Study of the Multiple Collision Tectonics of the South Fossa Magna

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The purpose of this research project was to examine the hypothesis of the multiple collision tectonics of the South Fossa Magna, in the central Japan. This hypothesis is that the some ancient volcanic islands or submarine volcanoes belonging to the paleo Izu-Ogasawara Arc collided and accreted to the Eurasia/North American Plate in the South Fossa Magna during Neogene and Quaternary. The results of this research are summarized as follows :1. Detailed geological mapping (1/25000) and the microbiostratigraphy of the Fujikawa Group, which is most widely distributed in the South Fossa Magna, has led to the discovery of two horizons of the thickly bedded conglomerates as indicators of collision of islands. One horizon which is in connection with the collision of the Misaka Block is at 9Ma-7Ma. Another horizon is at 5Ma-3Ma. This horizon indicates the time of the collision of the Tanzawa Block.2. The thickly bedded conglomerates of the Fujikawa Group is thought to be the trough-filled sediments based on the sedimentological data.3. Reviewing the geological literatures about the South Fossa Magna, two other horizone of thickly bedded conglomerates were recognized. One is related to the collision of the Kushigatayama Block (12Ma) and the other is to the collision of the Izu Block (1Ma).4. Paleo-stress pattern deduced from the minor-faults and dikes is consistent with the direction of the movement of the Philippine Sea Plate after the collision of the Izu Block.