LARGE DECKE STRUCTURES IN THE KUROSEGAWA AND SAMBOSAN TERRAINS, IN KYUSHU, SOUTHWEST JAPAN

Many large overthrusts have moved Paleozoic and Mesozoic strata toward south or southeast, producing decke structures in the Chichibu, Kurosegawa and Sambosan terrains in Southwest Japan. The decke structures of the Kurosegawa and Sambosan terrains in Kyushu have been clarified by the detailed analyses of geologic structures and conodont biostratigraphy. Two large deckes, the Shiraiwayama decke and the Gomayama decke which moved along the Shiraiwayama thrust and the Butsuzo Tectonic Line (thrust) respectively, occupy the most part of the terrains in Kyushu. The Shiraiwayama decke thrusts itself southeastward over the Gomayama decke which also thrusts itself southeastward over the Shimanto supergroup.
The Gomayama decke is a single decke, in which strata are continuously distributed in general, whereas the Shiraiwayama decke consists of four subdeckes, S1, S2, S3 and. S4 deckes from northeast to southwest. These four subdeckes have a common character that they have the Permian limestone layers at their southeastern frontal edges. Overlapping of subdeckes takes place in the area where two subdeckes adjoin. The western subdecke always overlie the eastern one there. Those four subdeckes moved southeastward with a slight clockwise rotation, and are arranged in a left-handed en echelon pattern as a whole.
The Kashimine decke moved along the Kashimine thrust in eastern Kyushu. The Kashimine decke covers both the Shiraiwayama decke and the Gomayama decke. Many minor thrusts are associated with the large overthrusts and produce an imbricate structure. Originally, therefore, these terrains must have been much wider.
The upper Paleozoic and lower Mesozoic strata change their lithofacies remarkably from the Kurosegawa islands-area to the Sambosan geosyncline. The sedimentary facies show that there were land masses and shallow submarine hills in the Kurosegawa islands-area and in the southern marginal Sambosan geosyncline, whereas there was a relatively "deep" sea-area between them. The geosyncline was a narrow sedimentary basin which lay probably on the continental side of a trench.