Abstract
The geological structure and tectonic evolution of the Paleozoic-Mesozoic accretionary complexes of the Ultra-Tamba and Tamba terranes in the Maizuru and Obama districts in SW Japan were clarified through geological surveys and investigations of fault rocks. A detailed geological map was created by precise field surveys and compilation of past investigation results. Based on the new geological map, we discuss the geotectonic history of the study area.
The Ultra-Tamba terrane, consisting of a late Permian-possibly Triassic accretionary complex, is subdivided into the Kozuki, Oi, and Hikami formations. The Kozuki Formation is newly identified in the study area and occurs sporadically along the boundary between the Yakuno Ophiolitic Complex of the Maizuru terrane and the Oi Formation of the Ultra-Tamba terrane. The Tamba terrane comprises mainly an Early-Late Jurassic accretionary complex and is subdivided into the Shuzan, Kumogahata, Haiya, Tsurugaoka, and Yuragawa complexes, and the Furuya Formation.
The complexes are separated by thrust faults, each comprising a single nappe, and display an imbricate nappe structure as a whole. Layer-parallel, brittle-ductile deformation structures are predominant in the Ultra-Tamba terrane and the Shuzan Complex of the Tamba terrane. These complexes were folded along E-W trending vertical axial planes during the Early Cretaceous. The Kanbayashigawa Fault is characterized by a NE-SW trending cataclastic shear zone with a sinistral shear sense that formed under a paleo-stress field cut-off at these fold structures. The southwestern part of this fault has been reactivated with a dextral shear sense under the current stress field. This slip-sense inversion resulted from the conversion of the stress-field and/or the rotation of regional blocks, and the opening of the Sea of Japan played an important role in the reactivation and slip-sense inversion of old fault structures in SW Japan.
