Basic concept of balanced cross section and duplex structures of the Shimanto Terrane, Kyushu, Japan

Petroleum exploration requires an exact cross section. Balanced cross section construction (Dahlstrom, 1969) is a very useful way to extrapolate surface structures to the depths. The basic concept of his method is that the geometric features of any constructed section must be retrodeformable to the restored section. Bed lengths and areas must be equal both in deformed cross section and undeformed restored section. Balanced cross section construction has been very successful in understanding complex structures of the fold and thrust belts of the orogenic belts, especially for duplex structures. But an application of the method to the accretionary terranes is very difficult, because original successions are not clarified. However in such a case, the concept of balanced cross section is essential to understand thrust structures. Some examples of the duplex structures of the Shimanto Terrane in Kyushu, Japan are described.Key words: balanced cross section, thrust, duplex, fault-bend fold; normal fault, Shimanto Terrane, Kyushu