New age constraints on counter-clockwise rotation of NE Japan
New age constraints on counter-clockwise rotation of NE Japan
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DOI：
10.1111/j.1365-246x.2007.03513.x
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发表时间：
2007-12-01
影响因子：
2.8
通讯作者：
Otofuji, Yo-Ichiro
中科院分区：
文献类型：
--
作者：
Baba, Ayumi K.;Matsuda, Takaaki;Otofuji, Yo-Ichiro
中新世玄武安山质熔岩和白垩纪花岗岩采样的Abukuma地体东北本州的地质年代学和古地磁调查，以确定日本东北部逆时针旋转的精确时间。从高田组和良善组的26个地点和白垩纪花岗岩的12个地点中分离出了解封温度为560-590 ℃的特征组分。在阿武熊地体的白垩纪花岗岩中观察到一致的西北古地磁方向（D = 265度-331度）。白垩纪的平均古地磁方向（D = 313.5 °，I = 54.6 °，α（95）= 0.3 °，N = 20）与日本东北部日本海一侧的渐新世-中新世（33-20 Ma）古地磁方向以及北上地体的白垩纪古地磁方向几乎一致。这种类型的行为清楚地表明，阿武熊地体经历了一致的逆时针旋转的一部分，东北日本后20马。对斜长石颗粒进行的常规钾（K）-氩（Ar）定年方法显示，高田组（11个地点）和良前组（8个地点）的独特年龄分别为14.44 +/- 0.88和16.52 +/- 0.82 Ma。这种类型的行为表明，两个独立的火山活动可能是负责生产火成岩在高馆和良善地区。高馆组的北向下沉（D = 355.3度，I = 39.4度，alpha（95）= 11.6度），而良善组的西向下沉（D = 333.5度，I = 58.0度，alpha（95）= 9.6度）。因此，这里提出的结果表明，东北日本省仍在经历逆时针旋转16.5 Ma，但14.4 Ma的旋转运动已经完全停止。与日本西南部现有的古地磁和地质年代学数据相比，日本东北部和东南部两省的旋转运动已经停止了14 Ma。因此，任何关于日本东北部旋转运动完全停止的额外证据都将为理解日本海开放的时间提供一把钥匙。
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, alpha(95) = .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 +/- 0.88 and 16.52 +/- 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, alpha(95) = 11.6 degrees), while slightly westerly deflected declination appears in the Ryozen Formation (D = 333.5 degrees, I = 58.0 degrees and alpha(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.