Archaeomagnetic Study for the Past 2,000 Years in Southwest Japan

To clarify the secular variation of the geomagnetic field in the Kinki District, about 2, 600 samples were collected from some 150 kilns and hearths excavated in the Kinki District and its vicinity. The age of these samples covers a time range from 100 A.D. to the present. As a result of the magnetic measurements of these samples, the secular variation of the geomagnetic field in the Kinki District was brought to light in detail in this time range. In the same time range, the positions of the virtual geomagnetic north pole were calculated by using the values of the declination and inclination of the geomagnetic field thus obtained. The aspect of the secular variation of the virtual pole position seems to show that the movement of the virtual pole consists of two kinds of motion, that is, one is a basic counter-clockwise rotation of the period of about 1, 500 years and the other is three clockwise rotations of the period of about 500 years

セイナンニホンニオケルカコ2000ネンカンノコウコチジキガクテキケンキュウ

京都大学0048新制・論文博士理学博士乙第1824号論理博第357号新制||理||145(附属図書館)3171UT51-46-E435(主査)教授 笹嶋 貞雄, 教授 中沢 圭二, 教授 亀井 節夫学位規則第5条第2項該当Kyoto UniversityDA

^<40>Ar-^<39>Ar AGES AND PALEOMAGNETIC DATA FOR METAMORPHIC AND IGNEOUS ROCKS FROM MAHANADI AND GODAVARI GRABENS, EAST INDIA

Age dating and Paleomagnetic studies were performed on metamorphic and igneous rocks collected from Mahanadi and Godavari Grabens, Eastern Ghats in India, in order to estimate the paleo-position of the Indian plate that was a part of Gondwana up to early Cretaceous. ^Ar-^Ar datings were performed for three biotites separated from gneisses and a granite of the Mahanadi Graben, seven biotite and whole rock samples from a granite, a khondalite, a charnockite and dolerites of the Godavari Graben. From five samples, ^Ar-^Ar plateau ages of about 500-580Ma were obtained. These ages are consistent with K-Ar and ^Ar-^Ar ages reported for metamorphic and granitic rocks from East Antarctica (e.g. Y. TAKIGAMI et al.; Recent Progress in Antarctic Earth Science, ed. by Y. YOSHIDA et al., Tokyo, Terra Sci. Publ., 61,1992). One biotite sample separated from a gneiss in the Mahanadi Graben shows a plateau age of 677.0±6.4Ma which had not been reported in the East Antarctica by the K-Ar and ^Ar-^Ar methods. This result may represent that the Pan-African Orogeny had not affected the Mahanadi Graben. However, further investigations should be done to clarify it. Age spectra of dolerite dike samples show features of excess Ar, which might be derived from old basement rocks. In the paleomagnetic studies, AF and Thermal demagnetizations of natural remnant magnetization (NRM), thermomagnetic analyses, magnetic hysteresis analyses were carried out for these samples. A synthetic result indicates that the samples from 2 sites have meaningful NRM\u27s for paleomagnetism. Their NRM directions consist with each other taking α_ values into consideration, and may simultaneously magnetize at 544Ma which is obtained by ^Ar-^Ar dating. The virtual geomagnetic pole position (latitude 36.4°S, longitude 166.6°E) was derived from the mean NRM direction (declination -53.1°, inclination 11.4°). The paleo-position of Indian plate at 544Ma supports previous results reported from East India

PALEOMAGNETIC STUDY OF RAJMAHAL TRAP IN INDIA : DISCUSSION OF GEOMAGNETIC DIPOLE MOMENT AND RECONSTRUCTION OF GONDWANALAND

A paleomagnetic study was conducted on the Rajmahal trap in northeastern India. The age of the trap was determined by ^Ar/^Ar dating as 125 to 130 Ma (H. SAKAI et al., J. Geol. Soc. Jpn., 103,192,1997). The VGP (vitual geomagnetic pole) of the Rajmahal trap was studied and compared with the APWP (apparent polar wander path) of India (J. E. BESSE and V. COURTILLOT, J. Geophys, Res., 96,4029,1991). The VGP is situated slightly apart from the position of 120 Ma in Indian APWP, which suggests that the Rajmahal trap is older than 120 Ma. That is, paleomagnetic study supports the ^Ar/^Ar age of 125-130 Ma. This result suggests that the Rajmahal trap was formed around the age of rifting of India from Antarctica. Thelliers\u27 paleointensity experiment was applied to the Rajmahal basalt and the paleointensity of 31.4±5.8 μT was determined. The geomagnetic dipole moment in Mesozoic was summarized using formerly reported paleointensities and data in this study. We can identify that the dipole moment from 180 Ma to 125 Ma before the period of Cretaceous quiet zone, oscillates between 30% and 70% of the present moment