2017 Volume 123 Issue 11 Pages 953-967
Rock magnetic and paleomagnetic data were obtained from early Miocene igneous rocks in central Honshu, Japan. At Takane in the Hida region, gabbro intrusions and older sedimentary rocks are intruded by numerous andesitic dikes that comprise a parallel dike swarm. The dikes were emplaced under two different sets of extensional paleostress conditions, which were interpreted from a method of clustering dike orientations. Cross-cutting relationships indicate that both paleostress conditions existed during the same period. More than 240 oriented cores for magnetic analysis were taken from 38 sites at two localities. The andesites and gabbros typically contain magnetite, but some andesites also contain pyrrhotite. The magnetite records easterly deflected remanent magnetization directions of dual polarities that pass the reversals test. Positive baked contact tests at two sites demonstrate that the easterly deflected direction is a thermoremanent magnetization acquired at the time of intrusion. The overall in situ (i.e., in geographic coordinates) mean direction for the andesitic dikes is judged to be highly reliable, although there are two possible scenarios for explaining the easterly deflection: (1) clockwise rotation and (2) tilting to the northwest. We prefer the former scenario and conclude that ~45° clockwise rotation occurred in Takane with respect to the North China Block of the Asian continent. This rotation must represent clockwise rotation of the whole of Southwest Japan during the opening of the Japan Sea. Very little difference is observed between the amount of easterly deflection in Takane and that in the Tokai and Hokuriku regions, indicating no significant relative rotation. Therefore, the crust beneath Takane has not undergone rotation caused by collision of the Izu-Bonin arc with Honshu. Statistical analyses of paleomagnetic directional data suggest that the two paleostress conditions during the intrusion of andesite dikes lasted for long enough to sample geomagnetic secular variation.
