Paleomagnetism of the Neogene Jiuxi Basin, Hexi Corridor, China: implications for a non- rigid deformation mechanism?

The uplift of the northeastern Tibetan Plateau has attracted wide attention recently. The ongoing sequence of uplift phases of the plateau has been suggested to be a young continuous outgrowth of the Tibetan Plateau. Hence, the research of the deformation and evolution of this part of the plateau will lead to the further understanding of the kinematics and mechanisms during the evolution of the entire Tibetan Plateau. However, so far the deformation history within the northeastern Tibetan Plateau is still not fully understood. Recent paleomagnetic data within the Qaidam-Altyn Tagh region are contradictory, in that they are being used as arguments for significant as well as insignificant rotations in the region. In addition, paleomagnetic studies in the Gonghe-Guide and Longzhong basins are marred by a lack of comprehensive understanding as to whether significant clockwise rotations of the region were diachronous or whether they all occurred before 29 Ma. We report here on a paleomagnetic study of two magnetostratigraphic sections sampled in the Jiuxi Basin in the western Hexi Corridor, a foreland basin of the northern Qilian Shan (Mts.) at the northern margin of the Tibetan Plateau, to help explain the deformation of the region. The two sections: Laojunmiao and Hongshuiba, yield magnetostratigraphic ages of about 13-0 Ma and 11-5.7 Ma, respectively. The declinations observed in the sedimentary strata of the Laojunmiao section show similar values with the mean of about 0.1 degrees, suggesting insignificant rotation; whereas those from the Hongshuiba section, with a mean declination of about 317.3 degrees, imply significant counterclockwise rotations of about 43 degrees. We attribute the difference in the mean declinations of the two sections in the Jiuxi Basin during the same period to local deformation. This and other apparently contradictory paleomagnetic data within the Altyn Tagh-Qaidam area may imply non-rigid deformation in the region.