Abstract
We report new paleomagnetic directions from six Middle Miocene andesite sites in Takanawa peninsula, Shikoku, Southwest Japan. These andesites were divided into three groups in terms of their K-Ar ages. One dyke is dated as 12.0+/-0.2 Ma. Two dykes and two necks are dated in the range between 14 to 15 Ma. Rest one shows relatively older age as 15.8+/-0.2 Ma. The paleomagnetic directions of the younger five sites (12 Ma to 15 Ma) do not show any significant deflection from that of the present geomagnetic field. On the other hand, a clockwise deflected declination (52.2°) is revealed at the older site. Local tectonic disturbances and geomagnetic secular variation are inadequate to explain the deflected direction of this site. These results, in conjunction with the previous study on the Miocene Mizunami/Kani Groups (18 Ma to 15 Ma), can delineate the continuous record of the clockwise rotation of the Southwest Japan block. We provide further evidence for beginning of the block rotation at about 16 Ma and its termination at about 15 Ma or 14 Ma.
