[S-CG59] 日本列島の構造と進化: 島弧の形成から巨大地震サイクルまで

[SCG59-09] 白亜紀前弧盆地の時空間範囲と弧-海溝系の二次改変:和泉層群の例

*磯崎 行雄¹、長谷川 遼¹、堤 之恭² (1.東京大学大学院総合文化研究科広域科学専攻広域システム科学系、2.国立科学博物館地学研究部門)

キーワード：弧-海溝系、前弧盆地、砕屑性ジルコン、中央構造線、古第三紀、白亜紀

The Upper Cretaceous Izumi Group represents a typical sedimentary package accumulated in a forearc basin; the extremely thick turbidite-dominated terrigenous clastic rocks occur in a narrow belt less than 15 km wide but continues for more than 300 km along a major fault called Median Tectonic Line (MTL) in SW Japan between the Cretaceous arc granitoid (Ryoke) belt and coeval high-P/T bluseschist (Sanbagawa) belt in SW Japan. Well-established ammonite and radiolarian biostratigraphy confirmed the Campanian-Maastrichtian age. The latest analysis of detrital zircon geochronology of sandstones of the Izumi Group claried several new aspects, which require essential re-considerations on the basin setting/development, and post-depositional tectonic modifications during the early Cenozoic. 1) The youngest part of the Izumi Group ranges up to the Paleogene. 2) Its eastern extension is laterally traced to the Mikawa area, northern Kanto Mtns., and the Pacific coast of northern Kanto. In short, the Izumi sedimentary basin was originally much greater in size both in across-arc width and in along-arc length, and the basin existed much longer than previsouly believed. Owing to the severe across-arc contraction during the Paleogene-Neogene, more than a half of the primary sediemntary entity has been lost.