阿蘇カルデラ西端，立野峡谷の地質と形成年代

抄録

Aso Volcano in central Kyushu, SW Japan, is one of the largest caldera volcanoes in the world. Aso caldera, 25 km north-south and 18 km east-west, was formed by four gigantic ignimbrite eruptions: Aso-1 (270 ka), Aso-2 (140 ka), Aso-3 (120 ka) and Aso-4 (90 ka). The only outlet of the caldera, named as Tateno gorge, is located at the western edge of the caldera. Since the gorge’s formation history remains poorly studied, a detailed geologic study has been conducted to clarify the stratigraphic relationships between lavas and Aso ignimbrites, and evaluate the formation age of the gorge. The gorge walls are composed mainly of pre-Aso volcanic rocks (0.8-0.4 Ma), and densely welded Aso-2 pyroclastic flow deposits occur on both northern and southern walls of the gorge. Andesitic lava flows (Hokamaki lava) are found between the Aso-1 and Aso-2 ignimbrites in the gorge, and the lavas show potassium-rich characteristics that are indicative of the whole-rock chemistry of magmatic products from Aso Volcano. Additionally, welded Aso-1 blocks were observed in the gravel beds (lahar deposits) underlying the Hokamaki lava from borehole cores. The evidences suggest that the origin of Tateno gorge, which was draining a pre-existing caldera, formed after the Aso-1 ignimbrite eruption (270 ka), and thereafter Hokamaki lavas and the Aso-2 pyroclastic flows (140 ka) passed through the gorge.