Paleomagnetic Study on the Dike Swarm in the Oga Peninsula, Northeast Honshu Island

抄録

A paleomagnetic study has been made on 14 dikes emplaced at two formations in the Oga Peninsula. Stable remanences of seven dikes are normally magnetized, six are reversely magnetized, and one has an intermediate direction. No significant difference in the directions is observed between the normally and the reversely magnetized rocks and between the intruded formations. The veins of dikes trend northeast with the standard deviation of 4°. Mean inclination and declination of remanence for 13 dikes excluding an intermediate one are calculated to be 58.3° and -2.8°, respectively. The pole position corresponding to the mean field direction is close to that observed at the Middle Miocene formations in the peninsula and in the northern part of NE Honshu Island. Dike rocks indicate the bimodal volcanism characteristic in Middle Miocene time. The direction of stable remanences and the bimodal distribution suggest that the dikes intruded in Middle Miocene time. The dominant orientation of strikes in the Oga dikes represents one of the general trend of the Middle to Late Miocene stress field after the tectonic rotation of NE Honshu Island had completed.