This paper reviews the geology, petrology, and tectonics of the Proto-Izu-Mariana arc against the Honshu arc, Japan, since 15 Ma, to formulate current topics related to the Pacific-type orogeny.
丹沢 大磯地域に分布する火山岩類の K Ar 年代と 南部フォッサマグナ地域における鮮新世火山フロントの西方移動 K Ar ages of volcanic rocks in the Tanzawa Oiso area, central Japan: Implication for westward migration of the Pliocene volcanic front in the southern Fossa Magna region 森 慎一(Shin'ichi MORI) 山下 浩之(Hiroyuki YAMASHITA) 有馬 眞(Makoto ARIMA) 藤岡換太郎(Kantaro FUJIOKA) We determined K Ar ages of the late Miocene to Pliocene andesitic to basaltic dykes and lavas distributed in the Tanzawa Oiso area in the southern Fossa Magna region, central Japan. Our data indicate two distinct periods of volcanic activity in this region; one took place during the middle Miocene (about 15 Ma) and another during the late Miocene to Pliocene (7.5 4.2 Ma). Combined the present results with the age data reported previously from volcanic rocks in the southern Fossa Magna region, the present study suggests that the volcanic front in this region during 8 to 4 Ma was located at about 40 km east of the present-day volcanic front. The volcanic front began to migrate westward to the present-day location at about 4 Ma. The westward migration of volcanic front was probably associated with an abrupt change of lateral motion of the Philippine Sea Plate from north-northwestward to northwestward occurred between 4 and 2 Ma.
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