Early Cenozoic Mega Thrusting in the Qiangtang Block of the Northern Tibetan Plateau

Recent mapping and seismic survey reveal that intensive compression during the Early Cenozoic in the Qiangtang block of the central Tibetan Plateau formed an extensive complex of thrust sheets that moved relatively southward along several generally north-dipping great thrust systems. Those at the borders of the ~450 km wide block show it overrides the Lhasa block to the south and is overridden by the Hohxil-Bayanhar block to the north. The systems are mostly thin-skinned imbricate thrusts with associated folding. The thrust sheets are chiefly floored by Jurassic limestone that apparently slid over Triassic sandstone and shale, which is locally included, and ramped upward and over Paleocene-Eocene red-beds. Some central thrusts scooped deeper and carried up Paleozoic metamorphic rock, Permian carbonate and granite to form a central uplift that divides the Qiangtang block into two parts. These systems and their associated structures are unconformably overlain by little deformed Late Eocene-Oligocene volcanic rock or capped by Miocene lake beds. A thrust system in the northern part of the block, as well as one in the northern part of the adjacent Lhasa block, dip to the south and appear to be due to secondary adjustments within the thrust sheets. The relative southward displacement across this Early Cenozoic mega thrust system is in excess of 150 km in the Qiangtang block, and the average southward slip-rate of the southern Qiangtang thrusts ranged from 5.6 mm to 7.4 mm/a during the Late Eocene-Oligocene. This Early Cenozoic thrusting ended before the Early Miocene and was followed by Late Cenozoic crustal extension and strike-slip faulting within the Qiangtang block. The revelation and understanding of these thrust systems are very important for the evaluation of the petroleum resources of the region.

Early Uplift History of the Tibetan Plateau:Records from Paleocurrents and Paleodrainage in the Hoh Xil Basin

Paleocurrent indicator data collected in field work were used to study the early Cenozoic regional paleodrainage patterns in the Hob Xil basin in northern Tibetan plateau. The paleocurrent directions of the Eocene Fenghuoshan Group obviously show that the flows were northward with a unidirectional dispersal pattern. This probably reflects the uplift of the Qiangtang terrain during the initial basin deposition period and indicates that the Tanggula Moutains occurred as topographic highlands at least in the Eocene. Paleoflows of the Oligocene Yaxicuo Group were dominantly oriented to the north and then flowed eastwards during its late deposition. This regional variability of paleodrainage patterns of the Yaxicuo Group is interpreted to record the dispersal style of sediments from transverse rivers to longitudinal river systems. It is inferred that the Oligocene uplift of the Kunlun Mountains obstructed by northward paleoflows and created longitudinal river systems parallel to the orogenic belts. The temporal and spatial changes of the paleodrainage patterns suggest that the northern boundary of the Tibetan plateau during the early Cenozoic was situated in the Hoh Xil area and its uplift has progressed northwards through time.

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.