Refined Paleomagnetic Direction of the Miocene Motegi Formation, Tochigi Prefecture, Central Honshu

To refine the early Middle Miocene (ca. 16 Ma) paleomagnetic direction in the Motegi area, we collected paleomagnetic samples from dacite domes and intrusives of the Motegi Formation. Characteristic remanent magnetization components were revealed by stepwise demagnetization, which were used to calculate site-mean directions for 10 sites. Together with published data from 11 andesite and basalt sites, we obtained a refined formation-mean direction of D = 188.0°, I = −59.5°, a 95 = 5.8° (21 sites), and a corresponding paleomagnetic pole at 82.0°N, 186.3°E, A 95 = 7.3°. All the 21 sites possess reversed polarity, suggesting geologically rapid formation of all lithologies of the Motegi Formation during a single polarity chron (C5Br: 15.16–15.97 Ma). We conclude that detectable latitudinal displacement and vertical-axis rotation with respect to the eastern part of the Asian continent have not taken place in the Motegi area since 16 Ma. This provides support for the idea (Hoshi and Takahashi, 1997) that no significant rotation has occurred in this area since 18 Ma.