PALEOMAGNETIC 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

Kinematical adjustment of tectonic blocks has occurred in the latest s tage of are formation of Japan within central Japan. Miocene welded tu ffs have been sampled for chronological and palaeomagnetic studies fro m the northern part of central Japan. Zircon fission-track dating conf irms Middle to Late Miocene ages (13.7-5.0 Ma) for the Yatakezawa, Shi obara, Kawaji, Kinugawa, Katashinagawa, Okkai and Ashio Formations. Ch aracteristic directions with high unblocking temperature component abo ve 560 degrees 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 degrees N, 139. 3 degrees E): D = 7.3 degrees, I = 50.3 degrees, alpha(95) = 6.6 degre es. Presence of normal and reversed polarities, and a fairly long time average exclude the effect of geomagnetic field secular variation. Co mparison with the Miocene palaeomagnetic poles of the neighbouring blo cks indicates that the northern part of central Japan and northeast Ja pan 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 Jap an and the Kanto Mountain was in a convergent regime with dextral stri ke slip. A southward force exerted from the northern part of central J apan, 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.