日本海拡大期の西南日本前弧域および中央構造線沿いの変形 : 予察  [in Japanese] Deformation of the forearc region and along the Median Tectonic Line in southwest Japan during the opening of the Japan Sea : A preliminary report  [in Japanese]

A review on the stratigraphy and structural development of the forearc region in southwest Japan indicates that compression tectonics prevailed in the area during the period from late Early-early Middle Miocene (20-15 Ma), which is coeval with the timing of the opening of the Japan Sea. Although the reactivation of the Median Tectonic Line (MTL) around this time is not certain, the normal faults formed in the Sambagawa belt very close to the MTL in western Shikoku, which prodeced the basins of the Eocene Kuma Group, could have been reactivated as thrusts (the Tobe and Hanayama thrusts). In Early Miocene, the Akaishi Tectonic Line (ATL) in the Chubu district was reactivated, which is indicated by the development of pull-apart basins of Early Miocene in age. The Lower Miocene series were strongly folded and trancated by the ATL at 16 Ma, which is probably caused by the collision between the southwest Japan arc and Izu-Mariana arc. Tectonics in southwest Japan during the opening of the Japan Sea is well explained by the trench retreat, which is assumed to be caused by the buckling of spherical shell (plate) (Yamaoka et al., 1987). Based on the model, it is suggested that the Japan arc bent and stretched in the direction parallel to the arc due to the opening of the Japan Sea. The shortening strain in the forearc region of southwest Japan during the back-arc opening could be due to the lateral contraction accompanied by the stretching of the arc, or this may be due to the convergence between the southwest Japan arc and Shikoku basin. Granites distributed in the outer belt of southwest Japan, and some of so-called Setouchi volcanic rocks may have intruded along the extension fractures caused by the stretching of the Japan arc. The strain rate of shortening at around 15 Ma in the forearc region exceeded 5×10^<-15>/s based on stratigraphical evidences, and that in the Kaikomagatake area, central Honshu, where the collision between the southwest Japan arc and Izu-Mariana arc had occured since 15 Ma, reached 10^<-13>-10^<-10>/s based on the microstructural evidence of deformed rocks. The rapid shortening strain in the forearc region in southwest Japan could have resulted from the rapid back-arc opening (opening of the Japan Sea).