Late Cretaceous–Palaeogene topography of the Chinese Tian Shan: New insights from geomorphology and sedimentology
Résumé
The Cenozoic growth of the intra-continental Tian Shan Range initiated during the late Eocene–Oligocene, and led to a tectonic reactivation of the complex Palaeozoic and Mesozoic lithospheric structure. Due to the very low erosion rates linked to the semi-arid climate that characterised the Tian Shan region during most of the Cenozoic, the topography of the range is not at equilibrium with deformation. Fragments of pre-orogenic low relief surfaces are preserved among the Late Cenozoic Alpine-type topography. Using field observations and satellite image mapping of those fragments, as well as sedimentology and biostratigraphic analysis, we show that the pre-Oligocene topography of the Tian Shan region was indeed complex, combining hundreds of metres to one-and-a-half kilometre-high reliefs with a multi-phased Mesozoic planation surface incised by Late Mesozoic paleo-valleys. The occurrence of several metres-thick Late Cretaceous–Palaeogene calcareous paleosols, in the basins surrounding the range further implies a semi-arid climate, very low subsidence rates and no uplift at that time. Late Cretaceous and early Miocene fossil records in the northern Tian Shan suggest a possible connection, even if episodic, between the drainage system of the south Junggar foreland basin and the proto-Paratethys Sea to the west. A renewed late Eocene–early Oligocene sedimentation probably marks the onset of the Tian Shan uplift. We argue that in addition to the growth of the Pamir and Western Kunlun ranges, this incipient uplift was one of the driving mechanisms for the final retreat of the proto-Paratethys Sea from the Tarim Basin. This regression apparently did not change the climate in the studied area because semi-arid conditions seem to prevail at least since the Late Cretaceous. Finally, the Tian Shan uplift remained very limited up to the Miocene as revealed by the occurrence of Burdigalian lake deposits preserved in the paleo-valleys inside the present day range. In contrast, post-early Miocene deformation of the northern Tian Shan has produced 4000 to 5000 m of differential vertical movement between the uplifted range and the subsiding proximal foreland basin.