History of basin formation and tectonic evolution at the termination of a large transcurrent fault system: deformation mode of central Kyushu, Japana

Abstract The central part of the Kyushu Island is a locus of active arc-volcanism associated with subduction of the Philippine Sea plate. Being located at a bend in the boundary between the Philippine Sea and Eurasian plates, a complicated lateral displacement has generated half-grabens, a zone of strike-slip, and rhomboidal basins filled with volcanic material in central Kyushu. Initial northward subduction in latest Miocene time activated the N-S-trending Kokura-Tagawa Tectonic Line (KTL) as a left-lateral slip fault that bounds the western margin of the area of volcano-tectonic depression, the Hohi volcanic zone, which was initially formed about 6 Ma ago. The depocenter, which is now called Kuju basin, in the Hohi volcanic zone was at the corner of the KTL and the E-W-trending Median Tectonic Line (MTL). Relative convergence direction of the Philippine Sea plate shifted counterclockwise at about 1.5 Ma, and active motion on the KTL declined. At the same time, west-northwestward subduction enhanced right slip on the MTL. Geologic, gravimetric, and seismic data indicate that the MTL has shifted its active trace northward in central Kyushu as far as 10 km. As a result, the depocenter adjacent to the transcurrent fault migrated northeastward in the Hohi volcanic zone, specifically, from Kuju basin (Pliocene) via Shonai basin (early Quaternary) to Beppu Bay basin (late Quaternary). The latest depocenter of the Beppu Bay is surrounded by active faults that clearly delineate a rhomboidal basin on the MTL. Central Kyushu exemplifies the basin forming history and tectonic evolution at the termination of a large transcurrent fault system.