日本地球惑星科学連合2022年大会

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[J] ポスター発表

セッション記号 S (固体地球科学) » S-MP 岩石学・鉱物学

[S-MP27] 変形岩・変成岩とテクトニクス

2022年5月29日(日) 11:00 〜 13:00 オンラインポスターZoom会場 (15) (Ch.15)

コンビーナ:中村 佳博(国立研究開発法人産業技術総合研究所 地質調査総合センター)、コンビーナ:針金 由美子(産業技術総合研究所)、座長:中村 佳博(国立研究開発法人産業技術総合研究所 地質調査総合センター)、針金 由美子(産業技術総合研究所)

11:00 〜 13:00

[SMP27-P09] 地震発生帯巨大分岐断層の残留ガス組成:九州四万十帯延岡衝上断層の例

畑 良太2、*亀田 純1濱田 洋平3山口 飛鳥4鈴木 德行1 (1.北海道大学大学院理学研究院自然史科学専攻、2.北海道大学大学院理学院自然史科学専攻、3.独立行政法人海洋研究開発機構 高知コア研究所 、4.東京大学大気海洋研究所 )

キーワード:岩石残留ガス、断層、ラジカル反応

Gases and fluids in the deep crust are thought to circulate through faults as their main flow channel in addition to diffusion in rock matrix (e.g., McCaig, 1997). It is also known that some types of gas are formed by the fault rupture and slip itself (e.g., Wakita et al., 1980). Therefore, clarification of the gas composition in the vicinity of the fault will be important not only for understanding the characteristics of fluid and gas movement on the fault, but also for understanding the rupture and sliding processes of the fault. In this study, we investigated the gas composition of the Nobeoka fault in the Kyushu Shimanto Belt, which is considered to be a mega-branching fault in the ancient plate subduction zone.
Core samples obtained by a drilling survey through the Nobeoka fault in 2012 (NOBELL; Hamahashi et al., 2013) were used for the experiment. The drilling core samples include the main slip zone of the thrust, ~40 m of the fracture zone on the hanging wall side (the Eocene Kitagawa Group), and ~200 m of the fracture zone on the footwall side (the Eocene to early Oligocene Hyuga Group). Residual gases from the rock chips were recovered by crushing and extraction using a ball mill (Saito et al., 2012; Suzuki et al., 2017), and the gas composition was measured using a gas chromatograph (7890A: Agilent) equipped with a pulsed discharge helium ionization detector.
Analysis showed that H2, CO2, and alkanes such as CH4, C2H6, and C3H8 were detected at all depths. Among these, CO2 variations showed a good correspondence with the internal structure of the fault zone. The compositions of H2 and CH4 are somewhat different between the hanging wall and footwall at the main slip zone of the fault, and this trend may reflect the difference in metamorphic to diagenetic grades, considering the results of previous studies on similar rock samples (Suzuki et al., 2017). On the other hand, H2 concentration normalized by the total organic carbon content is generally higher than the value obtained from the analysis by Suzuki et al. (2017), suggesting the possibility of traces of hydrogen production on the fault. In this presentation, we will discuss these gas compositions and their origins, with particular attention to their relationship to faulting.

Reference McCaig, A., 1997, Mineral. Soc. Ser., vol. 8, pp. 227–266. Wakita H., et al., 1980, Science, 210, 188–190. Hamahashi, M., 2013, G3, 13, 5354–5370. Saito, H. et al., 2012, Geochem. J., 46, 255–259. Suzuki, N., et al., 2017, Int. J. Coal Geol., 173, 227–236.