2006 Volume 112 Issue 2 Pages 153-165
The sediments and volcanic rocks of Shitara in Aichi Prefecture have a wealth of information regarding Miocene tectonic events of southwestern Japan such as clockwise rotational motion and vertical movement. In order to place geochronological constraints on such events, we have determined zircon fission track (FT) ages for 32 rock samples collected from lower clastic sediments of the Hokusetsu Subgroup and upper volcanic rocks of the Shitara igneous complex. FT ages for felsic tuff beds intercalated in background marine clastics show late Early Miocene (~17.5 Ma) deposition of the upper part of the Hokusetsu Subgroup, which is in accordance with the biostratigraphic age assignment. On the other hand, FT ages for a thick pile of felsic lavas and pyroclastic rocks of the Shitara igneous complex show clustering around 15 Ma, indicating geologically instantaneous emplacement at that time. Combining the present results with published lithological, paleomagnetic, and structural data, we propose a model in which (1) the Hokusetsu Subgroup was deposited on the subsiding, southward-migrating southwestern Japan microplate around 17.5 Ma, just before the main phase of the clockwise rotation; (2) the clockwise rotation had ended by 15 Ma; (3) collision of the southwestern Japan microplate with the Shikoku basin and the resultant crustal uplifting occurred immediately prior to 15 Ma.
