Geotectonic boundary corresponding to the Butsuzo Tectonic Line in the Nishime area of the Hokusatsu District, southern Kyushu, Japan

Abstract

The Nishime area of Hokusatsu District, southwest Kyushu, Japan, is underlain by a chert-clastic complex comprising the Nitao and Momigi Units of Chichibu Belt; a mélange of sandstone, chert, and limestone clasts in an argillaceous matrix, corresponding to the Omae Unit of Chichibu Belt; and coherent sandstone-mudstone formations that belong to the Saiki Subgroup of the Shimanto Belt. These pre-Paleogene bedrock formations are mostly covered by Neogene and younger formations. On the basis of detailed geologic mapping of this area, we propose two precise locations for the boundary between the Omae Unit and the Saiki Subgroup. This boundary is traditionally called the Butsuzo Tectonic Line (BTL). The first proposed location is on the Ushinohama coast, where a brittle shear zone separates the Saiki Subgroup from the mélange of the Omae Unit. Here, deformation structures in the shear zone indicate northeastward thrust of the hanging wall of the Omae Unit. The second location is at Chugenda. Although direct contact is not exposed at the Chugenda location, a mélange similar to the hanging wall of the Ushinohama coast shear zone is juxtaposed against coherent beds of sandstone and mudstone. We suggest that a geotectonic boundary corresponding to the BTL runs through the Ushinohama coast and Chugenda.