Study of the Multiple Collision Tectonics of the South Fossa Magna

这项研究项目的目的是检验日本中部大峡谷南侧多次碰撞构造的假说。这一假说是在新近纪和第四纪，属于古伊豆-小川岛弧的一些古火山岛或海底火山在南大峡谷与欧亚/北美板块碰撞并吸积而成。1.详细的地质填图(1/25000)和南大隐窝分布最广的藤川群的微生物地层学，导致发现了两个层位的厚层砾岩作为岛屿碰撞的标志。与三坂地块碰撞有关的一个层位为9 Ma-7 Ma。另一个层位在5 Ma-3 Ma。这一地平线指示了坦萨泽地块碰撞的时间。根据沉积学资料，认为富士川群的厚层砾岩为槽状充填沉积。回顾有关南大隐窝的地质文献，发现了另外两个层位较厚的砾岩。一种与草山地块的碰撞有关(12 Ma)，另一种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模式与伊豆地块碰撞后菲律宾海板块的运动方向一致。

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.