MULTIPLE COLLISION OR NOT BETWEEN THE IZU-BONIN AND HONSHU ARCS, JAPAN?

The South Fossa Magna, central Japan, has been regarded as a collision zone between the Izu-Bonin and Honshu arcs during the Neogene to Recent. This tectonic framework was proposed more than 30 years ago, and then several authors have examined the tectonic history. In the South Fossa Magna, four exotic blocks are recognized; Kushigata, Misaka, Tanzawa and Izu blocks. This zone is situated along the plate boundary between the Eurasian and Philippine Sea plates on its western part and between the North American and Philippine Sea plates on its eastern one. Only he Izu block is on the Philippine Sea plate, which is subducted beneath the Eurasian plate along the Suruga trough (= Nankai trough) and beneath the North American plate along the Sagami trough. On the western and northern sides of the South Fossa Magna, the Cretaceous to Paleogene Shimanto accretionary prism extends. This bend of the Shimanto accretionary prism, which is used to be the almost E-W trending, has been caused by the north-north-westward advance of the Izu-Bonin arc. The Kushigata, Misaka and Tanzawa blocks make an almost E-W trending arrangement. The chronological sequence of the multiple collision is as follows; the collision of the Kushigata block to the Honshu arc occurred around 12 Ma, the Misaka around 8 Ma, the Tanzawa around 4 Ma and the Izu around 1 Ma. The dating for each collision is based on the depositional ages of the sedimentary sequence which is composed of the lower volcaniclastics and lava flows and the upper deep-sea fan and delta fan deposits as the trench-fill. It is controversial whether the sequential collision of three blocks except for the Izu block occurred or not; the coalesced terrane of the South Fossa Magna including the Kushigata, Misaka and Tanzawa blocks might be accreted into the Honshu arc. We found the planktonic foraminifer dated as 14-15 Ma from the trench-fill sediments in the Tanzawa block recently. This appears to mean that parts of the sedimentary sequence were deposited since 15 Ma in the coalesced terrane of the South Fossa Magna. That is, it is impossible to discriminate three blocks as a separate block. It is probable that only the Izu and the South Fossa Magna collision occurred between the Izu-Bonin and Honshu arcs.