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

<正>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.