Abstract
The Miocene basaltic-andesitic lavas and Cretaceous granites were sampled in the Abukuma Terrane of northeast Honshu for geochronological and palaeomagnetic investigations to determine precise timing of counter-clockwise rotation of NE Japan. Characteristic component with unblocking temperature of 560-590 degrees C is isolated from 26 sites of the Takadate and Ryozen Formations and 12 sites of the Cretaceous granites. Concordant northwesterly palaeomagnetic directions (D = 265 degrees -331 degrees ) are observed in the Cretaceous granites of the Abukuma Terrane. The Cretaceous mean-palaeomagnetic direction from this terrane (D = 313.5 degrees , I = 54.6 degrees , varies as 95 = 6.3 degrees and N = 20) is almost identical to Oligo-Miocene (33-20 Ma) palaeomagnetic direction reported from the Japan Sea side of NE Japan as well as to the Cretaceous palaeomagnetic direction reported from the Kitakami Terrane. This type of behaviour clearly suggests that the Abukuma Terrane underwent a coherent counter-clockwise rotation as a part of NE Japan after 20 Ma. Conventional potassium (K)-Argon (Ar) dating method performed on the plagioclase grains revealed distinctive ages of 14.44 + or - 0.88 and 16.52 + or - 0.82 Ma for the Takadate Formation (11 sites) and the Ryozen Formation (eight sites), respectively. This type of behaviour indicates that two independent episodes of volcanic activities were probably responsible for the production of igneous rocks in the Takadate and Ryozen areas. Northerly declination is obtained from the Takadate Formation (D = 355.3 degrees , I = 39.4 degrees , varies as 95 = 11.6 degrees ), while slightly westerly deflected declination appears in the Ryozen Formation (D = 333.5 degrees , I = 58.0 degrees and varies as 95 = 9.6 degrees ). The here presented results, therefore, suggest that the province of NE Japan was still experiencing a counter-clockwise rotation at 16.5 Ma, however by 14.4 Ma the rotational motion has completely ceased. Compared with the available palaeomagnetic and geochronological data from SW Japan, rotational motion in both provinces of NE and SE Japan has already been ceased by 14 Ma. Thus any additional evidence regarding the complete cessation of rotational motions in NE Japan will provide a key to understand the timing of opening in the Japan Sea.