Paleomagnetism and Post-Middle Miocene Counter-Clockwise Rotation of Tanegashima Island off Kyushu, Japan

Abstract

A paleomagnetic study was carried out on sedimentary rocks from the Middle Miocene Kukinaga Group and the Paleogene Kumage Group in Tanegashima Island off the southern extremity of Kyushu. 173 specimens were collected from 15 stratigraphic levels of the Kukinaga Group and 113 from seven horizons of the Kumage Group. After thermal and alternating field demagnetizations, characteristic magnetization directions were determined using the combined analysis of direct line-fitting and demagnetization plane data. Reliable paleomagnetic directions were isolated for 11 horizons from the Kukinaga Group. Eight of these were of reversed polarity and the remaining three from the uppermost horizons were of normal polarity. The unfolded formation-mean direction was D=331.2°, I=41.3°, and α95=9.9°. Five horizons of the Kumage Group yielded stable magnetization components of reversed polarity which, however, were not regarded as primary because of the negative tilt test. The mean paleomagnetic direction for the Kukinaga Group is deflected westward from the direction predicted using the 10Ma paleopole of Eurasia by 33.0°±10.7°. This implies that Tanegashima Island rotated counter-clockwise about 30° with respect to Eurasia and Southwest Japan since the Late Miocene, probably during the last 10Ma.