Recognition of 'cryptochron' in the polarity subchron C3Ar: Palaeomagnetic results of the Late Miocene lava sequence from Noma Peninsula (Kyushu Island), Japan

• Published in: Geophysical Journal International
• Main Authors: Otofuji, Yo-ichiro, Zaman, Haider, Shimoda, Makiko, Aihara, Kazuyoshi, Kani, Munemoto, Yokoyama, Masahiko, Ikeda, Satoru, Ahn, Hyeon-Seon, Wada, Yutaka
• Format: Text
• Language: English
• Published: Oxford University Press 2013
• Subjects: Geomagnetism; rock magnetism and palaeomagnetism; Kamchatka Peninsula; Antarc*; Antarctica; Kamchatka
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/193/1/122; https://doi.org/10.1093/gji/ggs111

This study presents a newly discovered geomagnetic excursion in the andesitic lava sequence of the Kamegaoka Mountain (31°21’N, 130°13’E), Noma Peninsula, Kyushu Island. About 170 oriented samples were collected from 13 consecutive lava flows, covering an area from mountain top to sea shore. Thermal and alternating field demagnetizations of the studied samples generally revealed a univectorial magnetization, however, two components structure with minor viscous overprints is also observed in some samples. Remanent magnetization is generally unblocked between 560 and 590 °C, indicating magnetite as dominant remanence carrier. Reversed polarity directions are detected in the bottom and uppermost parts of the sequence, whereas anomalous directions with positive inclination are observed in the middle part of the sequence. This newly discovered anomalous palaeomagnetic direction, named as Noma excursion (C3Ar-1), has a well-defined K-Ar age of 6.66 ± 0.45 Ma. Comparison with the geomagnetic polarity timescale allow us to place this event within the polarity subchron C3Ar, in which no such cryptochron has been observed before by high resolution ODP study of the sedimentary cores. The virtual geomagnetic poles estimated for the studied lava sequence moved from Antarctica to Kamchatka Peninsula (60°N), swung back to New Guinea equatorial region and then followed a path to Antarctica again. These poles followed a swath between the 90°E and 140°E longitudes, which are almost identical to one of the preferred longitudinal bands for transitional poles at the times of polarity reversals and excursions in the Brunhes and Matuyama chrons. According to this study, this preferred pathway may have started prior to 6.7 Ma. If properly emphasized, identification of Noma excursion in the studied lava flows can facilitate more such discoveries in the Late Miocene.