Paleomagnetism of Miocene dikes in the Shitara basin and the tectonic evolution of central Honshu, Japan

Abstract

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• , α 95 = 5.2• , and a paleomagnetic pole at 82.3• N, 216.6Although 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.

Instantaneous paleomagnetic record from the Miocene Kozagawa Dike of the Kumano Acidic Rocks, Kii Peninsula, Southwest Japan: cautionary note on tectonic interpretation

“…Therefore, it is likely that clockwise rotation of Southwest Japan took place in the late Early to early Middle Miocene and was completed by 15 Ma, as suggested by Hoshi et al . () and Hoshi and Yokoyama (). Hoshi () inferred that the southwest and up, and northeast and down directions were both acquired in an extraordinary paleomagnetic field during a polarity transition or excursion.…”

“…On the Pacific side of Southwest Japan, late Early Miocene sediments ( ca 17 Ma) show magnetization directions consistent with clockwise rotation, whereas volcanic rocks dated at ca 15 Ma display north (normal polarity) or south (reversed polarity) directions (Torii ; Hayashida et al . ; Hoshi & Yokoyama ). On the northern Kii Peninsula (loc.…”

“…In Southwest Japan, remanent magnetization (or remanence) directions of Miocene age have been documented from a number of areas in order to constrain the tectonics of Southwest Japan, particularly those associated with the back‐arc opening of the Japan Sea (e.g., Otofuji ; Ishikawa ; Hoshi et al . ; Iwaki & Itoh ; Hoshi & Yokoyama ; Kojima et al . ; Shimada et al .…”

Reconstruction of northeast Asian deformation integrated with western Pacific plate subduction since 200 Ma

“…Paleomagnetic evidence has demonstrated opposite rotations for southwestern and northeastern Japan (e.g., Kawai et al, 1961;Otofuji, 1996;Hoshi and Yokoyama, 2001). SW Honshu and northern Kyushu consistently rotated clockwise.…”

Timing of clockwise rotation of Southwest Japan: constraints from new middle Miocene paleomagnetic results

“…The data from Izumo also suggest that clockwise rotation was complete by 15 Ma. Furthermore, studies conducted in other areas of Southwest Japan have provided timeaveraged mean directions suggesting little or no rotation since 15 Ma (Nakajima et al 1990;Itoh et al 2000;Hoshi et al 2000b;Hoshi and Yokoyama 2001;Tamaki et al 2006;Hoshi and Sano 2013).…”