Magnetostratigraphy and Rock Magnetism of the Neogene Yaha Section (Northwest China) and Possible Late Cenozoic Uplift of the Tianshan by 11 Ma
Abstract
In order to better constrain the timing of the Cenozoic uplift history of the Tianshan due to the India-Asia collision, we carried out a magnetostratigraphic study at the Yaha section, on the southern flank of the Tianshan range (northwest China). 1069 samples were collected from a 2814-m-thick section composed mainly of fluvio-lacustrine sandstone and conglomerate mapped as the Neogene Kuqa Formation. Rock magnetic experiments show that the magnetic remanence is principally carried by magnetite yet hematite is also abundant . Stepwise thermal demagnetization isolated a linear magnetization component that decays univectorally toward the origin. Based on a positive fold test and the equal proportion of magnetic polarities, the isolated direction likely represents a primary magnetization. Eighteen magnetic polarity changes were identified. They correlate between ~5.2 and ~12.6 Ma with the reference magnetic polarity time scale. We also measured the anisotropy of magnetic susceptibility of the samples. Before ~ 11 Ma the average shape parameter T of the AMS is around 0 meaning the grains are spherical in shape and far-traveled. By ~11 Ma, T strongly increases to values strictly greater than 0 meaning that the magnetic grains are oblate in shape and traveled less far. The timing of this change in the T parameter is strongly correlated to a decrease in the bulk susceptibility that likely signals a modification of the source rock. From the rock magnetic parameters, together with the acceleration of the sedimentation rate, we conclude that the uplift of the Tianshan mountains started by ~ 11 Ma.