伊豆大島周辺の海底地形
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Topographic features around Izu Oshima region were summarized with special references to the characteristics relating to subduction of the Philippine Sea plate (PHS) along the Sagami Trough. Several topographic highs having circular contours around Oshima Island may be the results of parasitic eruption of Oshima Volcano. Their trend is almost parallel to the elongation of the linearity of parasitic cones distributing onshore. Along the eastern trench slope off Oshima Island, large scale collapse structures are seen in the submarine topographic map. These are the result of the falling down mostly of pyroclastic materials (tephras) during historic ages. The Oshima Island shows eastward tilting owing to the bending resulted from the subduction of the PHS. Large scale of the collapse structures may be caused by the oversteepening of the eastern trench slope of Oshima Island and soft sediments consisting mostly of pyroclastics including a large quantity of pore water slide down along the trench slope as far as the Sagami Trough.<BR>Radial distribution both of saddles and valleys observed northeastern offshore part of the Oshima Island may be lava flows and/or dike swarms derived from the old Okata Volcano whose eruption center was estimated to exist at the northeastern extension of the Oshima Island.<BR>About 4 km south off the Oshima Island, large scale of the topographic breakes consisting of caotic highs and depressions are seen in the submarine topographic map. These may be debris flow deposits along eastern slope of Ohmurodashi which consists mostly of the Quaternary rhyolites geologically and chemically similar to those exposed on the Niijima and Kozushima Islands. These are the results of catastrophic phreatomagmatic eruption of the Ohmurodashi such as that of the Myojinsho. Mixture of pyroclastics and lavas will be found from these caotic areas.<BR>The upper steam of the Sagami trough shows notable meandering whose direction is almost parallel to that of the movement of the PHS east off Oshima Island and almost perpendicular east off Ohmurodashi. At the latter case, thrust structures may trace along the subduction zone at the landward slope of the Sagami trough. Just landward side of the thrust, curious round topographic highs can be observed. They may be mud volcanoes.<BR>Topographic features observed around Oshima Island may best be understood in term of the normal and oblique subduction of the Philippine Sea plate along the Sagami Trough.
- 公益社団法人 東京地学協会の論文
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