Abstract
Southwest Japan rotated clockwise during the late stage of the opening of the Japan Sea, although the exact timing of the rotation is controversial. A recent biostratigraphic study has revealed that the Miocene Oidawara Formation in eastern Southwest Japan was deposited just before 15 Ma; consequently, its paleomagnetic direction may help constrain the timing of rotation. For this purpose, we collected fine felsic tuffs and siltstones at 71 stratigraphic sites (horizons) in the Oidawara Formation. An analysis of alternating field and thermal demagnetization results yielded characteristic remanent magnetization (ChRM) directions for 177 samples. Approximately 80 % (142) of the samples exhibit reverse polarity ChRM directions that are thought to be paleofield directions of reverse polarity Chron C5Br. Normal polarity ChRM directions in 35 samples include primary paleofield records as well as records of secondary magnetization. The data suggest that a short normal polarity interval (microchron or cryptochron) at ~15.8 Ma is present within the dominantly reverse polarity interval of Chron C5Br. Reliable site-mean directions for 19 sites yield a tilt-corrected formation-mean direction of D = 10.5°, I = 41.1°, α 95 = 7.0°, and k = 23.9, indicating virtually no rotation with respect to a reference paleomagnetic direction for the Asian continent. A rotation versus age plot for Southwest Japan indicates that the clockwise rotation started after 17.5 Ma and ceased largely before 15.8 Ma, yielding a rotation rate of ~23°/Myr.
Highlights
The eastern Asian and western Pacific margins, including the Japanese islands, are the areas where relative plate motions caused several remarkable tectonic events during the Cenozoic, such as back-arc opening (e.g., Martin 2011; Sasaki et al 2014), arc-arc collision (e.g., Arai and Iwasaki 2014), and severe intra-arc crustal deformation (e.g., Cho and Kuwahara 2013; Itoh et al 2014)
Stepwise thermal demagnetization (ThD) of three-axis isotheral remanent magnetization (IRM) (Fig. 6a) indicates that magnetite is the main carrier of remanent magnetization in the tuff
It should be noted that during stepwise ThD of natural remanent magnetization (NRM), characteristic remanent magnetization (ChRM) directions were determined over a wide temperature range (100–500 °C) and that no significant directional changes were observed at around 300 °C
Summary
The eastern Asian and western Pacific margins, including the Japanese islands, are the areas where relative plate motions caused several remarkable tectonic events during the Cenozoic, such as back-arc opening (e.g., Martin 2011; Sasaki et al 2014), arc-arc collision (e.g., Arai and Iwasaki 2014), and severe intra-arc crustal deformation (e.g., Cho and Kuwahara 2013; Itoh et al 2014). The clockwise rotation of Southwest Japan (Otofuji and Matsuda 1983; Torii 1983) is a conspicuous example of such tectonic events. This model, hereafter called the 15-Ma rapid rotation model, is supported by several studies based on data from volcanic rocks (Otofuji et al 1991; Shimada et al 2001). This model has been questioned, given the results of a number of paleomagnetic and geochronological studies (e.g., Nakajima et al 1990; Jolivet et al 1995) that cite an absence or lack of data for the rotation of rocks dating from ca. Determination of the paleomagnetic direction of accurately dated rock units is critical for testing the 15-Ma rapid rotation model
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