1991 年 43 巻 4 号 p. 277-294
A paleomagnetic study was made for Miocene-Pliocene volcanic and sedimentary rocks from the Shakotan Peninsula, West Hokkaido, Japan (43.2°N, 140.5°E). Out of 20 rock units sampled, 18 sites were successful to reveal stable remanence but three sites were rejected because of their anomalous directions. Although most of the sites showed in situ directions coinciding with the direction of the present earth's field (PEF), we did not adopt the usual rejection criterion; the NRM in which the in situ direction is close to the PEF should be automatically rejected. Our opinion is that as long as the original stable remanence is detected there is no need to, rather better not to, reject such NRM directions. To support this idea we compared decay curves of remanence in AF demagnetization among NRM, IRM, and ARM. The total mean paleomagnetic direction (I=55.6°, D=14.7°, α95=9.5°) is slightly eastward and gives a paleomagnetic pole (77.9°N, 249.5°E, α95=12.1°) which might suggest a clockwise block rotation of the Shakotan Peninsula. However, we concluded that this pole for middle Miocene to Pliocene is not significantly apart from the present geographical pole on 95% confidence level. On the same confidence level, the pole position agrees with some of the contemporary poles from Northeast Japan implying the same block of West Hokkaido and Northeast Japan. This pole position also indicates that the Shakotan Peninsula did not suffered much tectonic deformation from the collision of East Hokkaido and the Kuril arc to the middle part of Hokkaido which is considered to have occurred after the late Miocene (KIMURA and TAMAKI, 1985). The northerly paleodirection revealed by the lowest sedimentary rock (-15Ma) might be an indication of the timing of rotation of Northeast Japan earlier than Southwest Japan.