New glass fission-track ages of Middle Pleistocene tephras on Yakushima Island, southern Japan

Abstract

The ages of two hornblende-bearing Middle Pleistocene rhyolitic tephra beds on Yakushima Island, southern Japan, have been determined by the glass-ITPFT method. Koseda pyroclastic flow deposit is 0.58±0.08 Ma and Anbo tephra (An) is 0.78±0.09 Ma. The normal geomagnetic polarity of An demonstrates a stratigraphic position just above the Bruhnes/Matuyama Boundary. Both volcanic units are the result of large-magnitude eruptions from a source that is most probably located at one of the calderas south of the central part of the Kagoshima graben, southern Kyushu, given the low total alkali content of their glass shards. They must have been widely distributed across Kyushu and central Japan, although no other occurrences are presently known. New information on their mineralogy, glass composition, and age will greatly facilitate recognition of their distal correlatives. Although not explored in this study, these two tephra beds will undoubtedly play a useful role in future studies aimed at elucidating the Quaternary paleoenvironmental history of Yakushima Island.