Tectonic and Environmental Evolutions of the Northern Tibetan Plateau Prior to the Collision of India with Asia

Tectonic and environmental patterns and evolution of the present North Tibetan Plateau （NTP） prior to the India collision with Asia is significant to understand the formation of the Tibetan Plateau and its influence on the environment.In this study,we integrated and analyzed the tectonostratigraphy and the special sedimentary layers whose climatic implications are clear in the NTP.Additionally,we stressed the tectonic and environmental events and their evolutions from the Mesozoic to the Early Cenozoic.Our results show that four tectonic phases,which sequentially took place during the Triassic,Jurassic,Cretaceous and Paleogene,played an important role on the formation of the North Tibet.The climate was basically dry and hot from the Triassic to the Eocene and became dry and cool since the Oligocene in this region.The climatic evolution was characterized by a transition from a wet and hot phase during the Triassic-Middle Jurassic,to a dry and hot phase during the Late Jurassic-Eocene.Both phases encompassed 5 wet and hot periods followed by 5 dry and hot climate events,respectively.In addition,we found that the tectonic deformation and the climatic conditions were spatially and temporally different.In detail,in the regions north of the PaleoTian Shan and Paleo-Qilian Mts.the tectonic deformation and climatic condition were stronger and wetter than in regions south of the Paleo-Tian Shan and Paleo-Qilian Mts.during the Late Triassic-Jurassic.Whereas in the Cretaceous,the tectonic movement was intensive in the west but steady in the east,and climate was dry in the south but wet in the north of NTP.The formation of the tectonic and climatic patterns in NTP were the consequence of either global climate change or regional tectonics,including the Paleo-Asian Ocean closure and the Qiangtang block,Lhasa block and India plate collision subsequently to Asia.Furthermore,the regional tectonic events occurred before any global climate change and drove the climatic change in the NTP.

3D palaeogeographic reconstructions of the Phanerozoic versus sea-level and Sr-ratio variations

A full global geodynamical model over 600 million years （Ma） has been de- veloped at the University of Lausanne during the past 20 years. We show herein how the 2D maps were converted into 3D （i.e., full hypsometry and bathymetry）, using a heuristic-based approach. Although the synthetic topography may be viewed as relatively crude, it has the advantage of being applicable anywhere on the globe and at any geological time. The model allows estimating the sea-level changes throughout the Phanerozoic, with the possibility, for the first time, to flood accordingly continental areas. One of the most striking results is the good correlation with ＂measured＂ sea-level changes, implying that long-term variations are predominantly tectonically-driven. Volumes of mountain relief are also estimated through time and compared with strontium isotopic ratio （Sr-ratio）, commonly thought to reflect mountain belt erosion. The tectonic impact upon the general Sr-retio trend is shown herein for the first time, although such influence was long been inferred.

Did the modern Yellow River form at the Mid-Pleistocene transition?

The incision of the Sanmen Gorge marks the birth of the modern Yellow River,but its timing varies from the late Miocene-early Pliocene to the late Pleistocene(~0.15 Ma),and the suggested forcing mechanisms vary from the uplift of the Tibetan Plateau to global climate change.Here,we report sedimentologic,geochronologic,and provenance data from a drill core near the Sanmen Gorge,the last gorge along the main course of the Yellow River.Our results indicate that typical river channel deposits,with detritus from the Ordos Block in the upstream regions,started to accumulate in the Sanmen Gorge at~1.25 Ma.When integrated with river terrace evidence from the upstream and downstream regions,the results provide robust evidence that the final integration of the modern Yellow River occurred at~1.25 Ma,consistent with the beginning of the Mid-Pleistocene transition(MPT).We propose that the accelerated lowering of eustatic sea level during the MPT may play as important a role as tectonism in driving the birth and evolution of the modern Yellow River.