四万十帯のデュープレックスと低角ナップ構造

There are many shallowly-dipping or nearly horizontal thrusts in the Shimanto Terrane of Kyushu. Some of them are accompanied by duplex structures and thin thrust sheets of red and green siliceous mudstones, which were formed during underplating.
In the Shimanto Terrane of east Kyushu, the Cretaceous sandstone-dominated Morotsuka Group is thrust toward SE over the Cretaceous phyllite-dominated Makimine Group along the Tsukabaru Thrust. The Makimine Group and Paleogene Kitagawa Group are thrust toward SE over the Paleogene Hyuga Group along the Nobeoka Thrust. The Nobeoka and Tsukabaru Thrusts are nearly horizontal as a whole, and form giant nappes and some klippen. Their minimum horizontal displacements amount to 60km and 30km, respectively.
The nearly horizontal nappe structures of the Shimanto Terrane in Kyushu are inferred to be traced to Shikoku. In Shikoku, the Cretaceous Lower Shimanto Group, which is correlative with the Morotsuka Group, is in fault contact with the mainly Paleogene Murotohanto Group. The boundary fault is called the Aki Tectonic Line in east Shikoku and the Nakasuji Tectonic Line in west Shikoku. These faults dip steeply or nearly vertical, However, the originally low-angle Aki Tectonic Line possibly became nearly vertical in east Shikoku, and it possibly occurs near the Nakasuji Tectonic Line in west Shikoku. The low-angle nappe structures are possibly the fundamental structures of the Shimanto Terrane.

九州の四万十帯には，多くの低角な衝上断層が存在し，それらはデュープレックスや，赤・緑色珪質泥岩の衝上シートを伴うことがある．九州の四万十帯では，砂岩優勢の白亜系諸塚層群が，塚原衝上断層によって，千枚岩優勢の槙峰層群の上に衝上している．また，槙峰層群と古第三系北川層群は，延岡衝上断層によって，古第三系日向層群の上に衝上している．延岡衝上断層及び塚原衝上断層は，離れた位置にクリッペが存在することから，全体としてほぼ水平で，変位量はそれぞれ60km,30kmに達する．四国では高角な安芸構造線，中筋構造線が白亜系と古第三系を境しているが，安芸構造線はもともと低角であり，中筋構造線付近には低角な衝上断層が存在する可能性がある．九州で確認された低角なナップ構造は四国へも追跡され，西南日本の四万十帯の基本構造である可能性がある．