滇藏高原中、新生代沉积岩石古地磁方向的准确性及其构造意义

Paleomagnetism has been extensively used to reconstruct the Mesozoic and Cenozoic paleolatitudes for the Tibetan plateau and nearby crustal blocks, and to describe their relative latitudinal movements. The majority of the paleomagnetic data, which were derived from sedimentary rocks, suggest about 1500 km northward translation for the Tibetan plateau in the Cenozoic. However, recent paleomagnetic studies revealed that the paleolatitude derived from the Cretaceous red beds from the Linzhou basin, Lhasa block, was shallowed by 10 due to deposition and compaction processes. The results were corroborated by volcanic paleomagnetic data, which are immune from inclination shallowing. Therefore, the amount of crustal shortening was significantly decreased, and the initial collision age appeared to be much younger than previously thought. Probably, crustal shortening has not played a dominating role in accommodating the India-Asia continental collision. To better decipher the processes of crustal shortening and extrusion tectonics caused by the collision, we propose to test the accuracy of paleomagnetic directions carried by Mesozoic and Cenozoic sedimentary rocks in the Lhasa,Qiangtang,and Lanping-Simao blocks,by applying the inclination correction approach based on magnetic fabric analysis and the Elongation-Inclination analysis based on geomagnetic secular variation model. The results will provide better constraints on the mechanisms of continental collision.

古地磁学被广泛地用于研究青藏高原及邻近一些地壳块体中、新生代所处的古纬度以及南北方向的位移量。以沉积岩石为主体、大量的结果显示青藏高原整体在新生代发生约1500公里的北向运动。然而，最近的古地磁研究揭示拉萨地块林周盆地白垩纪红层的古纬度由于沉积和压实效应而系统偏低了10 ，该结果得到同时代火山岩的佐证，因此地壳缩短的规模大幅度减小，同时获得的印度-亚洲大陆碰撞的年龄也趋于年轻。也许亚洲大陆地壳缩短在印度-亚洲大陆碰撞机制中所起的作用并不如以前认为的占主导。为了更好地描述印度-亚洲大陆碰撞引起的大陆缩短以及挤出构造，本项目拟综合以磁组构物理模型为基础和以地磁场统计模型为基础的两种倾角矫正方法，在拉萨、羌塘、兰坪-思茅地块对中、新生代沉积岩开展古地磁方向准确性的研究，结果将更好地约束大陆碰撞的机制。

以古地磁方向准确性为目标，在滇藏高原开展了白垩纪和第三纪古地磁研究，在以下几个方面获得了新的进展：从帕米尔高原东部晚白垩世塔里木盆地西南缘的浅海碳酸盐岩石获得了可靠的原生剩磁，磁化率各向异性及非磁滞剩磁各向异性测试显示弱的沉积组构特征，平均古地磁倾角和参考方向一致，证明中亚古地磁倾角异常是由于一些红层记录受到沉积和压实作用导致的倾角偏低，而并不存在长期倾斜的偶极子或者长期的非偶极子场。对江孜晚白垩世床得组大洋红层的古地磁研究，揭示出由细颗粒色素赤铁矿携带的可靠的特征剩磁，通过褶皱检验和B级倒转检验，其形成的古纬度位于北纬10°（误差2°），远离于印度大陆边缘，而更靠近亚洲大陆，结果首次证实一些大洋红层并不发育在印度大陆边缘。对萨嘎—吉隆交界的桑丹林剖面红色硅质粉砂、泥岩的古地磁研究，获得了可靠的特征剩磁，通过99%置信水平下的褶皱检验，揭示其形成于南纬8°（误差2°），而不是在亚洲或者印度大陆边缘，结果确认沉积物源区分析约束印度-亚洲大陆碰撞时限的方法在该剖面不适用。对石炭-二叠纪泛大陆再造进行了古地磁倾角偏低的研究，结果显示倾角偏低是造成劳亚和冈瓦纳大陆古地磁极差异的部分原因，主要原因是两大陆之间围绕一个位于板块边界的欧拉极小规模的旋转（5—13°）。我们的再造类似于Pangea A, 所以不再需要一个石炭-二叠纪的Pangea B。