Late Cenozoic Building History of the Chinese Tianshan Seen From The Rock Magentic Record
Résumé
The Tianshan range is among Asia's largest mountain chains, with summits higher than 7000 m spread over an E-W distance of 2500 km. The range's geologic record attests to a complex history of Paleozoic subduction-related processes, with a new phase of Cenozoic reactivation induced by the India-Asia collision. The exact timing of reactivation is poorly constrained, which inhibits a better understanding of the evolution of continental deformation due to an indenting India. In addition, Tianshan is a case example of an active intracontinental mountain belt where the history of mountain building might be deciphered from the sediment accumulation in the adjacent Tarim and Junggar basins. In the Neogene sediments of these basins, we carried out three magnetostratigraphic studies at the Yaha section located on the southern piedmont and the Kuitun and Jingou He sections on the northern piedmont. Stepwise demagnetization isolated a stable magnetic component of dual polarity that passes the fold test in all three sections. We found 18, 16, and 22 polarity chrons at the Yaha, Kuitun and Jingou He sections, which we correlated with a reference polarity time scale to date the sections between 5.2 and 12.6 Ma, 3.1 and 10.5 Ma, and 8.5 Ma and 22 Ma, respectively. At Yaha, sedimentation rate increases markedly at ~ 11 Ma, while sedimentation rate is constant at Kuitun; Jingou He is characterized by two important accelerations at ~15 Ma and 11 Ma. We also performed rock magnetic measurements to track time-transgressive changes in the sedimentary record. Several magnetic parameters, such as the anisotropy shape parameter (T) and bulk susceptibility (k) show marked changes at around 21, 15 and 11 Ma on both sides of the range. Based on the changes in the rock magnetic parameters and in the sedimentation rate, we conclude that the central Tianshan Mountains likely began uplifting by 21 Ma and then underwent accelerated uplift at 15 and 11 Ma.