EARLY MIOCENE PALEOMAGNETIC RESULTS FROM THE NINOHE AREA, NE JAPAN - IMPLICATIONS FOR ARC ROTATION AND INTRAARC 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 weld ed tuffs ranging from 24 to 21 Ma possess westerly paleomagnetic direc tions with normal polarity (formation-mean: D/I -294.5 degrees/44.2 de grees with alpha(95) = 8.3 degrees, 8 sites), while the andesite flows formed at 17 Ma exhibit southerly directions with reversed polarity ( D/I = 186.6 degrees/-61.9 degrees, 2 sites). A positive conglomerate t est assures the stability of high-temperature components of remanent m agnetization. Our results demonstrate that northern NE Japan rotated c ounter-clockwise through more than 60 degrees between 21 and 17 Ma, mo st likely in association with the opening of the Japan Sea. The rotati on 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 fur ther suggest that intra-arc block rotations occurred in the back-are r egion of NE Japan during and after the are rotation.