Magnetostratigraphy of Mesozoic and Paleozoic Systems in Japan

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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.