[S-EM15] Geomagnetism, paleomagnetism and rock magnetism

[SEM15-10] Paleomagnetism of dikes intruding bedded cherts in the Inuyama Section of the Mino Belt, central Japan

*Taizo Uchida^1,2, Hiroyuki Hoshi¹ (1.Aichi University of Education, 2.Graduate School of Integrated Arts and Sciences, Kochi University)

Keywords:Paleomagnetism, Rock magnetism, Dike, Inuyama Section, Mino Belt

Triassic bedded cherts in the Inuyama section in the Mino Belt have multicomponent remanent magnetizations; a (near-)primary component with low inclinations and several secondary components acquired during and after the formation of a synclinal structure called the Sakahogi Synform. The acquisition of the secondary components has been discussed based on paleomagnetic fold tests and rock magnetic experiments. Still, little is discussed on the possible link between the acquisition of secondary magnetizations and Cretaceous and later igneous activity occurring in and around this area. We examined the paleomagnetism of igneous dikes intruding the bedded cherts in the Inuyama section to investigate this possible link. The dikes are of the basaltic composition and have K-Ar whole-rock ages of about 90 Ma. Detailed stepwise demagnetization experiments show that the dikes possess a characteristic remanent magnetization (ChRM) with a normal-polarity northerly direction. Rock magnetic experiments show that magnetite is the primary carrier of magnetization. At several sites on both limbs of the synform, cherts very close to the dikes have a relatively strong magnetization with a single northerly component similar to the ChRM in the dikes; in contrast, cherts far apart from the dikes have a weaker, multicomponent magnetization. This shows a positive baked contact test and suggests that the northerly direction of the dikes is a primary thermoremanent magnetization. We conclude that the intrusion of dikes has affected only the magnetization in cherts close to the dikes. The northerly direction postdates the formation of the synform because it was observed in both limbs. Because the direction is similar to those for middle Miocene rocks in Southwest Japan, it is likely that, contrary to the previous K-Ar data, the age of formation of the basaltic dikes is middle Miocene or later.