火山フロントの火砕物質のゆくえ : 伊豆・小笠原弧北端部と房総半島南部間の相鴨トラフでの付加

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So-o Trough which is situated at the northeastern tip of the Philippine Sea Plate (PHS) is only the place where the PHS is going to subduct underneath the Honshu Arc. The trough is located also near the volcanic front of the Izu-Bonin arc, and Fujioka (1988) suggested that on the landward slope of the trough, volcaniclastic materials derived from the present Izu-Bonin arc are transported, deposited and accreted. Submarine topographic maps and Precise Depth Record (PDR) profiles revealed the following evidence ; (1) A topographic high, similar to the outer swell which is recognized in normal trenches, can be seen at the south of the So-o Trough, (2) Gently seaward dipping sediment surfaces are seen in the several lines on the landward portion of the bottom of the trough, and (3) Steep slopes with several sharp inflection points are clearly seen on the landward slope of the trough. Single seismic profile lines across the So-o Trough also offer the following evidence ; (1) A topographic high is recognized at the south of the trough (outer slope : So-o Knoll), (2) Landward dipping trench wedge structures are clearly seen along the bottom of the trough. Inclination of the reflectors becomes steeper towards the deeper part of the trench sediments. Gently seaward-dipping surface sediments are seen on the landward portion of the trough. These are a kind of trench fill turbidite sequence derived from the steep landward slope, (3) Low angle thrust structures on the landward slope of the So-o Trough are recognized in several profiles. Sediment columns obtained across the So-o Trough by the transect piston cores are turbidites with remarkable Bouma sequences which consist of a large amount of volcanogenic materials. Volcaniclastic materials which are the dominant constituents of the sediments represent chemical feature similar to those of the present volcanic front of the Izu-Bonin arc, that is low-alkali tholeiitic affinity like the basalts of Oshima Island, which is situated just west of the trough. The Neogene Nojimazaki Conglomerate of the southern Boso Peninsula has sedimentary structures similar to those of the So-o Trough and the volcanic fragments of the conglomerates show also chemical affinity similar to the low-alkali tholeiitic series. The above evidence strongly suggests that the volcaniclastic materials of the So-o Trough were derived from the present Izu-Bonin volcanic front. They were deposited on the bottom of the Trough floor anjd were accreted by the subduction of the PHS plate. Volcaniclastic materials distributed onshore south Boso Peninsula may also have been settled by similar accretion processes as proceeded in the So-o Trough.
特定非営利活動法人日本火山学会の論文
1991-04-15

火山フロントの火砕物質のゆくえ : 伊豆・小笠原弧北端部と房総半島南部間の相鴨トラフでの付加

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