Was the final phase of the Oshima Orogeny (mid Early Cretaceous) accompanied by extensional stress in the Kitakami district?

Abstract

The Early Cretaceous Oshima Orogeny in NE Japan was characterized by extensive magmatism and intense tectonic deformation, including folding before the late Aptian. However, the stress inversion of dike orientations in Jurassic and older accretion complexes in the Kitakami Mountains recently revealed an extensional setting during the mid Early Cretaceous. Unfortunately, it is difficult to evaluate the influence of the post-intrusion tilting of the dike attitudes, because the host of the dikes has complicated geologic structures. It is, accordingly, difficult to interpret the inversion result. Here, we inverted orientation data from quartz veins in the basement of the Upper Cretaceous formations that deposited shortly after the vein formation. As the data were collected near the base of the formations with dip angles smaller than ~20°, we could ignore the tilting of the veins to qualitatively interpret the extensional stresses that were determined by the inversion. The tectonostratigraphy and magmatic history of the northern Kitakami region suggest that the stresses were of the Aptian time. Thus, it was confirmed that the Kitakami region was subjected to extensional stress conditions. The stress change from compression to extension occurred around the Barremian-Aptian boundary.