日本地球惑星科学連合2014年大会

講演情報

ポスター発表

セッション記号 S (固体地球科学) » S-CG 固体地球科学複合領域・一般

[S-CG67_2PO1] 海洋底地球科学

2014年5月2日(金) 16:15 〜 17:30 3階ポスター会場 (3F)

コンビーナ:*沖野 郷子(東京大学大気海洋研究所)、田所 敬一(名古屋大学地震火山研究センター)、石塚 治(産業技術総合研究所地質情報研究部門)、土岐 知弘(琉球大学理学部)、高橋 成実(海洋研究開発機構地震津波・防災研究プロジェクト)

16:15 〜 17:30

[SCG67-P18] 九州南方沖鬼界カルデラにおける, カルデラの陥没に伴う堆積盆の進化

*池上 郁彦1蓑和 雄人1倉冨 隆1清川 昌一2大岩根 尚3中村 恭之4亀尾 桂5 (1.九州大学理学府地球惑星科学専攻、2.九州大学理学研究院地球惑星科学部門、3.三島村、4.海洋研究開発機構、5.東京大学大気海洋研究所)

キーワード:カルデラ, 反射法探査, 鬼界カルデラ, アカホヤ

Kikai caldera (Matsumoto, 1943) is a mostly submerged highly active caldera volcano located in 40 km off Kyushu Island. The caldera is recognized as the product of 7300 cal. BP super-colossal eruption with Akahoya tephra (Machida and Arai, 1978; Fukuzawa, 1995) which is widely distributed along the western part of Japan. Previous studies for near-vent onshore geology strongly suggests such a large eruption was not occurred only once, but multiple times in the Kikai caldera (Ono et al., 1982).
In Kikai caldera, 24 lines of multi-channeled seismic reflection surveys were held in two survey cruises (KT-10-18 and KT-11-11) in 2010 and 2011 using R/V Tansei-maru of JAMSTEC (Japan Agency for Marine-Earth Science and Technology). The acquired seismic data for subseafloor structures spotted thick sedimentary basin at the eastern margin of the caldera. The basin covers 70 square km of the 20 km-wide caldera and is next to caldera rim fault. The infill of the basin is characterized by the group of onlapping stratified deposits named B which maximum thickness is more than 600 m. The B-sequence has two major depositional discontinuities in the middle and the top. The lower one is paraconformity and the upper one is disconformity though, the both of them are associated with similar deformation of the basin itself. The deformation is characterized by 1. Dragged-up reflectors along the caldera rim fault, and 2. Slight outward rotation of the deposits. Both characteristics intensify along the depth, which means lower deposits were experienced much more deformation.
The both two types of the deformation suggest the basin was experienced at least two major subsidence event. The former dragged-up structure is interpreted as the incomplete slip of the caldera rim fault for the relief of the subsidence, while the latter rotation shows the slippages were slightly listric. The displacements of the subsidence events could be estimated from the top and bottom of the dragged-up structures, as more than 100 m in the lower-older event and more than 50 m in the upper-newer event. The subsidence would be an abrupt event, as the paraconformity was formed in the lower-older event. The most likely candidate for such a significant subsidence is caldera collapse. As therefore, the basin might be the one of the pre-caldera structure, and it has been experienced multiple caldera collapse events in the past.