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Evolution tectonique du Tianshan au Cénozoïque liée à la collision Inde-Asie Apports de la magnétostratigraphie
et de la géochronologie isotopique U-Th/He.

Abstract : The Tianshan range is one of the largest Asia's mountain ranges, with summits higher than 7000 m extending over an E-W distance of 2500 km. The Tianshan is the most active intracontinental range, however, its geology records a complex history of Paleozoic subduction-related processes, with a recent phase of Cenozoic reactivation induced by the India-Asia collision. The exact timing of this later reactivation is poorly constrained, which inhibits a better understanding of the evolution of continental deformation due to the indenting India. In addition, on both its northern and southern sides the Tianshan present piedmonts composed by series of fold and thrust belts with excellent surface exposures of Permian to Quaternary sediments in the adjacent Kuche and Junggar basins that present case examples of foreland basins. The history of mountain building with its related foreland sedimentation and erosion rate might be deciphered from the sediments well exposed along the piedmont.
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. From stepwise demagnetization we found 18, 16, and 22 magnetic 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 remarkably at ~ 11 Ma; Sedimentation rates areconstant at the Kuitun section while; the Jingou He is characterized by two important accelerations at ~15 Ma and ~11 Ma. We also performed measurements of rock magnetism to track time-transgressive changes in the sedimentary record. Several magnetic parameters, such as the anisotropy shape parameter (T) and bulk susceptibility (Κ) show obvious 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 15 Ma and then underwent accelerated uplift at 11 Ma.
To describe the kinematics and the kinetics of both the piedmonts and the high range we also combine structural model of the piedmont and foreland basin, with chronological constraints on the stratigraphy derived from these magnetostratigraphic sections and U-Th/He dating of detrital apatite and zircon grains to constraint : (1) the timing of development and rate of folding of the piedmont, (2) the flux history and propagation rates of sediment and (3) the erosion rates history of the high range.
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Julien Charreau. Evolution tectonique du Tianshan au Cénozoïque liée à la collision Inde-Asie Apports de la magnétostratigraphie
et de la géochronologie isotopique U-Th/He.. Tectonique. Université d'Orléans, 2005. Français. ⟨tel-00011572⟩

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