LARGE-SCALE FOLD-THRUST STRUCTURE IN ACCRETIONARY COMPLEX, SOUTHWEST JAPAN -TECTONIC MODIFICATION OF ACCRETIONARY COMPLEXES BY OROGENIC MOVEMENT AT CONVERGENT MARGINS-

Convergent margins are the most active tectonic regions on Earth due to the subduction or collision of two plates. The island arcs such as Japanese Arc have formed by ‘accretion’ of material at the frontal edge of subduction zones. As mentioned in the Himalaya collision zone, most of the matured island arcs have been tectonically modified by orogenic movement. However, deformations occurred during the accretion process and orogenic process have not been well distinguished because of their similar features and complicated structures. Former tectonic models of southwest Japan insisted that accretion processes were essential to understand the formation of Japan.

This research provides new geological and structural data as well as development of measuring method of grading structure based on grain-size analyses to assess the tectonic modification of accretionary complexes by orogenic movement of the Chichibu Belt which consists of the low-grade metamorphosed Jurassic-Permian accretionary complexes and the overlying Cretaceous fore-arc sedimentary rocks.

The results show the framework of geological structure is not low angle as previously thought but shows large-scale overturned folds and thrusts which dips back-arc ward. The folds bent the primary piled nappe structure of accretionary units and the overlying Cretaceous rocks. The higher-grade metamorphosed rocks overlie the lower-grade rocks bounded by a thrust.

Post-late Cretaceous shortening in the fore-arc region in the southwest Japan is indicated by several lines of evidence, including compressional activity of ocean ward-verging thick-skinned fold and thrust belt. This tectonic deformation should be related to uplifting of high-pressure metamorphic rocks of the Sambagawa Metamorphic Belt which distributes back-arc side of the Chichibu Belt. Thus, the geological structure of the Chichibu Belt has been tectonically modified by post-accretion orogenic movement. Some of deformation structures observed in accretionary complexes which have been interpreted as results of accretion processes must be explained as orogenic movement of post-accretion. This will be a remarkable improvement on the current model which contributes to understand the tectonic evolution and deformation processes of subduction zones.