No abstract

[1] The present-day topography of the Tian Shan range is considered to result from crustal shortening related to the ongoing India-Asia collision that started in the early Tertiary. In this study we report evidence for several episodes of localized tectonic activity which occurred prior to that major orogenic event. Apatite fission track analysis and (U-Th)/He dating on apatite and zircon indicate that inherited Paleozoic structures were reactivated in the late Paleozoic-early Mesozoic during a Cimmerian orogenic episode and also in the Late Cretaceous-Paleogene (around 65-60 Ma). These reactivations could have resulted from the accretion of the Kohistan-Dras arc or lithospheric extension in the Siberia-Mongolia zone. Activity resumed in the late Mesozoic prior to the major Tertiary orogenic phase. Finally, the ongoing deformation, which again reactivates inherited tectonic structures, tends to propagate inside the endoreic basins that were preserved in the range, leading to their progressive closure. This study demonstrates the importance of inherited structures in localizing the first increments of the deformation before it propagates into yet undeformed areas.

In order to better constrain the tectonic evolution of central Asia under the influence of the India-Asia collision, we carried out a magnetostratigraphic study at the Kuitun He section, on the northern flank of the Tianshan range (northwest China). A total of 801 samples were collected from a 1559-m-thick section, which is composed mainly of fluvio-lacustrine sandstone and conglomerate. Stepwise thermal and alternating field demagnetization isolated a linear magnetization component that decays univectorally toward the origin and likely represents a primary magnetization principally carried by magnetite. From this component, 29 magnetic polarity intervals were identified. They correlate between 3.1 and 10.5 Ma with the reference magnetic polarity time scale, indicating a relatively constant sedimentation rate with an average of 0.21±0.01 mm/year. We also performed a suite of rock magnetic experiments designed to track time-transgressive changes in the sedimentary record. From the rock magnetic parameters, together with the constant sedimentation rate, we conclude that the Tianshan mountains were actively uplifting by 10.5 Ma.