Kaisho-Kamitakara tephra erupted from the Hida Mountains in the early half of the Middle Pleistocene and its significance for the geomorphic chronology of central Japan

Abstract

The Kaisho-Kamitakara tephra (KMT) erupted from the Kaisho source vent located in the southwest part of the Hida Mountains in central Japan is composed of a pyroclastic flow deposit (> 40 km3) and a fallout tephra (>40 km3) distributing in the Chubu to south Tohoku areas (300 km from the source). The Volcanic Explosivity Index (VEI) of the eruption forming KMT is 6, indicating a paroxysmal eruption. The purpose of this paper is to describe the characteristic properties, distribution, and stratigraphic positions in several areas and age of this tephra, and to show the chronology of the geomorphic surface and paleogeography before and after the eruption. The followings characteristic of this tephra are used for the identification: abundant content of biotite and quartz as phenocryst; the chemical composition of glass shards showing a rhyolitic tephra with much content of alkali; chemical composition of titanomagnetite; the presence of zircon with a high uranium content. Radiometric ages obtained by the fission track dating method with the zeta calibration and by the K-Ar dating method in previous studies, and the stratigraphic position in the Kasamori formation of the Kazusa group distributed in the Boso Peninsula show that the deposition of KMT occurred between the marine isotope stage 17.3 and 15.2 (0.58-0.69 Ma). Chronological applications in several areas using the KMT as a datum plane are as follows: 1) Dissected fluvial terrace surfaces forming hills in and around the Kanto Plain that is the Sayama, Asuyama, and Kitsuregawa upper surfaces, and the depositional surfaces of the Nashinoki gravel in the southern part of the Matsumoto basin, were correlated, and they were formed a few tens of thousands of years preceding the deposition of the KMT. Alluvial fans, which are the origin of these surfaces, probably developed in the transitional stage from the interglacial stage of MIS 17.3 to the glacial stage of 16.2. The presence of these well-developed alluvial fans seems to indicate that there was a fairly long duration of favorable conditions for the formation of broad alluvial fans. 2) Along the west margin of the Kanto Plain, several hills in addition to those mentioned above are well developed, such as the Koma, Kusahana, Kasumi, Moroyama, Iwadono, and Hiki hills. The depositional surfaces of these terraces, which originated from fluvial terraces older than the Sayama surface, are not preserved. This means that the oldest hills with preserved depositional surfaces in and around the Kanto Plain are the Sayama surface and its correlates, showing that depositional surfaces older than 0.7 Ma are not preserved. 3) The distribution of the pyroclastic flow deposit of KMT suggests that the main part of the Hida Mountains was sufficiently high to form a barrier to the pyroclastic flow. 4) It is clarified that the Nihonmatsu surface, the lowest erosional low-relief surface in the western part of the Abukuma hihglands in southern Tohoku, is covered by the KMT. This means that the erosional surfaces were formed before 0.7 Ma.