[S-GL23] Geologic structure and tectonic history of East Asia and Japanese Islands

[SGL23-06] The Tokunioshima belt: a new geotectonic unit in northern Ryukyus

*Yukio Isozaki¹, Hiroshi Yamamoto², Yukiyasu Tsutsumi³, Kenichiro Tani³ (1.Department of Earth Science and Astronomy, Multi-disciplinary Sciences - General Systems Studies, Graduate School of Arts and Sciences, The University of Tokyo, 2.Faculty od Science, Kagoshima University, 3.Deprtment of Geosciences, National Museum of Nature and Science)

Keywords:Tokunoshima belt, Paleogene, schist-gneiss, amphibolite facies, Shimanto belt, zircon U–Pb age

The pre-Miocene basement geology of the Tokunoshima Island in central Ryukyus is two-fold; i.e., the Late Cretaceous accretionary complex (Amagidake and Omo units) and the high-grade metamorphic unit newly named the Inokawadake metamorphic complex (IMC) (Yamamoto et al., 2024; Fig.). The former, intruded by ca. 60 Ma granitoids, is correlated with the northern subbelt of the Shimanto belt in Kyushu, SW Japan, whereas the latter is extremely unique in the Shimanto belt not only in Ryukyus but also in SW Japan. IMC occurs only in the southern half of the island, in a domain of ca. 10 km wide in N-S and E-W directions, of which dostribution was lately named the Tokunoshima belt. The ca. 500 m thick IMC structurally overlies the Cretaceous AC as a klippe, which is composed mostly of pelitic–psammitic schists and mafic amphibolite, with minor lenses of serpentinite and dioritic gneiss. IMC suffered from considerable deformation and high-grade metamorphism up to the amphibolite facies. The youngest detrital zircon U–Pb ages (ca. 60 Ma) of the protolith sandstone and the ca. 17 Ma granitoid intrusion constrain the age of the metamorphism within the Eocene-Oligocene interval. The possible coeval heat source in SW Japan includes the San-in granitoid (Paleogene batholith) belt in Chugoku-N. Kyushu on the north, and an unknown collided oceanic arc system from the south, similar to the Tanzawa block in S. Kanto. Based on the newly obtained zircon U-Pb ages from psammitic schists, amphibolite, and granitoids, the geological significance of IMC and the Tokunoshima belt in East Asia is discussed in the tectonic framework of Paleogene East Asia.