Study on the Tectonics and Sedimentation of the Arc-arc Collision Type sedimentary Basins

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The south Fossa Magna is well known as a zone consisting of islands which belonged to the paleo-Izu-Bonin arc and collided and accreted to the Honshu arc due to the movement of the Philippine Sea plate in a north-west direction. There were four times after 15Ma. The purpose of this study is to clarify the relationship between multiple collision tectonics and sedimentation in the South Fossa Magna. The results are summarized as follows.(1) The trough-fill sediments related to the collision of the Tanzawa block are mainly composed of turbidite and debrite. They show a trend of change of clast type and upward coarsening sequences reflecting the upheaval movement of the hinterland according to the collision of the Tanzawa block.(2) The geological features of the Misaka block (second collision) was not well understood. Sedimentological studies clarified that the Misaka block constitutes a set of forearc, volcanic arc and backarc rift zone.(3) The Kushigatayama block that was thought to be the first collided massif in the South Fossa Magna is mainly composed of re-sedimented hyaloclastite and mass flow deposits. These facts show that the Kushigatayama block must have been a forearc or backarc region.(4) some of the dykes older than 2Ma, distributed in the area to the north of the Tonoki-Aikawa tectonic line, rotated clockwise about 34 degrees based on paleo-magnetic studies. This indicates that part of the Honshu arc rotated clockwise accompanied by the blocks.