Late Pleistocene and Holocene tephras in the Tokara Islands, southern Japan

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  • トカラ列島における後期更新世以降のテフラ
  • トカラ レットウ ニ オケル コウキ コウシン セイ イコウ ノ テフラ

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Abstract

Numerous Quaternary tephras occur in the Tokara Volcanic Islands—Kuchierabujima, Kuchinoshima, Nakanoshima, Suwanosejima, and Akusekijima islands—situated in the center of the back arc of the Ryukyu Islands arc, southern Japan. We have elucidated the stratigraphy and distribution of late Pleistocene and Holocene tephras on these volcanic islands, including the detection of two interbedded, foreign, widespread marker tephras, Kikai-Akahoya (7.3 cal ka) and Aira-Tn (29 cal ka). These foreign tephras allowed us to place the local tephras into a widespread tephrostratigraphic framework, and thus to examine the volcanological history of the islands using tephrochronology. One distinct feature is that no large-scale pumice eruptions occurred after the Kikai-Akahoya eruption, which differs from the Holocene eruption record of volcanoes in the gigantic caldera volcanic fields, southern Kyushu, that lie north of the Tokara Islands.<BR> We have recognized large-volume pumice fall deposits from Kuchierabujima, Kuchinoshima, and Akusekijima islands, and these are expected to be widely distributed. We have obtained major element compositions of glass from each of these tephras, using wavelength dispersive electron microprobe analysis at Kagoshima University to accurately characterize or fingerprint them. The glass shard compositions of these three tephras are sufficiently different to enable them to be distinguished from one another, thus allowing us to undertake correlations. Of the three pumice-fall tephras, a late Pleistocene deposit from Kuchierabujima Island has very distinctive glass composition. The glass fingerprint data will thus greatly facilitate recognition of distal correlatives.

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