Spatial and temporal variation of Neogen magmatism in the arc-arc junction

点击翻译按钮获取中文摘要

The Cenozoic volcanism in Hokkaido has been strongly influenced by tectonic events in the arc-arc junction. Main research results are summarized as follows.1.Detail geological investigations and newly determined K-Ar ages of Cenozoic volcanics reveals the migration and/or the convergence of volcanism in Hokkaido island since Miocene. During Pliocene and early Pleistocene, convergence of volcanism has also continued. Finally, three quaternary volcanic fields has constructed. Echelon arrangements of volcanoes in Kuril arc would be formed in early to middle Pleistocene.2.The distribution of the Quaternary volcanoes and mode of spatial variations in major element chemistry suggest the presence of three isolated Quaternary volcanic fields, which have developed under different arc-trench system. It should be possible that mode of spatial variations of chemical compositions of arc volcanics is strongly affected by tectonic setting and/or geological structure.3.Middle Miocene volcanism in northern Hokkaido is characterized by a short term activity (4 My) and weak arc signatures in volcanics. Considering tectonics arround Hokkaido island, the volcanism should be related with the formation of back-arc basins, Japan and Kuril basins.4.We have found unique volcanisms, (Takikawa volcanic field and North Taisetsu volcano) at the junction in Pliocene and early Pleistocene age. They are characterized by basaltic monogenetic volcano group and the presence of icelendite-like dacite. They suggest that the junction has been under the extensional stress field related with the oblique subduction of Pacific plate since late Miocene.