佐藤 利典

Abstract The Conrad Rise (CR), located midway between Antarctica and the Southwest Indian Ridge (SWIR), remains one of the least explored submarine large igneous provinces (LIPs) in the Indian Ocean to date. Relying on only seafloor paleomagnetic records, early studies hypothesized that the formation of the CR occurred during the Late Cretaceous. Here, we present new geochemical and geochronological data, including Sr‒Nd‒Pb‒Hf isotopes and ⁴⁰Ar/³⁹Ar data. Our results indicate that the uppermost part of the CR (Ob and Lena seamounts) unexpectedly formed later than previously predicted, at approximately 40 Ma in an intraplate setting. Another small seamount north of the Ob seamount formed later, at 8.5 Ma. The isotopic composition of lava from the small seamount north of the Ob seamount overlaps with that commonly defined by the Indian plume component. Overall, the isotopic variations defined by the volcanic suite from the CR could be accounted for by a three‐component mixing model involving the common component, lower continental crust, and depleted mantle endmembers. The newly obtained ⁴⁰Ar/³⁹Ar ages imply that the CR volcanism might have been triggered by major regional plate reorganizations during the middle to late Eocene and the late Miocene, inducing the release of a small upwelling rising from the African large low‐velocity province.

Abstract The Aira caldera, located in southern Kyushu, Japan, originally formed 100 ka, and its current shape reflects the more recent 30 ka caldera-forming eruptions (hereafter, called the AT eruptions). This study aimed to delineate the detailed two-dimensional (2D) seismic velocity structure of the Aira caldera down to approximately 15 km, by means of the travel-time tomography analysis of the seismic profile across the caldera acquired in 2017 and 2018. A substantial structural difference in thickness in the subsurface low-velocity areas in the Aira caldera between the eastern and western sides, suggest that the Aira caldera comprises at least two calderas, identified as the AT and Wakamiko calderas. The most interesting feature of the caldera structure is the existence of a substantial high-velocity zone (HVZ) with a velocity of more than 6.8 km/s at depths of about 6–11 km beneath the central area of the AT caldera. Because no high ratio of P- to S-wave velocity zones in the depth range were detected from the previous three-dimensional velocity model beneath the AT caldera region, we infer that the HVZ is not an active magma reservoir but comprises a solidified and cool remnant. In addition, a poorly resolved low-velocity zone around 15 km in depth suggests the existence of a deep active magma reservoir. By superimposing the distribution of the known pressure sources derived from the observed ground inflation and the volcanic earthquake distribution onto the 2D velocity model, the magma transportation path in the crust was imaged. This image suggested that the HVZ plays an important role in magma transportation in the upper crust. Moreover, we estimated that the AT magma reservoir in the 30 ka Aira caldera-forming eruptions has the total volume of 490 km³ DRE and is distributed in a depth range of 4–11 km. Graphical Abstract