2014 Volume 120 Issue 8 Pages 255-271
We report on a new paleomagnetic direction from early Miocene (18-17 Ma) sedimentary rocks of the Tomikusa Group in southern Nagano Prefecture. Rock samples collected from 24 sites were demagnetized stepwise by means of alternating-field and thermal methods. Site-mean directions of normal, reverse, and intermediate polarities were determined for 23 sites. Rock magnetic experiments suggest that the remanent magnetization is carried by magnetite, as well as by iron sulfide (pyrrhotite or greigite) or maghemite. The directional data pass a reversals test and a baked contact test, indicating they are reliable and can therefore be used to investigate magnetic polarity stratigraphy and tectonic rotation. Magnetostratigraphic correlation shows that the Tomikusa Group is younger than the base of Chron C5Er (18.7 Ma) and older than the top of Chron C5Dn (17.2 Ma). The north-seeking overall mean direction is indistinguishable from the geocentric axial dipole field direction, as well as from the direction calculated from the paleomagnetic pole for the North China Block in the Asian continent. The paleomagnetic orocline test was applied to a data set consisting of the Tomikusa and other early Miocene directions reported from near the curved Median Tectonic Line (MTL) in central Honshu. The result demonstrates that the surface trace of the MTL was straight in the late early Miocene (ca. 17 Ma).
