サザンクロス潜航調査報告:フィリピン海プレート南端部、ヤップ、パラオ海溝およびアユトラフの横断潜航

Abstract

フィリピン海プレート南端部を横断潜航するサザンクロス航海の概要を報告する。この航海ではパラオ海溝陸側斜面、アユトラフ軸部、ヤップ海溝陸側、海側斜面と九州-パラオ古海嶺に潜航した。パラオ海溝陸側斜面からは表面に条痕を持つ巨大な石灰岩の岩帯が発見され、これが地質時代の珊瑚礁起源で浅所から来たことがわかった。パラオ海溝の南につながるアユトラフ中軸部には自然地震はほとんどなく、軸上に位置する海丘は堆積物に厚く覆われており、熱水チムニーは発見されず代わりに古い鉱化作用を受けた玄武岩が得られた。このことからアユトラフは過去に拡大したが現在は拡大を停止している可能性が高い。マリアナ海溝の南につながるヤップ海溝陸側斜面では地形の大きな傾斜変換点に広い平坦面があり、その平坦面より浅いところではガブロや変成した玄武岩が得られ、平坦面より下からは蛇紋岩化したカンラン岩が得られた。このことは地形の傾斜変換点がモホ面である可能性を示す。ヤップ海溝海側斜面では顕著な地塁･地溝構造が見られ、その急崖は新鮮な玄武岩で覆われていた。また地塁･地溝の中には三陸沖に見られたと同様の新鮮な海底の裂け目が認められた。九州-パラオ古海嶺の潜航では火山岩の水深による形態の変化と、CCD(炭酸塩補償深度)を確認する潜航が行われた。これらの新知見はフィリピン海プレート南端部のテクトニクスを考えるうえで重要である。

In this report, preliminary results of the Southerncross cruise which is the southern tip of the Philippine Sea Plate transect dives across the Palau, Yap trenches and Ayu trough were presented. Huge limestone block was found at the landward slope of the Palau Trench. The limestone is old reef origin and is transported from a much shallower part. Ayu Trough which is the southern extension of the Palau Trench has a few natural earthquakes. Knolls which are located in the axial part of the trough are covered with thick sediments and no active hydrothermal chimneys were found. Old ore deposits were taken instead of active chimney. These results may indicate the past spreading and no spreading now. Yap trench landward slope being connected with the Mariana Trench, has a notable flat plane which shows topographic inflection point. Gabbros and metabasalts were collected above the plane and peridotites and serpentinites were obtained below the palne. This means that the plane is possibly the petrologic Moho based on the topography and petrology. On the contrary, at the seaward slope, notable horst and graben structures were recognized on which walls were covered with fresh basalts. In these horst and graben, many fresh fissures which are similar to those found at the seaward slope of the Sanriku area. Morphologic changes with depth of the volcanic rocks were seen along the Palau-Kyushu remnant arc and CCD (Carbonate Compensation Depth) was traveled by submersible here. These new findings give a new insite for the better understanding of the tectonics of the southern tip of the Philippine Sea Plate.