Sedimentological, geochemical, and chronological analyses were carried out on 19 carbonate rock samples collected from the submarine slope to the west of Minamitorishima (Marcus Island) located near the western margin of the Pacific Plate.Four groups of carbonate rocks were distinguished: mollusk-rich carbonates, coral-rich carbonates, foraminiferal-nannofossil packstone, and mudstone/wackestone. The mollusk-rich carbonates are characterized by dominance of bivalve (including rudist) and gastropod shells. Skeletal grains are extensively bioeroded, some with thick micrite envelopes. Sr isotope ratios ( 87 Sr/ 86 Sr) and Mesorbitolina ex gr. texana (large benthic foraminifer) indicate that the shallow-water carbonates were deposited in the late Aptianearly Albian (~122-111 Ma). The coral-rich carbonates are characterized by abundant scleractinian corals and nongeniculate coralline algae associated with encrusting acervulinid foraminifers. The biotic composition indicates that the carbonates were deposited in a coral reef setting during the Oligocene-Miocene. Geochemical data show that the coral-rich carbonate were dolomitized at 6.8-9.5 Ma (Tortonian-Messinian) and that normal seawater is the likely parent fluid. The foraminiferal-nannofossil packstone is semi-consolidated foraminiferal-nannofossil ooze, deposited in the Pleistocene (0.99-0.45 Ma). The mudstone/wackestone is marked by absence of macrofossils and is phosphatized: its age and depositional environment could not be assessed. The Cretaceous mollusk-rich carbonates are distributed to shallower depths than expected following standard seafloor subsidence, clearly showing that Minamitorishima has undergone not continuous thermal subsidence but significant episodic uplifts probably by Eocene volcanism.

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.

Estimates of uplift rates for lithospheric forebulges are needed to understand exact plate motions at plate convergence zones and to delineate the fate of coral reefs atop the forebulges. A carbonate island on a lithospheric forebulge can provide excellent materials for estimating uplift rate because carbonate sediments can be dated and their paleo-water depth determined. We estimated the uplift rate of Kitadaito Jima Island, a carbonate island on the lithospheric forebulge of the Philippine Sea Plate that is subducting beneath the Eurasian Plate. Marine skeletal sands containing pebble-sized bioclasts and filling the concavity of an intertidal erosional notch at an elevation of ~ 71 m were found near the top of this island. Strontium isotope ages ranging from 1.78 to 2.01 Ma with an average of 1.89 Ma (standard deviation = 0.07 Ma) were obtained from these deposits. As global sea level at 1.89 Ma was ~ 21 m lower than the present, the mean uplift rate was estimated at ~ 49 m/million years (Myr) with a compounded uncertainty of ± 2.6 m/Myr. This rate is comparable to, or up to ~ 140 m/Myr less than, rates reported from other Indo-Pacific carbonate islands in similar tectonic settings. This study illustrates how contemporary Sr isotope age models and careful considerations of limestone sample depositional depths can yield more accurate and precise uplift rates of modern forearc bulges than possible in many older studies.