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

[SGL23-05] Reconstruction of Stratigraphy and Tectonic History of the Goto Islands, Nagasaki Prefecture, Southwest Japan

*Shoichi Kiyokawa¹, Hiroaki Takahashi^2,1, Masaru Yasunaga^3,1 (1.Department of Earth and Planetary Sciences Faculty of sciences, Kyuushu University, 2.West Japan Engineering Consultants, Inc., 3.Goto Geopark (Shimo Goto))

Keywords:Opening of sea of Japan, Middle Miocene, Goto Islands, Extention

The Goto Islands, located at the westernmost tip of the Japanese archipelago, preserve a record of extensional tectonic evolution and volcanic activity from the period of the Japan Sea opening to the post-opening continental margin extension. The region is characterized by the middle Miocene Goto Group and associated volcanic rocks, which provide key insights into the tectonic and magmatic processes during this time (Kiyokawa et al. 2022).
Stratigraphy and Depositional Environment
The Goto Group has a total thickness of 2000–3000 m and is divided into three major stratigraphic units. Lower Unit (ca. 22–17.6 Ma): Composed of thick, greenish volcaniclastic rocks containing basaltic to dacitic volcanic materials, this unit represents the initial stage of continental rifting. Middle Unit (ca. 17.6 Ma): Consists of alternating sandstones and shales deposited in lacustrine and meandering fluvial environments within a syn-rift sedimentary basin, coinciding with a period of volcanic activity. Upper Unit (ca. 17.6–16.8 Ma): Composed of thick sandstones of fluvial–deltaic facies, which were deposited during rapid subsidence at the continental margin. This unit indicates the presence of a large river system that drained into the Sea of Japan. The depositional environment of the Goto Group records a transition from relatively cooler flora to warmer flora (Daijima-type), reflecting climatic changes associated with the Miocene Climatic Optimum. The Goto felsic volcanic rocks (ca. 16.8–15.4 Ma) unconformably overlie the Goto Group. Subsequently, granitic magmatism occurred between 16 and 14.5 Ma.
Structural Geology
Two distinct faulting phases are recognized: D1: Northeast–southwest trending normal fault activity. D2: Northwest–southeast trending normal fault activity.The westernmost part of the Goto Islands is influenced by left-lateral movement along the Tsushima–Goto Tectonic Line, which was active between D1 and D2.
Tectonic Evolution
ca. 22.0–16.8 Ma: The Goto Group was deposited during the rifting of the volcanic arc along the eastern margin of the Eurasian continent. Rapid subsidence due to D1 faulting resulted in the formation of a horst-and-graben basin. The anticline structures observed in the Goto Islands are associated with the flexural bending of strata due to D1-related normal faulting.
ca. 16.8–15.4 Ma: D2-related northeast–southwest extensional faulting led to the deposition of marine-influenced sediments (Kounoura Formation, in Kami-Goto), intercalated with explosive rhyolitic volcanic layers that overlie the Goto Group. The D2 faults show vertical displacements of at least 800 m. During this period, the Goto region is thought to have been at a low enough elevation to allow marine incursions. The current five major islands (Fukue, Hisaka, Narushima, Wakamatsu, and Nakadori) are separated by D2 fault boundaries.
In the Quaternary, alkaline volcanic activity (700000-18000 years) is observed along the D1 and D2 fault zones.
The geology of the Goto Islands reveals the evolutionary process of extensional tectonics and associated volcanic activity in the continental marginal arc region at the westernmost edge of the Japan Sea extensional field during the mid-Miocene.