Paleomagnetism of Tertiary rocks from the OGA Peninsula and the rotation of northeast Japan

Abstract

Attslract. We have made a paleomagnetic study of Tertiary rocks from the Oga Peninsula, northers Honshu Island. Vokanic and sedimentary rocks were sampled from 26 sites in the peninsula. The ages of the rocks range from 62 Ma to the present. The resuRs indicate a couaterclockwise rotation of Northeast Japan with respect to eastern Asia around a pole at 58øN, 149øE, between about 22 Ma and 15 Ms. The amount of the rotation is about 20 ø. Before the rotation, Northeast Japan was situated along the east coast of the Asian continent. INTRODUCTION More than two decades ago, Kawai et aJ. [1961] demonstrated the bending of Honshu Island on the basis of the comparison of the paleomagnetic field directions from Northeast and Southwest Japan. Recent paleomagnetic studies in Japan have aimed at determining the mode and the timing of the bending and at relating the bending to the opening of the Sea of Japan. For Southwest Japan, Otofuji and Matsuds [1983] performed paleomagnetic investigations on the acidic rocks exposed on the Sea of Japan side of Southwest Japan and concluded that the clockwise rotation of that region took place from 28 to 12 Ms. The timing of the rotation was narrowed to around 15 Ma by Otofaji and Matsuds [1984]. For Northeast Japan, Tosha [1983] and Otofuji et aL [1985] recently made paleomagnetic studies. Otofuji et aL [1985] collected samples from a wide area of Northeast Japan, and their results indicate that the region north of the Tanakura Tectonic Line (see Figure la) behaved as one block, at least during the Tertiary period. However, the timing of the rotation of Northeast Japan is not clear from their data, since the relative ages of the rocks are usknown, partly because of the wide spread of the sampling sites. On the other hand, Tosha [1983] collected samples from the Oga Peninsula, where the sequence of Tertiary rocks is well preserved. Therefore his data are suitable for determining the chronologic progression of the movement of Northeast Japan. Recently, Celaya and McCabe [1987] and Morean et aL [1987] used Tosha's [1983] data and interpreted the tectonic evolution of Japan. These workers discussed the opening of the Sea of Japan by using the temporal declination changes in Japan, and later workers argued the remagnetization of pre-Tertiary rocks. Since the thesis by Tosha [1983] is not widely known and the above authors' iaterpretation of his data is quite different from his own interpretation, we here present the original data and results of recent studies, and discuss the tectonic implications. Some geological and geophysical studies suggest the tectonic continuity between the northeastern part of Honshu Island and the western part of Hokkaido Island in Cretaceous time [Takahashi, 1983; Takigaml; 1983; Segawa and Furuta, 19T8]. In the present study we assume the area north of the Tanakura Tectonic Line and the western part of Hokkaido to be one tectonic block since the Early Cretaceous, and we call the block Northeast Japan. GEOLOGICAL SETTING, AND SAMPLINGIn Northeast Japan, Tertiary an...

Neogene Kinematics in the Japan Sea Region and Volcanic Activity of the Northeast Japan Arc

“…Other data confirm the clockwise rotation of Southwest Japan, such as studies of the deformation associated with the bending of Kyushu (Murata, 1987;Fabbri et al, 1988). Paleomagnetic data in Northeast Japan also suggest counterclockwise rotations during the Japan Sea opening (Otofuji et al, 1985;Tosha and Hamano, 1988). We showed (Jolivet et al, 1991) that such a rotation is necessary to properly close the basin.…”

“…As discussed in several papers (Otofuji and Matsuda, 1983,1984,1987Otofuji et al, 1985Otofuji et al, , 1991Tosha and Hamano, 1988;Hayashida, 1986;Hayashida et al, 1991), it is classically admitted that Northeast Japan underwent a counterclockwise rotation and Southwest Japan a clockwise rotation during the Japan Sea opening (Fig. 6).…”

“…Otofuji et al (1985Otofuji et al ( , 1991 proposed that both Southwest and Northeast Japan rotated at very high rates about two poles located at either ends of the Japan arc at about 15 Ma. Tosha and Hamano (1988) proposed instead that Northeast Japan underwent a slower rotation between 22 and 15 Ma. Figure 6 shows the mean values of declination for various sites in Northeast and Southwest Japan after these authors.…”

“…The apparent rotation of Northeast Japan seems progressive, while that of Southwest Japan seems instantaneous at 15 Ma. If one considers, like Otofuji et al (1991), (2) Hayashida et al (1991), (3) Hayashida (1986), (4) Otofuji and Matsuda (1983), (5) Otofuji and Matsuda (1984), (6) Otofuji and Matsuda (1987), (7) Otofuji et al (1985), and (8) Tosha and Hamano (1988). Lower: rotation path for Southwest and Northeast Japan and the period of Japan Sea opening.…”

Paleomagnetism of miocene sedimentary rocks around Matsushima bay, Northeast Japan and its implication for the time of the rotation of Northeast Japan.

“…The declinations from the strata of 12 Ma or younger at Oga Peninsula, Northeast Japan (TOSHA and HAMANO, 1988) and the strata of 15 Ma or younger at Shakotan Peninsula, western Hokkaido (TANAKA et al, 1989) have the same tendency. There is a possibility that Northeast Japan was slightly rotated clockwise after its counterclockwise rotation with the opening of the Japan Sea.…”