11:15 AM - 11:30 AM
[SEM16-09] Paleomagnetism of Miocene volcanic rocks from the Shimokita Peninsula and tectonic rotation of Northeast Japan
Keywords:Japan Sea opening, Miocene, Northeast Japan, Paleomagnetism, Tectonic rotation, Tectonics
This study presents new paleomagnetic data from the Miocene Tomari volcanic rocks on the Shimokita Peninsula and discusses the timing of the counterclockwise (CCW) rotation of Northeast Japan (NE Japan) associated with the back-arc opening of the Japan Sea. During this back-arc opening, NE Japan migrated with a CCW rotation from the eastern margin of the Asian continent to its present location. However, understanding of this rotation has not significantly progressed in the past 40 years. Some researchers suggest that the rotation ceased around 17 to 18 Ma, while others propose it continued until around 15 Ma. The Tomari volcanic rocks are dated to approximately 16 Ma, based on previous biostratigraphic and radiometric data. Therefore, paleomagnetic data from these rocks may help constrain the timing of the CCW rotation of NE Japan.
For magnetic measurements, basaltic and andesitic rocks were collected from 30 sites of lavas and dikes. The characteristic remanent magnetizations (ChRMs) were determined for all sites. Twenty-six sites have ChRM directions classified as normal or reverse polarity, while four sites have intermediate polarity directions. The normal and reverse directions are nearly antiparallel, and the directional concentration improves with tilt correction for the lavas. Thus, the ChRMs are interpreted as primary thermoremanent magnetizations. Demagnetization data indicate that magnetite is the main magnetic mineral carrying the ChRMs. Comparing the overall mean direction for the Tomari volcanic rocks with the reference Miocene direction of the Asian continent suggests that the study area experienced a small (approximately 15–20°) CCW rotation relative to the continent after the formation of the volcanic rocks. Assuming this observed rotation reflects the CCW rotation of NE Japan, it suggests that the CCW rotation of NE Japan was ongoing at the time of the formation of the Tomari volcanic rocks and may have been nearing its end.
For magnetic measurements, basaltic and andesitic rocks were collected from 30 sites of lavas and dikes. The characteristic remanent magnetizations (ChRMs) were determined for all sites. Twenty-six sites have ChRM directions classified as normal or reverse polarity, while four sites have intermediate polarity directions. The normal and reverse directions are nearly antiparallel, and the directional concentration improves with tilt correction for the lavas. Thus, the ChRMs are interpreted as primary thermoremanent magnetizations. Demagnetization data indicate that magnetite is the main magnetic mineral carrying the ChRMs. Comparing the overall mean direction for the Tomari volcanic rocks with the reference Miocene direction of the Asian continent suggests that the study area experienced a small (approximately 15–20°) CCW rotation relative to the continent after the formation of the volcanic rocks. Assuming this observed rotation reflects the CCW rotation of NE Japan, it suggests that the CCW rotation of NE Japan was ongoing at the time of the formation of the Tomari volcanic rocks and may have been nearing its end.