Cenozoic volcanic and tectonic history of the northeast Japan.

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The stratigraphy, volcanic geology, sedimentology, petrology, magnetostratigraphy and K-Ardating dating of the Cenozoic volcanic and sedimentary sequences in the northeast Japan have been studied in order to know volcanic and tectonic evolution of the northeast Honshu arc.Tectonic evolution of the northeast Japan in the past 25 m.y. is divided into four stages from the type of volcanism, sedimentation and variation of the tectonic stress regime.1)Rifting Stage (RS) (around 25Na to 16 Na).2)Back-arc Basin Opening Stage (BBOS)(16 Ma to 12Ma).3)Transitional Stage (TS)(12 to 3 Ma).4)Shortening Deformation Stage (SDS)(after 3 Ma).Description of the mode of occurrence and facies analysis of the volcanic sequences formed in the RS and the early BBOS revealed eruption and emplacement environment of the sequences.Development of the biostratigraphy contributed to the precise correlation and age determination of the Lower to Middle Miocene, and so temporal changes of the paleoclimate and paleobiogeography became clear in detail. The calcareous nannofossil biostratigraphy showed that the first marine environment invaded the area along the Japan Sea in the Early Miocene NN 4 Zone of Martini's zonation.Petrochemical result on the Cretaceous to the Recent volcanic rocks clarified the variation of the chemical composition of the rocks and mantle in time and space, AND relationships between volcanism and tectonics. During from the Late Cretaceous to the Early Miocene late, the lateral variation of K20 content is similar to the Recent, the content increase toward back-arc side from the trench side. However, in the period between 17-14 Ma, a low alkali content zone appeared in the eastern margin of the Japan Sea. After the period, axial part of the zone migrated toward east and situated near the recent volcanic front at around 8 Ma.