LARGE DECKE STRUCTURES AND THEIR FORMATIVE PROCESS IN THE SAMBAGAWA-CHICHIBU, KUROSEGAWA AND SAMBOSAN TERRAINS, SOUTHWEST JAPAN

Many large overthrusts have moved Paleozoic and Mesozoic strata toward south or southeast, producing decke structures in the Sambagawa-Chichibu, Kurosegawa and Sambosan terrains in Southwest Japan. The decke structures and their thrusting process in Kyushu, through Shikoku, to Kii Peninsula have been clarified by the detailed analyses of geologic structures.
Four large deckes, all trending in the ENE-WSW to NE-SW direction, are arranged from north to south in Kyushu to Shikoku; the Kashimine-Kitatada-Ikegawa, Nanokawa-Onoyama, Shiraiwayama-Uonashi-Kambaradani, and Gomayama-Unomachi deckes, which moved along the Kashimine-Kitatada-lkegawa, Nanokawa-Onoyama, Shiraiwayama-Uonashi-Kambaradani, and Butsuzo thrusts respectively.
The four large overthrusts have south- or southeastward vergence and are accompanied with smaller thrusts. The overthrusts observed in the earth surface probably join to form a master decollement in a deeper level of the crust. A northern thrust was always formed after the formation of a southern thrust. The master decollement or the Butsuzo thrust formed probably at first, and then the northern thrusts formed successively, branching off the master decollement. Along the master decollement, strata of the Sambagawa-Chichibu, Kurosegawa and Sambosan terrains moved southward over the Shimanto supergroup at certain ages between Late Cretaceous and Paleogene.