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.