碰撞前印度大陆北缘古地理轮廓——藏南晚白垩世堆拉灰岩古地磁研究
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Paleogeography of the Northern Margin of Indian Continent Prior to Its Collision: An Investigation of the Late Cretaceous Limestones in South Tibet
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    摘要:

    堆拉灰岩的古地磁数据为重建碰撞前印度大陆北缘轮廓提供了重要制约,但也留下了一些疑点,为此,本文对该地区晚白垩世海相沉积开展了进一步的古地磁研究。通过对10个采点样品的系统退磁及统计分析,得到宗山组最下段经地层校正后的特征剩磁分量为D=168.2°,I=-7.3°,α95=7.5°。该分量与前人从该地区宗山组中、上段得到的古地磁结果无显著区别,并在95%的置信水平上通过褶皱检验,表明剩磁是褶皱前获得的。尽管如此,堆拉晚白垩世灰岩统一的负极性特征与其沉积期间(Turonian-Maastrichtian)对应的古地磁标准极性柱不符,表明该地区宗山组不同层位可能普遍遭遇了重磁化,不能用来约束喜马拉雅地体的古地理位置。基于以上考虑,本文仅用前人从岗巴宗山地区获得的古地磁结果对晚白垩世期间特提斯喜马拉雅地体的古地理位置进行了计算和修正,结果显示该地体在宗山组上段沉积期间(约71~65Ma)的古纬度约为 9°S,计算得到当时大印度的北向延伸量为1500km,与前人从宗浦组得到的古地磁结果一致。

    Abstract:

    The available paleomagnetic data from Duila limestones provide important constraints on paleogeography of the northern margin of Greater India but controversy still remains. Hence we carried out a further study on the late Cretaceous marine sediments in this area. On the basis of systematic demagnetization and statistical analysis on samples collected from 10 sites, characteristic remnant magnetizations (ChRMs) were successfully isolated with tilt-corrected direction of D=168.2°,I=-7.3°,α95=7.5°, which is insignificantly different from the one obtained from the middle and upper part of the Zongshan Formation in the area of Duila and have passed a positive fold test suggesting a pre-folding in nature. The uniform reversed polarity of the ChRMs for the Zongshan and Gamba CunKou formations is, however, incompatible with the geomagnetic polarity time scale during the time interval from the Turonian to Maastrichtian suggesting that the primary remanence of the rocks had most probably been completely overprinted and hence could not be used for paleogeographic reconstruction of Greater India. Excluding the data obtained from the Zongshan Formation in the Duila area, the remaining 8 sites obtained from the area of Gamba suggest that the Tethyan Himalayan Terrane was located at ~9°S, and the amount of the northern extension of Greater India is recalculated to be ~1500 km, which is compatible with the one obtained from the Zongpu Formation by former studies.

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易治宇,梁雅伦,赵杰,闫永刚,陈力为,唐祥德.2016.碰撞前印度大陆北缘古地理轮廓——藏南晚白垩世堆拉灰岩古地磁研究[J].地质学报,90(11):3282-3292.
YI Zhiyu, LIANG Yalun, ZHAO Jie, YAN Yonggang, CHEN Liwei, TAN Xiangde.2016. Paleogeography of the Northern Margin of Indian Continent Prior to Its Collision: An Investigation of the Late Cretaceous Limestones in South Tibet[J]. Acta Geologica Sinica,90(11):3282-3292.

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  • 在线发布日期: 2016-11-01
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