北海道苫小牧沖海域の地質構造形成過程の特性  [in Japanese] Tectonic analysis of the Tomakomai offshore area, Hokkaido  [in Japanese]

北海道南部苫小牧沖海域には並走する数条の褶曲構造が発達し, 石油・天然ガスの賦存が期待されている。この地域の地質構造形成過程を仮想基盤変位法(KODAMA et al., 1985)により実験的に再現した。本地域は褶曲構造付近を境界として地層の層厚が急変したり, 浅層と深層で褶曲形態が異なる現象が見られる。実験の結果, これらの褶曲構造は隣合う翼部で形成時期が違うこと, あるいは浅層と深層で褶曲の形成時期が異なることがわかり, 地層の堆積過程における下位層の累積変形として乏くの褶曲構造の形成を説明できると考えた。また, 褶曲の形態ならびに形成過程が東部と西部で異なっており, これは隣接したふたつの堆積盆地の異なった造盆地運動を反映した結果であると総括された。なお垂直運動と水平運動による変形像の比較は, この両者は互いにエンドメンバーであり, 実際の地質構造は両者の特徴を合わせ持ったものであると考察された。

This paper presents the analysis of the tectonic process of the Tomakomai offshore area, southern Hokkaido, based on tectonic simulation. According to the recent progress of exploration for oil and gas in this region, its geologic framework has been clarified more than before. The geologic structure of this area is charactarized by the N-S to NNW-SSE trending folds. Based on the analysis of structural configuration and thickness changes of sediments, it is demonstrated that the sedimentation rate shows local variation. The location of these changes of sedimentation rate seems to be geographically coincide with the present anticlinal structures. Such structural feature mentioned above was examined by computer simulation using the Virtural Basement Displacement Method (KODAMA et al., 1985). The results of simulation indicate that the anticlines of shallow and deep zones were constructed disharmoniously and each limb of present anticline was formed different geologic period. This simulation leads to the conclusion that the anticlinal structure was formed through the process of accumulation of the incremental deformation. Fracture systems were also formed by the same process. The result of simulation comparing horizontal tectonics and vertical tectonics reveals that the actual structural styles in the studied area is situated between them.