Basin evolution within the Kitakami Massif, northeast Japan: relationship between sedimentation, tectonics and volcanism in an incipient Neogene continental back-arc basin

Abstract

Abstract The Ichinohe basin in the western margin of the Kitakami Massif, northeast Japan, consists of the Early to Middle Miocene Yotsuyaku and Keiseitoge formations and represents the incipient stage of Neogene back-arc sedimentation that extends toward the north and northwest. Megaclast breccias derived from Mesozoic basement rocks of the Kitakami Massif were transported from southern areas as debris flows. The debris-flow deposits of the Yotsuyaku Formation grade northwards along the basin axis into clast-supported, braided-stream deposits, where floodplains developed. In the center of the basin voluminous volcanic eruptives of the Keiseitoge Formation were derived from eastern and southeastern volcanic ranges and formed transversal distributaries. The deposition of both clastic and pyroclastic sediments suggests that volcanism and tectonic uplift took place simultaneously during basin evolution. Parallel-laminated, fine-grained sediments intercalated with lignite beds suggest lacustrine deposition, and large cross-beds of volcanic breccias indicate Gilbert-type progradation into the lakes. A number of small lakes in the center of the basin was probably formed as the result of damming tributary mouths by the main drainage from eastern and southeastern volcanoes or the reciprocal blocking by lahars and non-volcanic debris flows derived from the southern uplift. Damming episodes produced by thick lava flows were not recognized in this study. The evolution of the N–S trending, longitudinal Ichinohe Basin indicates that E–W extensional tectonism related to the opening of the Sea of Japan affected not only the Green Tuff Region, to the west of the studied basin, but also even the Mesozoic basement during the Miocene. Although the mingling of volcanic and basement-derived deposits indicates the complexities of sedimentary facies and provenances, our view of the stratigraphic architecture has relevance to the interpretation of the evolution of other extensional basin systems.