10:45 〜 12:15
[SMP26-P04] Clockwise rotation of SW Japan at Izanagi–Pacific ridge subduction time revealed by arc migration
キーワード：火成活動、イザナギプレート、白亜紀、古第三紀、拡大海嶺沈み込み、マイクロプレート回転
Prominent belts of Cretaceous high-P/T metamorphism and high-T/P metamorphisms with its associated magmatism observed in the paleo-Japan arc are conventionally understood as the result of interactions between an active spreading ridge and the east Asia continental margin. The overall west to east younging of arc-related igneous rocks in SW Japan parallel to the Median Tectonic Line has been taken by many workers to reflect the oblique subduction of a spreading ridge with a south to north component. Recent reconstructions of oceanic plate movements in the western pacific realm that incorporate newly constrained oceanic paleomagnetic anomalies and mantle tomography suggest that ridge subduction only took place in the Paleogene. A new compilation of geochronological and geochemical data related to the evolution of felsic to intermediate igneous rocks in SW Japan indicates that the ridge subduction most likely occurred in the Eocene, not the Cretaceous. Furthermore, the inferred ridge subducted subparallel to the trench and then formed a slab window, resulting in the cessation of arc magmatism. The trend of the volcanic arc, defined by the spatial distribution of contemporaneous igneous activity, shows a c.20° counterclockwise rotation after ridge subduction. Prior to the ridge subduction, the arc shows progressive inland migration of about 400 km. We propose a microplate rotation model that incorporates ridge subduction-related tectonics, and is compatible with known rotations about vertical axes of Cretaceous terranes indicated by paleomagnetic data and the newly compiled geochronological and geochemical data for Cretaceous igneous bodies. The southern and eastern margin of the microplate is bounded by the Median Tectonic Line and the Tanakura Tectonic Line, which can be interpreted as the pre-ridge subduction paleo-arc being cross-cut by these tectonic lines. Oceanic plate reconstructions show counterclockwise rotation in the subduction direction after the ridge subduction phase and this change in plate convergence vector is a likely cause for the microplate rotation.