北部フォッサマグナ,上田市南部の中新統小川層から見出された安山岩溶岩のK-Ar年代

概要

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The Miocene Ogawa and Nashidaira Formations, which are distributed in the eastern margin of the Utsukushigahara Belt of the Northern Fossa Magna, consist mostly of sandstone, conglomerate and andesitic volcanic rocks. The K-Ar age for whole rock sample of the andesitic lava taken from the basal part of the Ogawa Formation indicates 7.6±0.2Ma in this study. This K-Ar age and another previous works suggest that the Ogawa and Nashidaira Formations in the study area was deposited during the late Miocene to the Pliocene, the period ranging from 8 to 4Ma. The lithofacies change in the eastern margin of the Utsukushigahara Belt indicates that the emergence age is about 7.6Ma, where as that in the western margin shows that age is ranging from 7.5 to 6Ma. It's concluded that the emergence of the Utsukushigahara Belt was during from 7.6 to 6Ma in age.
地学団体研究会の論文
2006-01-25