Résumé
In order to better constrain the Tertiary reactivation of the different mountain ranges in Central Asia under the influence of the India-Asia collision, we carried out a magnetostratigraphic study along the Kuitun He section on the northern flank of the Tian Shan range. The Kuitun He section is composed of upper Tertiary sediments that were folded in a ramp anticline, then incised by a Holocene river. Sediment deposition was likely coeval with the building of the Tian Shan mountains, which coincided with major climatic change across Central Asia. About 1200 core samples were collected along the 2000-m-thick section, which is composed mainly of sandstone and conglomerate. In the laboratory, we performed both thermal and AF demagnetization, as well as magnetic mineralogical studies such as IRM, magnetic hysteresis, Curie point analyses, etc. Preliminary results show that magnetite is the main carrier of the magnetic remanence. A tenth of magnetic polarity intervals have been identified along the section. Preliminary correlation of this polarity sequence with the reference curve suggests an age span between 7 and 3 Myr with an average sedimentation rate of about 0.4 mm/yr. The high sedimentation rate can be interpreted in terms of facies migration, which owes to both tectonic and climatic forcing. The overall mean direction shows no significant inclination shallowing as is generally observed in Tertiary sediments throughout Central Asia.