Paleomagnetism of the Nijo Group and its implication for the timing of clockwise rotation of southwest Japan.

Abstract

A paleomagnetic study was conducted for well-dated volcanic rocks from the Middle Miocene Nijo Group in the Mt. Nijo area, southwest (SW) Japan. Samples were collected from 33 sites. Detailed stepwise demagnetization with both alternating-field and thermal techniques revealed well-determined site-mean directions from 29 sites. Tilt correction was applicable to 13 sites, yielding an overall mean direction of D=352.2°, I=51.7°, and α95=7.4° and a paleomagnetic pole at 84.3°N, 35.5°E, and A95=8.5°. The bedding-tilt test passed with 95% probability. Although the reversal test was negative, the presence of dual polarities and a study of angular standard deviation of virtual geomagnetic poles imply that the overall mean direction averages out, to a certain extent, the past geomagnetic secular variation. Magnetic polarity stratigraphy determines the age of the Nijo Group; the younger age limit is 14.6 Ma (top of polarity subchron C5ADr) and the older age limit is 15.2 Ma (base of polarity subchron C5Bn.2n). Comparison of the paleomagnetic pole with those from the Asian continent shows no rotation in the Mt. Nijo area since 14.6 Ma, suggesting that SW Japan has not experienced rotation since that time. We conclude that the clockwise rotation of SW Japan, thought to have occurred in Miocene time in relation to the opening of the Japan Sea, had already ended by 14.6 Ma. Two clockwise-deflected directions of the Middle Miocene rocks (Muro Pyroclastic Flow Deposit and Kumano Acidic Rocks) in the Kii Peninsula are probably instantaneous recordings of a geomagnetic field excursion or transition.