Study of the Multiple Collision Tectonics of the South Fossa Magna

本研究项目的目的是检验日本中部南大窝的多次碰撞构造假说。这一假说认为，古伊豆-小笠原弧的一些古火山岛或海底火山在晚第三纪和第四纪在大窝南碰撞并增生到欧亚/北美板块上。本研究的主要成果如下：1.详细的地质测绘（1/25000）和Fujikawa组，这是最广泛的分布在南窝Magna的微生物地层学，导致了厚层状砾岩的两个层面的发现，作为岛屿碰撞的指标。与Misaka地块碰撞有关的一个层位为9 Ma-7 Ma。另一层位为5 Ma-3 Ma。这一水平线表明了Tanzawa块体碰撞的时间。根据沉积学资料，认为藤川群厚层砾岩为槽充填沉积.通过查阅有关南马格纳窝的地质文献，又发现了两个厚层砾岩。一个与串山地块的碰撞有关（12 Ma），另一个与伊豆地块的碰撞有关（1 Ma）.由小断层和岩脉推断的古应力场与菲律宾海板块在伊豆地块碰撞后的运动方向一致。

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.