紀伊半島東部秩父南帯の犬戻峡コンプレックス : 放散虫化石年代と野又衝上断層  [in Japanese] The Inumodorikyo Complex of the Southern Chichibu Belt in the eastern Kii Peninsula, Southwest Japan : radiolarian ages and the Nomata Thrust  [in Japanese]

紀伊半島東部秩父南帯の犬戻峡コンプレックスは,珪質粘土岩,チャート,暗緑色珪質泥岩,および陸源砕屑岩から構成され,北傾斜の野又衝上断層を介して北側の野又帯と南側の桑木谷帯に区分される.陸源砕屑岩から産する放散虫化石年代は,野又帯でジュラ紀中世Bathonian中期〜Callovian後期(ジュラ紀中世後期)を,桑木谷帯でジュラ紀中世Callovian後期〜新世Oxfordian(ジュラ紀中世末〜新世前期)を示す.野又衝上断層は,破砕帯と断層岩の構造解析に基づくと,明瞭なスラスト変位を有する.北北東-南南西ないし北東-南西走向の高角断層は,犬戻峡コンプレックスの西南西-東北東走向の地質構造,および南側の白亜紀新世の付加複合体を切っている.さらに,犬戻峡コンプレックスの構造的下位の地層が,高角断層の1つの西山川-笠木川断層の東側に広く露出する.故に,高角断層は東側上昇の変位成分を示し,白亜紀末〜古第三紀にかけて活動した.

The Inumodorikyo Complex of the Southern Chichibu Belt in the eastern Kii Peninsula, Southwest Japan, consists of siliceous claystone, chert, dark green siliceous mudstone, and terrigenous clastic rocks, and is subdivided into the Nomata Belt on the north and Kuwanokidani Belt on the south by the north-dipping Nomata Thrust. Radiolarians recovered from the terrigenous clastic rocks show that the ages of the complex in the Nomata and Kuwanokidani Belts are, respectively, middle Bathonian to late Callovian (late Middle Jurassic) and late Callovian to Oxfordian (latest Middle to early Late Jurassic). The structural analysis of the crush zone and fault rocks suggest that the Nomata Thrust really has a dominant reverse-slip component. NNE- to NE-trending high-angle faults cut the ENE-trending structures of the Inumodorikyo Complex as well as the middle Late Cretaceous accretionary complexes on the south. Moreover the strata of lower structural horizon widely expose on the east of the Nishiyamagawa-Kasagigawa Fault, which is one of the high-angle faults. Hence the high-angle faults are east-side-up, dip-slip faults and were likely active during latest Cretaceous to Paleogene time.