Fission Track Ages of Tephras from Daisen and Sambe Volcanoes and Their Volcanological Implications.

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  • 大山および三瓶火山起源テフラのフィッショントラック年代とその火山活動史における意義
  • ダイセン オヨビ サンベ カザン キゲン テフラ ノ フィッショントラック ネンダイ ト ソノ カザン カツドウシ ニ オケル イギ

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Abstract

Zircons from tephras from Sambe and Daisen volcanoes, in the San-in district, Japan, have been dated by fission track method. The Daisen tephras dated are DNP (80±40ka), DHP (140±50ka), DMP (180±60ka), HdP (170±60ka), DOP (190±60ka), hpm 1 (230±70ka), and cpm (330±90ka) plinian pumice fall deposits and a NwF (100±40ka) block and ash flow deposit. Those dated from Sambe are the K 2 (100±30ka) ignimbrite and the K3 (100±20ka) pumice fall deposit. Errors are large due to the young ages; mean ages obtained are consistent with tephra stratigraphy and chronology using depositional rates of interbedded loess. DMP and DHP are exceptions, however. Their mean ages are older than the expected ages deduced from paleoclimatology, which suggests eruption at the beginning of the Late Pleistocene (-120ka). Based on the stratigraphy and ages obtained, it is concluded that the Middle-Late Pleistocene boundary is located between HdP and DMP. Explosive volcanism at Daisen volcano is considered to have begun not later than 350ka, with more than thirteen plinian tephra eruptions taking place by about 100ka, followed by seven or more plinian eruptions up to the present. Plinian eruptions at Sambe volcano commenced at about 100ka, and five tephras erupted in the Late Pleistocene.

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