Intra-arc deformations with vertical rotation axes: the case of the pre-Middle Miocene terranes of Southwest Japan

Abstract

Various folds (bends) with vertical or subvertical rotation axes occur in the pre-Middle Miocene terranes of Southwest Japan. They are variable in shape (conical folds, accordion folds, megakink bands and so on), and some are accompanied with strike-slip faults. They range from several kilometers to more than 100 km in width, and some of their hinge surfaces are traceable more than a few tens of kilometers. Their shapes, which resemble megakink band or buckle folds, indicate that they were produced by horizontal compression with vertical or subvertical σ 2; axes in a shallow level of the crust. They were superimposed on the pre-Middle Miocene fold-and-thrust structures or nappe-melange complexes, and are regarded to have been formed mostly during the Middle Miocene, contemporaneous with the clockwise rotation of Southwest Japan related to the fan-shaped opening of the Sea of Japan. Geometrical analysis clearly suggests that the drifting Southwest Japan behaved as a non-rigid body, and the formation of these structures resulted from the differential rotation of the terranes and the collision of the Izu-Bonin Arc with eastern Southwest Japan.