[S-CG66_30AM1] プレート収束帯における地殻変形運動の統合的理解

★ [SCG66-06] 中期中新世におけるTTT三重会合部の移動と西南日本地殻の急成長

*木村 学¹、橋本 善孝²、北村 有迅³、山口 飛鳥⁴、高下 裕章¹ (1.東京大学大学院理学系研究科、2.高知大学、3.鹿児島大学、4.東京大学大気海洋研究所)

We review recent progress in geological and geophysical investigation in SW Japan, the Nankai Trough and the Philippine Sea Plate (PSP), and propose a comprehensive hypothesis for the Miocene tectonics of the Nankai Trough. New interpretations are as follows: Near-trench magmatism in the outer zone of SW Japan might have various reasons. The possibility of an arc-arc collision in particular should be examined, in addition to the previous model of an oceanic ridge and hot PSP subduction. The indentation structure at Capes Ashizuri, Muroto in Shikoku, and Shiono on the Kii Peninsula may be explained by the collision of the active arc or topographic peaks such as seamounts, contrary to the previous "kink-folding" model due to recent E-W compression. This inference is drawn from comparison between the many modern examples of seamount collision and sandbox analogue experiments. Crustal components of SW Japan might consist mainly of igneous plutonic rocks, in contrast to the previous inference of Cretaceous to Tertiary accretionary complexes. This is especially the case in the outer zone to the north of Capes Ashizuri, Muroto and Shiono. This is inferred from geophysical observation of gravity anomalies, velocity and resistivity, together with geological estimations of caldera age and the size of its root pluton. Episodic crustal growth due to intrusion of igneous rock and subduction of the PSP may have stopped after ~11 Ma and restarted at ~7-8 Ma. New accretionary prism was again developed after ~6 Ma. This inference is suggested by recently conducted ocean drilling program.