Determination of Ages of Middle- to Late-Pleistocene Volcanism and Marine Terrace Formation In the Shimokita Peninsula, Japan, on the Basis of New Fission-Track Ages of Proximal Tephra Units

New fission-track ages of zircon crystals have been obtained as 0.18 ± 0.04 (1ó) Ma and 0.52 ± 0.09 (1ó) Ma for two proximal tephra units appearing in the Shimokita Peninsula, northern end of Honshu, Japan. Both the ages are concordant with tephrostratigraphy. Mt. Osore-yama, a Quaternary caldera volcano, is located near the center of the northern half of the Shimokita Peninsula. The history of volcanism on this mountain consists of three phases: lava eruption, explosive eruption ejecting pyroclastic material, and mixed eruption in a chronological order. The caldera was formed due to the explosive removal of the upper part of the volcanic body at the end of the second phase. The dated tephras erupted as the products of the second phase. The K-Ar ages are ~750 ka and ~800 ka for the two lava flow units of the first phase. These ages show that the first phase continued until ~750 ka, the second phase continued from ~500 ka to ~200 ka, and the caldera was formed at ~200 ka. More than 10 marine terraces fringe the Shimokita Peninsula. The older of the dated tephras was deposited before the formation of the fourth-lowest marine terrace. The younger of the dated tephras was deposited between the formation of the second-lowest marine terrace and the formation of the third-lowest marine terrace. A distal tephra with a stratigraphical age of 112–115 ka was deposited after the formation of the first-lowest marine terrace. These tephra ages show that the third- and fourth-lowest marine terraces were formed with two of the high sea-level stands from ~500 ka to ~200 ka, and that the first- and second-lowest marine terraces were formed with two of the high sea-level stands from ~200 ka to ~100 ka. The dating has been conducted as the JNES-commissioned research.