Quaternary volcanism along the volcanic front in Northeast Japan

東北日本の火山フロント付近の第四紀火山活動

梅田 浩司; 伴 雅雄*

Umeda, Koji; Ban, Masao*

東北日本の火山フロント付近の火山からのマグマ噴出量及び年代のコンパイルを行い、過去200万年間の島弧スケールでの火山活動の特徴を明らかにした。その結果、約1Ma(現在から100万年前頃)を境に、(1)噴火様式の変化(カルデラを伴う噴火成層火山を伴う噴火),(2)マグマ噴出量の増大(0.19km/ky0.64km/ky),(3)マグマ化学組成の変化(ソレアイト系列カルクアルカリ系列)が認められる。また、火山フロント付近の断層運動の開始も約1Ma頃であり、逆断層の発達により第四紀火山の基盤となる脊梁山地も急速に隆起した。なお、今回の解析にあたっては第2次取りまとめの際に作成した第四紀火山カタログのデータを使用した。

The Northeast Japan arc is located on a convergent plate boundary along the subducting Pacific plate and the overriding North American plate. From a compilation and analysis of stratigraphy, radiometric age and eruptive magma volume data for 139 volcanic events along the volcanic front, notable changes in eruption style, magma compositions, variation in eruptive volume, and distribution of volcanic centers can be recognized around 1.2 Ma. Before ca. 1.2, felsic caldera-forming volcanism are thought to occur in regions of neutral stress regime with low crustal strain rate. From ca. 1.2 Ma to the present-day, the crustal stress regime seems to have changed to compression yielding the formation of stratovolcanoes all the volcanic front. It has become apparent that stratovolcanoes lie along major thrust faults associated with uplift of the Ou Backbone Range since the Middle Pleistocene. Although it is widely assumed that magma cannot rise so easily in compressional setting, the increase of erupted magma volume since ca. 1.2 Ma may have been caused by an increase in subduction rate of the Pacific plate between 5 and 2 Ma.