Paleomagnetic constraints on Miocene rotation in the central Japan Arc



We present new Middle Miocene paleomagnetic data for the central Japan Arc, and discuss their implications for Miocene rotation. To obtain a refined paleodirection, we made magnetic measurements on basaltic to andesitic lavas and intrusive rocks from 12 sites in the Tsugu volcanic rocks (ca 15 Ma) in the northern part of the Shitara area, Japan. Significant secondary magnetizations in samples with strong magnetic intensities are interpreted as lightning-induced components. Mean directions carried by magnetite and/or titanomagnetite were determined for all sites. An overall mean direction with a northerly declination was obtained from dual-polarity site means for nine sites. This direction is indistinguishable from the mean direction for coeval parallel dikes in the northern part of the Shitara area, and also indistinguishable from the Miocene reference direction derived from the paleopole for the North China Block in the Asian continent. These comparisons suggest little or no rotation or latitudinal motion in the study area with respect to the North China Block since 15 Ma. We obtained a refined early Middle Miocene paleodirection (D = 9.7°, I = 52.5°, α95 = 4.8°; 30 sites) and paleopole (82.0°N, 230.8°E, A95 = 5.6°) for Shitara by combining data from the Tsugu volcanic rocks and a coeval dike swarm. An anomalous direction found at three sites could be a record of an extraordinary field during a geomagnetic polarity transition or excursion. Paleomagnetic data from Shitara suggest that: (i) the western wing of the Kanto Syntaxis, a prominent cuspate geologic structure in central Honshu, underwent a counterclockwise rotation with respect to the main part of the southwestern Japan Arc between ca 17.5 Ma and 15 Ma; (ii) collision between the Japan and Izu–Bonin (Ogasawara) Arcs began prior to 15 Ma; and (iii) clockwise rotation of the entire southwestern part of the Japan Arc had ceased by 15 Ma.