Palaeomagnetic and chronological study of Miocene welded tuffs in the northern part of Central Japan: tectonic implications for the latest stage of arc formation of Japan

Abstract

Kinematical adjustment of tectonic blocks has occurred in the latest stage of arc formation of Japan within central Japan. Miocene welded tuffs have been sampled for chronological and palaeomagnetic studies from the northern part of central Japan. Zircon fission-track dating confirms Middle to Late Miocene ages (13.7-5.0 Ma) for the Yatakezawa, Shiobara, Kawaji, Kinugawa, Katashinagawa, Okkai and Ashio Formations. Characteristic directions with high unblocking temperature component above 560°C are isolated from 36 sites. Palaeomagnetic directions after tilt correction provide a Miocene characteristic palaeomagnetic direction of the northern part of central Japan (36.8°N, 139.3°E): D = 7.3°, I = 50.3°, α 95 = 6.6°. Presence of normal and reversed polarities, and a fairly long time average exclude the effect of geomagnetic field secular variation. Comparison with the Miocene palaeomagnetic poles of the neighbouring blocks indicates that the northern part of central Japan and northeast Japan behaved as a unified block since 15 Ma, whereas the Kanto Mountain region moved kinematically as an independent block from the northern block comprised of central Japan and northeast Japan at least between 16 Ma and 12 Ma. The boundary between the northern part of central Japan and the Kanto Mountain was in a convergent regime with dextral strike slip. A southward force exerted from the northern part of central Japan, as well as a northward force by the Izu-Bonin ridge, induced the clockwise rotation of the Kanto Mountain region. The northern part of central Japan had welded completely with the Kanto Mountain region by 6 Ma.