Tectonic implications of carbonate deposits on the eastern slope of the Hahajima Seamount in the collision zone between the Izu-Bonin Arc on the Philippine Sea Plate and the Ogasawara Plateau on the Pacific Plate

摘要

The Hahajima Seamount is located at the junction between the Izu-Bonin and Mariana forearc slopes in the northwest Pacific Ocean, and between the Philippine Sea Plate and the Ogasawara Plateau on the Pacific Plate. Despite numerous previous studies, the origin of the Hahajima Seamount remains controversial. Here we constrain its origin based on sedimentological and chronological analyses of shallowwater carbonates collected from the eastern slope of the seamount. The carbonates are dominated by floatstones with numerous mollusks. Gravel-sized bioclasts include mollusks (e.g. nerineids and rudists) and local calcareous spines and minor amounts of corals. Sand-sized components include bioclasts of benthic foraminifers and dasycladalean algae, and non-skeletal grains of intraclasts and peloids, with no ooids. Most of the bioclasts are bioeroded and coated with micrite envelopes. The Sr isotope ages of the samples fell into two ranges: Berriasian or Aptian (140.3 Ma or 118.3-113.2 Ma, respectively) and Coniacian to Campanian (86.5-77.2 Ma). Shallow-water carbonates on the Hahajima Seamount are similar in lithology and Sr isotope ages to those on the Ogasawara Plateau located to the east, on the opposite side of the Izu-Bonin Trench (i.e. on the Pacific Plate). This indicates that the shallow-water carbonates on the eastern slope of the Hahajima Seamount were not deposited in situ, but instead originated from the Ogasawara Plateau. The eastern section of this seamount can thus be interpreted as an accretionary wedge.