Fault structures and K-Ar dating of the Northern part of Koshiki Islands, west of Kyushu, Southwest Japan

Abstract

The Koshiki Islands in western Kyushu consist of the Upper Cretaceous to Paleogene sedimentary rocks and some igneous rocks. This area is one of the best places to understand the stratigraphy and structures of western Kyushu area. These structural data enable us to reconstruct the tectonic history of this area.
Based on the characteristics of faults and their crosscutting relationship, we identified two fault systems in the study area. One is the NW-SE trending normal fault system (F₁ faults) and the other is the NNE-SSW trending oblique-slip normal fault system (F₂ faults). K-Ar ages from one of F₁ faults range 17.9-24.8 Ma.