Paleogene and Plio‐Pleistocene basin formation around northwestern Kyushu, Japan

Abstract

Cenozoic basin‐forming processes in northwestern Kyushu were studied on the basis of geological and geophysical data. Gravity anomaly analysis delineated four sedimentary basins in the study area: Goto‐nada, Nishisonogi, Amakusa‐nada, and Shimabara. Borehole stratigraphy and reflection seismic interpretation suggest that the Goto‐nada Basin was subdivided into the Paleogene and Plio‐Pleistocene depocenters (Goto‐nada 1 and 2). In the Paleogene, Amakusa‐nada Basin was rapidly subsiding together with the Shimabara Basin as part of a large graben. Goto‐nada 1 and Nishisonogi basins belonged to another depositional area. After stagnant subsidence stage in the early Miocene, the study area became a site of basaltic activity (since 10 Ma) and vigorous subsidence in the Plio‐Pleistocene. Goto‐nada 2 Basin is accompanied with numerous east–west active faults, and separated from the Amakusa‐nada Basin by a northeast– southwest basement high, Nomo Ridge. Plio‐Pleistocene subsidence of the Amakusa‐nada Basin is related with low‐angle normal faulting on the eastern flank of the Nomo Ridge. Shimabara Basin is a composite volcano‐tectonic depression which is studded by east–west faults. Focal mechanism on active faults suggests transtensional stress regime in the study area.