Collision Tectonics
Recent geological and geophysical investigations in and around the South Fossa Magna, central Japan, revealed that the Izu Peninsula, on the northwestward migrating Philippine Sea Plate, has been collided with the Tanzawa Mountains since 1 Ma. The Tanzawa Mountains has also collided with the Kanto Mountains at about 5 Ma. The South Fossa Magna is, therefore, thought as arc-arc collision zone between the Eurasian Plate and subducting Philippine Sea Plate since Miocene.
Collision of the Tanzawa Block
Tanzawa Block is composed of strongly altered and deformed volcalic rocks, which differ from non-volcanic marine sediments of the adjacent ares, such as the Chichibu, Itsukaichi Basins and the Boso Peninsula. Not only the lithological features, but also the absence of the intrusive rocks along the Tanzawa/Kanto Mountains boundary, it is strongly suggested that the volcanic activities of the Tanzawa Block were occurred extremely far from the Kanto Mountains. Based on the microfossil evidence, the Tanzawa Mountains is thought to have been collided with the Kanto Mountains at ca. 5 Ma, because there was a deep trough at 6 Ma between the Kanto Mountains and migrating Tanzawa Block.
Both the Misaka and the Kushigatayama Mountains, located west of the Tanzawa Mountains, are also discussed as the exotic segments on the Philippine Sea Plate, collided prior to the Tanzawa Block, while detailed timings of these two blocks are still controversial.
Origin of the scoria
The location of the Izu-Ogasawara arc during Miocene is one of the most important problems concerning with the Neogene tectonics in Japan. Some geologists thought that the Izu-Ogasawara arc was located far-east from the present location, and others insisted the westerly shifted location. The absence of the geophysical constraints, such as the hot spot chains or mid-ocean ridges, for determining the past plate motion of the Philippine Sea Plate, is owing to this controversy. The most critical evidence for discussing the location of the Izu-Ogasawara arc during Miocene is the scoria within the hemipelagic sediments in the Boso fore-arc basin sediments.
The photograph shows the frequent interbedded of the scoria in the hemipelagic sediments in the Boso Peninsula. The scoria beds are well developed in the marine sequence in the Miura and Boso Peninsulas, close to the present Izu Peninsula. In contrast, there is no scoria bed in the Miocene sediments in the sedimentary basins around the South Fossa Magna. This implies that the source volcano of these scoria beds located quite near the Miura and Boso Peninsulas. Based on the sedimentological features of the scoria beds and the size of these scoria, the source vent located almost the same position of the present Izu Peninsula or Izu-Oshima island.
As the presence of the scoria beds indicates the basaltic activity of the subducted Izu-Ogasawara arc (proto-Izu arc) at near the Boso peninsula, the age of the lowest scoria bed in the Boso marine sequence probably indicates when the basaltic volcanism started and/or when the active volcanic arc migrated to near the peninsula. Based on the biostratigraphic constraints, the age of the lowest scoria bed (Am-1 Tuff in the Boso Peninsula) can be estimated as ca. 13 Ma. Therefore, it can be concluded that the active basaltic volcanism has occurred since ca. 13 Ma until recent at quite near the Boso Peninsula. This implies that the proto-Izu arc has situated at almost the same locality of the present Izu-Ogasawara arc since 13Ma.
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