Early Miocene paleomagnetic results from the Ninohe area, NE Japan: Implications for arc rotation and intra-arc differential rotations

We have carried out a paleomagnetic study on well-dated Early Miocene volcanic rocks from the Ninohe area in northern NE Japan. Dacitic welded tuffs ranging from 24 to 21 Ma possess westerly paleomagnetic directions with normal polarity (formation-mean: D/I = 294.5°/44.2° with α95 = 8.3°, 8 sites), while the andesite flows formed at 17 Ma exhibit southerly directions with reversed polarity (D/I = 186.6°/−61.9°, 2 sites). A positive conglomerate test assures the stability of high-temperature components of remanent magnetization. Our results demonstrate that northern NE Japan rotated counter-clockwise through more than 60° between 21 and 17 Ma, most likely in association with the opening of the Japan Sea. The rotation of northern NE Japan thus preceded the about 15 Ma rapid clockwise rotation of SW Japan, and was synchronous with the possible pre-16 Ma southward translation of it. In comparison with published data, we further suggest that intra-arc block rotations occurred in the back-arc region of NE Japan during and after the arc rotation.