Recognition of ‘cryptochron’ in the polarity subchron C3Ar: Palaeomagnetic results of the Late Miocene lava sequence from Noma Peninsula (Kyushu Island), Japan
Recognition of ‘cryptochron’ in the polarity subchron C3Ar: Palaeomagnetic results of the Late Miocene lava sequence from Noma Peninsula (Kyushu Island), Japan
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DOI：
10.1093/gji/ggs111
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发表时间：
2013-04
影响因子：
2.8
通讯作者：
Y. Otofuji;H. Zaman;M. Shimoda;Kazuyoshi Aihara;M. Kani;M. Yokoyama;S. Ikeda;H. Ahn;Y. Wada
中科院分区：
文献类型：
--
作者：
Y. Otofuji;H. Zaman;M. Shimoda;Kazuyoshi Aihara;M. Kani;M. Yokoyama;S. Ikeda;H. Ahn;Y. Wada
本文研究了九州岛野马半岛龟冈山(31°21‘N，130°13’E)安山期熔岩序列中新发现的地磁漂移。从13个连续的熔岩流中采集了约170个定向样品，覆盖了从山顶到海滨的区域。研究样品的热退磁和交变退磁一般表现为单向磁化，但在一些样品中也观察到有少量粘性重叠的双组分结构。剩余磁化强度在560°C到590°C之间基本不受阻碍，表明磁铁矿是主要的剩磁载体。在层序的底部和最上部检测到反极性方向，而在层序中部观察到正倾角的异常方向。这一新发现的古地磁异常方向命名为诺玛漂移(C3Ar-1)，其K-Ar年龄为6.66 Ma±0.45 Ma。通过与地磁极性时间尺度的比较，我们可以把这一事件放在C3Ar的极性亚时线内，在C3Ar中，通过对沉积岩心的高分辨率的ODP法研究，以前没有观察到这样的隐时器。对所研究的熔岩序列估计的虚拟地磁极从南极洲移动到堪察加半岛(60°N)，然后摆动回到新几内亚赤道地区，然后再次沿着路径到达南极洲。这些极点沿着90°E和140°E经度之间的带状分布，这几乎与布鲁内斯和松山金龙发生极性反转和远足时过渡极点的首选纵向频带之一相同。根据这项研究，这种首选路径可能在6.7 Ma之前就开始了。如果得到适当的重视，在所研究的熔岩流中识别NOMA漂移可以促进在中新世晚期更多这样的发现。
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.