Magnetostratigraphy of Mesozoic and Paleozoic Systems in Japan

1. 1 .古磁测量设备：1987年安装了全自动超导岩石磁强计（2G760R），实现了对沉积岩极弱残余磁化的测量。北海道第三系：对北海道中部第三系沉积的古地磁研究表明，在晚白垩世至早第三纪期间，北海道中部经历了沿垂直轴逆时针（CCW）旋转90°的构造旋转，之后在中新世中期经历了顺时针（CW）旋转60°的构造旋转。九州南部第三系：对九州南部种子岛和日南地区中新世矿床的古地磁研究表明，九州南部在过去5 Ma期间发生了约30^O的CCW旋转。这表明南九州是一个独立的构造单元，不属于日本西南地块。北海道白垩系：在北海道中部白垩系埃佐群最上层地层中，发现了一个与坎帕尼亚早33世相关的极性反转带。所有下序均显示正极性，属于白垩纪正极性超时。赤纬向西偏转，表明该地区有一个中央涡旋。日本西南部白垩纪体系：对四国岛和浅路岛上白垩统泉群的磁地层学研究确定了松山市西部、峨山东部和浅路岛南部三个极性反转带。它们分别与晚白垩纪33r、32r和32.1r亚时相关。据估计，泉群的沉积速率非常高，高达0.7至1厘米/年。

1. Equipment for paleomagnetic measurement : An automated superconducting rock-magnetometer (2G760R) was installed in 1987, which made it possible to measure extremely weak remnant magnetization of sedimentary rocks.2. Tertiary System in Hokkaido : A paleomagnetic study of the Tertiary deposits in central Hokkaido demonstrated that central Hokkaido had experienced a counter-clockwise (CCW) tectonic rotation by 90^O about the vertical axis during the period between late Cretaceous and early Tertiary and, afterwards, suffered a clockwise (CW) rotation by 60^O during the middle Miocene.3. Tertiary System in south Kyushu : Paleomagnetic studies of the Miocene deposits in Tanegashima Island and Nichinan district in south Kyushu indicated that a CCW rotation by about 30^O had taken place in south Kyushu during the last 5 Ma. This suggests that south Kyushu is an independent tectonic unit which does not belong to the Southwest Japan block.4. Cretaceous System in Hokkaido : A reversed polarity zone, correlatable to chron 33r in the early Campanian, was found from the uppermost successions of the Cretaceous Ezo Group in central Hokkaido. All the lower successions showed normal polarity which can be assigned to the Cretaceous normal polarity superchron. Declinations were deflected westward, suggesting a CCW rotation for the area.5. Cretaceous System in southwest Japan : Magnetostratigraphic studies of the Upper Cretaceous Izumi Group in Shikoku and Awaji Islands identified three reversed polarity zones from the west of Matsuyama City, eastern Asan Mountains, and southern Awaji Island. They are correlatable to chrons 33r, 32r, and subchron 32.1r in the latest Cretaceous, respectively. The estimated sedimentation rate of the Izumi Group is extremely high, as much as 0.7 to 1 cm/yr.