Paleomagnetism of a wide spread tephra as a precise index for the block rotation.

Rotation of tectonic blocks was mostly detected by paleomagnetism. The precision of the rotation angle, however, has been limited by paleosecular variation (PSV). The dispersion of the declination by PSV is around 25^o, which should be averaged out taking multiple sites. It is more difficult determining the boundary of the blocks, since the direction of the single site include the influence of the tectonics and the PSV. Here we present a paleomagnetism of a wide spread tephra for the tectonic study. The tephra means the precisely same time everywhere, thus the same geomagnetic direction. Thus, if we could find the same tephra in the different tectonic blocks, the difference of the paleomagnetic declination would readily be thought as the consequence of the block rotation. Its another advantage is that it also usually have stable magnetization because of its volcanic origin. Izaku-Hisamine tephra of 3.3Ma in age (Torii and Oda, 2001) widely covers southern Kyushu area, southwest Japan. The area was shown to consist of a few rotated blocks by Kodama et al. (1995), and their paleomagnetic studies showed that the western part of the area was rotated counterclockwise for about 30 degrees at about 2Ma. The error of the rotation angle was large ( ~ 20^o) because partly of insufficient number of sites and partly of the magnetic instability of the samples. We made a paleomagnetic study of the tephra from the sites on those blocks to determine the precise rotation angles, and the extent of each blocks. The samples were taken from four sites on the blocks of the eastern part of the area, which were claimed to be rotated in Kodama et al. (1995). Three were from welded pyroclastics and the other was from co-ignimbrite ash fall deposits (i.e. they carried TRM and DRM, respectively). And as the reference, 9 sites of welded pyroclastics were taken from three regions in the western part of the area, which were on the block that Kodama et al. (1995) implied no roation. Taking the region at the center of the southern Kyushu as the reference direction, all of the four eastern sites showed counterclockwise rotation of about 20^o (+/- 10^o). However, the western area, which supposed to have coherent declination, grouped into two. One out of 3 regions of the area showed counterclockwise rotation of about 20^o. The distribution of the regions of counterclockwise rotation was, as a whole, rather complicated. The tectonics of the area was not as simple as that proposed by Kodama et al. (1995).