房総半島，新第三系安房層群上部のthin-skin変形と褶曲

Abstract

褶曲軸に直交する平面内での2次元的thick-skin変形を想定した，基盤の鉛直昇降による被覆層の褶曲形成モデルを検証すべく，千葉県富津市南西部で地質調査を行った．この地域では，安房層群上部(上部中新統～鮮新統)が，東西に併走する背斜と向斜をなしている．調査は約3 km四方の範囲で行い，地層厚の側方変化・地質図規模の構造・小断層を検討した．調査の結果，同モデルの想定に反して，堆積時には現在の褶曲構造と不調和な地質図規模の3次元的昇降運動があったことがわかった．さらに，小断層群が示す露頭規模の変形も3次元的であった．したがって，同モデルは反証された．また，地質図規模のフラット・ランプ構造を含む14条の層面滑り断層と露頭規模の後背地傾斜デュープレックスも発見され，これらは北東～北北東バージェンスのthin-skin変形を示していた.

The folding of Neogene–Quaternary strata on Boso Peninsula, central Japan, has been attributed to differential vertical movements of basement blocks. Previous researchers have suggested that the folding resulted from two-dimensional, thick-skinned deformations along a vertical cross-section perpendicular to the fold axes. To test this hypothesis, we investigated the geologic structures in an area with dimension of 3×3 km2 in the southern part of the peninsula, where the upper Awa Group (upper Miocene–Pliocene fore-arc basin fill) makes E–W trending folds. We found that lateral variations in stratal thicknesses record three-dimensional syn-depositional mapscale movements that are incompatible with the model of two-dimensional fold structures; moreover, outcrop-scale faults also record patterns of three-dimensional deformations. Bedding faults, including a map-scale flat-ramp structure and an outcrop-scale hinterland-dipping duplex, both show NNE vergence, indicating that the strata were subjected to thin-skinned rather than thick-skinned deformation.