Stratigraphy and chronology of silicic tephras in the Shikotsu-Toya volcanic field, Japan: Evidence of a Late Pleistocene ignimbrite flare-up in southwestern Hokkaido

Abstract

The Shikotsu-Toya volcanic field (STVF) in southwestern (SW) Hokkaido (Japan) comprises three caldera volcanoes (Toya, Kuttara, and Shikotsu calderas and their post-caldera volcanoes) and Yotei and Shiribetsu stratovolcanoes. The stratigraphy, chronology, petrography, and glass geochemistry of tephra layers in the Ishikari pyroclastic terrain are carefully examined and reassessed. On the basis of the presence of volcaniclastic soil horizons and common petrological features, individual eruptive episodes are defined and the source volcanoes of each tephra layer are identified by correlation with proximal deposits. 28 eruptive episodes are distinguished, including six that are newly discovered. Based on K-Ar ages of older effusive lavas around the STVF, effusive andesitic volcanism, which was active since the Middle Miocene in SW Hokkaido, would have converged in the STVF by 0.9–0.5 Ma. After a dormant period of 400 kyr, explosive eruptions began at Toya 122.5 ka, and at Shiribetsu before 120–115 ka. A catastrophic caldera-forming eruption ca. 106 ka resulted in the formation of Toya caldera. Explosive activity then migrated eastward, and Kuttara and Shikotsu volcanoes began erupting explosively before 90 ka and 85 ka, respectively. Subsequently, the large stratovolcano Yotei was constructed after ca. 75 ka in the back-arc side of the STVF. Following repeated explosive eruptions of these five volcanoes, the largest caldera-forming eruption occurred ca. 43.8 ka at Shikotsu volcano. The STVF has produced VEI* (volcanic explosivity index) 7 eruptions every 20–40 kyr since explosive eruptions began, and the total volume of magma erupted in the STVF is estimated to be ca. 700 km3 DRE (dense rock equivalent). These findings suggest that the STVF is one of the youngest ignimbrite flare-up suites in Japan, and that its eruption rate is up to ca. 5.6 km3 DRE/kyr since 125 ka.