Polarity Transitions of the Geomagnetic Field Deduced From the Natural Remanent Magnetization of Tertiary and Quaternary Rocks in Southwest Japan

Abstract

Miocene, Pliocene and Pleistocene volcanic and plutonic rocks were collected from the southwestern part of Japan, and directions of natural remanent magnetization were measured. Rocks with stable magnetization from various tests in the laboratory are classified into four large groups according to geological sequence or radiometric datings; the groups being of upper Miocene, Mio-Pliocene, middle Pliocene and Plio-Pleistocene. It was observed that the rocks belonging to Mio-Pliocene, middle Pliocene and Plio-Pleistocene have both the normal and reversed magnetizations almost in the same numbers, but the reversed polarity was not found in the rocks belonging to the upper Miocene time.Results of paleomagnetic observations obtained from the rocks in southwest Japan show that the geomagnetic field during the Miocene and Pliocene ages had close to the field of the present axial dipole with exception of duration of polarity changes. However, the field has often changed its polarity since the Miocene time. A transition which occurred in Mio-Pliocene is unusual as compared with that in the other ages, and the paleomagnetic poles during the transition are situated only in a definite zone along a great circle around the earth (about 10°E and 170°W in longitude). On the other hand, transitions in middle Pliocene and Plio-Pleistocene appear to have been abrupt, because no intermediate direction of magnetization, suggesting that the geomagnetic field changed its polarity gradually, was found in every section of rocks.