Paleomagnetic Study of Pleistocene Lavas and Dikes of the Zao Volcano Group, Japan

Abstract

Paleomagnetic studies, both directions and intensities, were made on lavas and dikes of Pleistocene age collected at the Zao Volcano Group, Japan. All the rock units collected have radiometric ages obtained by K-Ar method which cover the last one million years. Among 16 paleodirections obtained after cleaning by alternating field and thermal demagnetizations, eleven belong to the Brunhes chron, four to the Matuyama chron and one probably to the Jaramillo subchron. Angular standard deviation calculated from eleven sites of Brunhes chron was 11.9° with upper and lower 95% confidence limits of 16.8° and 9.2° respectively. This value is not inconsistent with the result from 113 poles in the Brunhes chron for Central Japan, although the confidence interval of our result is too large to assess any regional difference in paleosecular variation. Paleointensity experiment by the Thelliers' method was applied to a total of 70 specimens from nine cooling units. Among them, 31 specimens gave successful results and four and two paleointensities were obtained for the Brunhes and Matuyama chrons, respectively. Some correlations were found between the success rate of paleointensity experiment and reproducibility of the continuous measurements of magnetic susceptibility versus temperature. Such measurement of susceptibility and repeating it stepwisely by increasing the maximum temperature attained is proposed as a preceding technique to select suitable samples for thermal demagnetization and the Thelliers' experiment.