Abstract
Tectonic inversion commenced in the NE Japan arc sometime after the late Middle Miocene. This paper concerns the controversial timing of the inversion by dealing with the geology of the Higashiyama Hill in the southern part of the inner arc. The overturned Plio-Pleistocene formations make up the western limb of the north-south trending Higashiyama Anticline, which has a horizontal hinge line and a gently dipping eastern limb. Based on detailed geological mapping, paleocurrent and fault-slip analyses, we show that the anticline began uplifting no earlier than the deposition of the middle part of the Ushigakubi Formation, i.e., <3 Ma. Specifically, it was found that the hinge line largely coincided with one of the main stream of the Pliocene paleocurrent system of this area. The stream branched off to the both sides of the present hinge line. This pattern was persistent over a few million years until∼3 Ma, during which sandstone layers of the Araya Formation were deposited in the Higashiyama area including the hinge zone. This long-standing pattern evidences flat paleo topography or even a shallow paleo trough along the anticline. Our geological mapping and fault-slip analysis suggest that the broader exposure of the Ushigaskubi and older formations to the east of the hinge line than the western limb is the surface expression of the north-south extensional deformation of the strata, resulted from northward gravitational spreading of the culmination of the anticlinorium that is bounded at its western margin by the Higashiyama Anticline.
