青藏高原东北缘柴达木盆地路乐河地区新生代构造变形的古地磁证据

青藏高原东北缘是高原由西南向东北方向扩展的前缘位置，其新生代构造变形对揭示青藏高原隆升、扩展的过程与动力学机制具有重要的意义。柴达木盆地是青藏高原东北缘最大的新生代沉积盆地，发育巨厚的新生代地层，这些地层所记录的古地磁极旋转信息是定量约束柴达木盆地新生代以来构造变形发生的时间、方式与幅度的载体。本文以柴达木盆地北缘新生代地层出露良好、具有精确地层年代控制的路乐河剖面为研究对象，开展了古地磁极旋转研究，统计分析路乐河剖面24. 6~5. 2 Ma之间1477个可靠古地磁样品的特征剩磁方向(ChRM)，发现柴达木盆地北缘路乐河地区在24. 6~16. 4 Ma发生小幅度(不显著)的逆时针旋转，旋转角度约为8. 4°±6. 1°；16. 4~13. 9 Ma路乐河地区发生显著的顺时针旋转，旋转角度可达36. 1°±6. 0°；13. 9~5. 2 Ma 该地区未发生明显的构造旋转；5. 2 Ma以后路乐河地区逆时针旋转了~6°。结合柴达木盆地北缘区域构造变形的分析，我们提出柴达木盆地北缘路乐河地区在16. 4~13. 9 Ma 之间发生强烈的顺时针旋转构造变形(~36°)可能代表了盆地北缘中中新世遭受强烈的地壳差异缩短变形，从而成为高原最新形成的部分。

Paleomagnetic evidence of Cenozoic tectonic deformation in Lulehe area, Qaidam basin, northeastern margin of Tibet Plateau

The northeastern margin of the Tibetan Plateau is the leading edge of the expansion of the plateau from south to northeast, and its Cenozoic tectonic deformation is of great significance for revealing the upward and outward expansion processes and dynamic mechanism of the Tibetan Plateau. The Qaidam basin is the largest Cenozoic sedimentary basin along the northeastern margin of the Tibetan Plateau, it contains huge thickness Cenozoic strata with bulk information for plateau growth. The vertical- axes rotations recorded by these strata carry quantitative constraints on the time, mode and amplitude of Cenozoic tectonic deformation in the Qaidam basin since Cenozoic. Here, we studied vertical- axes rotations of the Lulehe section in the northern margin of the Qaidam basin, where Cenozoic strata are well exposed and controlled by precise stratigraphic age. The Characteristic remanent magnetization directions (ChRM) of 1477 reliable paleomagnetic samples from 24. 6 to 5. 2 Ma along the Lulehe section was statistically analyzed. It is found that the Lulehe area had undergone a small (insignificant) counterclockwise rotation with 8. 4°±6. 1° during 24. 6 to 16. 4 Ma. From 16. 4 to 13. 9 Ma, the Lulehe region was significantly clockwise rotated with 36. 10°±6. 0°. No significant tectonic rotation occurred in the region during 13. 9 to 5. 2 Ma and ~6° counterclockwise rotation recorded after 5. 2 Ma. Combined with the analysis of deformation in the northern Qaidam basin, we attribute middle Miocene ~36°clockwise rotation in the Lulehe area at the northern Qaidam basin to crustal differential shortening deformation, which drove the outward growth of the Tibetan Plateau to its north margins.