Paleomagnetism and fission-track ages of the Mt. Wasso moonstone rhyolitic welded tuff in the Ishikawa Prefecture, central Japan

摘要

The Mt. Wasso Moonstone Rhyolitic Welded Tuff (hereafter referred to as Mt. Wasso Rhyolite) is assigned to the lowermost part of the Miocene Hokuriku Group in the eastern part of southwest Japan. In an effort to clarify the rotational movements of southwest Japan associated with opening of the Japan Sea back-arc basin, we conducted paleomagnetic measurements and fission-track (FT) dating of the Mt. Wasso Rhyolite. Stable site-mean paleomagnetic directions and zircon FT ages were determined for five and three sites, respectively. The FT ages range between 20 and 22 Ma, and accord with a previous age determined by Rb-Sr whole rock isochron method. The numerical ages around 20 Ma correspond to timing of a hydrothermal alteration event, which may have caused remagnetization of the Mt. Wasso Rhyolite. Tilt-corrected paleomagnetic directions are clustered with a large easterly deflection (ca. 50 deg), indicating a clockwise rotation of the study area since 20 Ma. Compared with paleomagnetic directions of the Iozen Formation (15 approx 16 Ma) in adjoining hills, the present result implies that the study area did not significantly rotate during the early Miocene.