Tectonostratigraphic study of the Chichibu and Shimanto Belts in the Kanto Mountains, central Japan.

Abstract

On the basis of 120 radiolarian biochronologic data and detailed geologic mapping, fifteen tectonostratigraphic units have been recognized in the Southern Chichibu and Northern Shimanto Belts of the Kanto Mountains, central Japan. All units are in thrust contact with each other and are numbered from north to south as Units 1 to 15. The fifteen tectonostratigraphic units are composed of accretionary deposits which were successively accreted southward during Early Jurassic to Late Cretaceous time. Each unit in the study area primarily formed a stratigraphic succession from greenstone or limestone through chert, siliceous mudstone, and mudstone to sandstone (=oceanic plate stratigraphy). The combination of those rocks is recognized not only in coherent units but in melange units. Reconstruction of the plate stratigraphy of each unit helps our understanding of the pulse of the formation of accretionary complexes in the study area. There is a lack of Early Cretaceous accretionary complexes of the Hauterivian and the Aptian to middle Albian. The drastic changes from the formation of coherent units (Middle-Late Jurassic) through the formation of melange units (latest Jurassic-earliest Cretaceous) to the accretionary hiatus (Early Cretaceous) are recognized near the boundary between the Chichibu and Shimanto Belts. The formation process for the Jurassic to Cretaceous accretionary complexes in the Kanto Mountains is therefore divided into the following three tectonic phases : the Early to middle Late Jurassic (Phase I), late Late Jurassic to Early Cretaceous (Phase II), and Late Cretaceous (Phase III). This is possibly a record of the process and the historical changes in the ancient accretionary tectonics and the relative plate motions. Phase I (the accretion of complexes in the Chichibu Belt) and Phase III (the accretion of complexes in the Shimanto Belt) may be attributed to a difference in the two subducted oceanic plates : the former was old (Izanagi Plate) while the latter was young (Kula Plate) ; moreover, the conversion of the subduction of the different oceanic plates is likely to have caused the accretionary hiatus (Phase II) in the study area.