Late Cenozoic Geohistory in the Northern Fossa Magna Region, Central Japan

In the northern Fossa Magna region situated at a junction of the Honshu Arc and the Izu-Ogasawara Arc, an extremely thick series of Upper Cenozoic strata is extensively developed. In this paper, the stratigraphy is described in detail, and the sedimentary and tectonic history is reconstructed on the basis of the analyses of sedimentary environment and geologic structure.
The Upper Cenozoic system in this region is divided into seven formations, namely, the Uchimura, Bessho, Aoki, Ogawa, Shigarami, Sarumaru and Toyono Formations in ascending order. They are moderately deformed by folds and faults. Six types of fold and six systems of fault are discriminated. The subsidence pattern and sedimentary environment are analyzed through examining the change in thickness and litho- and bio-facies, as well as paleocurrents.
The Late Cenozoic geohistory in the northern Fossa Magna region is divided into two stages, though they overlap each other in their transitional period. The first stage during Miocene to Early Pleistocene is characterized by a series of movements including initial volcanism and subsequent subsidence, plutonism and folding. The sedimentation of preflysch, flysch and molasse facies have taken place. The history in this stage may be com-parable with that of "geosyncline". The development of sedimentary basin with a polarity toward back-arc side and the folding of strata have essentially resulted from the growth of regional asymmetric upwarping of NE-SW trend under the influence of gravity. The second stage during Pliocene to Quaternary is characterized by block faulting associated with uplifting of island arc. Volcanic chains and volcano-tectonic depressions of N-S and partly NW-SE trends were superimposed upon the structural framework constructed in the first stage. The migration of depocenters of sedimentary basins newly generated in this stage is oblique to that in the first stage, and seems to have continued until the present.
It is considered that the Late Cenozoic geohistory in the northern Fossa Magna region has been tectonically controlled by superposition of the block-faulting with a trend parallel to the Izu-Ogasawara Arc upon the asymmetric upwarping with an axis parallel to the Northeast Japan Arc.