The Tibetan plateau, which results from the collision between the Indian and Eurasian plates for about 50 million years (or My) constitutes, with the Himalayan chain on its southern edge, the largest high-altitude area of the globe. This unique topography is the cause of the monsoon phenomenon: the humid air, rising north from the Indian Ocean, is blocked by the Himalayan chain and the Tibetan plateau leading to diluvial rains in Southeast Asia and India. With very little humidity to cross this barrier, further north in Central Asia, the climate is dry, favoring the formation of deserts such as the Takla Makan or the Gobi.
Understanding the long-term climate change in Asia therefore implies knowing the topographic evolution of Tibet since 50 Millions years. Several models of plateau construction exist which differ in particular on the initial topography of the region: was the Tibet region flat and at low altitude at the onset of continental collision or did there exist, as some researchers think, a proto Tibetan plateau associated to mountain ranges?
The international team that Marc Jolivet participates in is working to reconstruct this initial topography of Tibet, looking for geological clues to reconstitute the early Cenozoic landscapes (~50 Ma). These indices are mainly contained in sediments deposited over time in sedimentary basins distributed on and around the plateau. The type of sediment and the associated fossil remains vary depending on the local slope (blocks and coarse sand for a mountain stream, fine sand and clay for a lake), the type of rocks eroded at the source, the climate or even the altitude.
As such, the Qaidam Basin, located more than 1000 km from the Himalayas in the northern part of Tibet, represents a unique archive of the topographical, climatic and tectonic evolution of the northern edge of the Tibetan plateau. This basin is entirely surrounded by mountain chains and therefore represents a trap without escape for the sediments resulting from the erosion of the plateau. With an accumulation of Cenozoic sediments of up to 15 km thick , it is one of the largest sedimentary basins in Asia. The team focused on the study of the oldest Cenozoic sediments deposited in the basin, known as the Lulehe Formation (“Lule River”), dated from the early Eocene (~55 My), the very beginning of the collision between India and Asia.
The team obtained seismic imaging profiles and exploratory drilling samples from the Qinghai Oilfield Company, an oil company associated with PetroChina. These data show a series of rivers flowing from the edges of the Qaidam Basin toward a plain with shallow lakes in the central area. These findings suggest that 50 million years ago, the Qaidam Basin was a closed, relatively wide, flat depression surrounded by eroding reliefs. Faults visible on the seismic profiles also show that the north and east edges of the basin were probably subjected to tectonic deformations leading to relief growth.
In order to get an idea of the importance of the reliefs that surrounded the basin at the beginning of the Cenozoic history, the team sought to “restore” the initial shape of the base of the basin. Indeed, in a context of tectonic compression of the type induced by the collision between India and Asia, the Earth’s crust that supports sedimentary basins will deform due to several factors: the load represented by the sediments deposited in the basin will result in a subsidence (a downward displacement) of the crust; the increasing weight of the mountain ranges that grow close to the basin also induces a downward movement of the basin edge; Finally, the crust, due to its mechanical resistance to flexure, will oppose this deformation.
Based on seismic data and several types of mechanical models of ground crust deformation, the team showed that at the time of the Lulehe Formation sediment deposition, the northern edge of the Qaidam Basin, which represents the extreme north of the Tibetan plateau, was already formed by a chain of high mountains. To the south of the basin, the Kunlun range already existed but was located further south than its current location.
This study shows that the topography of Tibet before the collision between India and Asia was already complex, with high mountains. 50 million years ago, at the beginning of the confrontation between the two plates, the northern edge of the plateau, located at least 1,500 km from the collision zone, already deformed tectonically, increasing the altitude of the pre-existing mountain ranges and probably disrupting the atmospheric circulation and thus the regional climate very early.
>>> rererence : Cheng, F., Garzione, C. N., Jolivet, M., Guo, Z., Zhang, D., Zhang, C., & Zhang, Q. (2019). Initial deformation of the northern Tibetan Plateau: Insights from deposition of the Lulehe Formation in the Qaidam Basin. Tectonics, 38. Doi : 10.1029/2018TC005214