Paleomagnetism of Miocene dikes in the Shitara basin and the tectonic evolution of central Honshu, Japan
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences. 2001
Received: 2 June 2000
Accepted: 14 March 2001
Published: 21 June 2014
Central Honshu, Japan, is conspicuous for a northward-convex form of zonal geologic structures. To clarify the formation of curvature of the zonal structures in the west of the convex form and further to discuss the tectonic evolution of central Honshu, we carried out a paleomagnetic study of dated (∼15 Ma) dike rocks of the Shitara basin. Samples were collected from 25 basalt dikes trending north-south with a nearly vertical intrusion surface. After stepwise alternating-field and thermal demagnetization, 24 site-mean directions were determined. Three dikes gave anomalous directions, probably resulting from instantaneous recording of a field transition or excursion. The other 21 site-means produced a mean direction, D = 9.7°, I = 54.5°, α95 = 5.2°, and a paleomagnetic pole at 82.3°N, 216.6°E, A95 = 6.2°. Although the reversal test is negative at the 5% significance level, the mean direction and pole are time-averaged ones in which the secular variation is averaged out. This was confirmed by studying the angular standard deviation of virtual geomagnetic poles. We conclude that the central Honshu curvature formed when the southwest Japan arc rotated clockwise between 17 and 15 Ma in relation to the opening of the Japan Sea, associated with differential rotation of the eastern part of the arc with respect to the central part. The differential rotation probably resulted from a sinistral shear on the eastern margin of the rotating arc. The formation of the curvature seems not to have borne on the collision of the Izu-Ogasawara arc with Honshu, although the collision probably caused large-scale clockwise rotation of the Kanto Mountains in the east of the northward-convex structure.