INDIA-ASIA COLLIISION AND HIMALAYA-TIBET OROGENESIS SENSU STRICTO

We integrate new data from the Himalaya and Tibet system that shed light on the relation between plate kinematics and the evolution of the Tibetan Plateau. Collision in the region north of Mount Everest is now dated biostratigraphically on the south flank of the Zhepure Shan as during plankton zone P8 in the late Ypresian. P8 is the shortest planktonic zone in the early Eocene and essentially constrains the age of initiation of collision at 50.6±0.2 Ma. This dates the time at which Asian margin derived sediments were first deposited above unequivocally Indian passive margin sediments. This is synchronous within P8 with the onset of collision in the Zanskar region to the west constraining initiation of collision along greater than 50% of the length of the suture. Onset of collision at 50.6 Ma thus predates the 50% reduction in India-Eurasia convergence by more than 4 m.y. at Chron 21, implying greater than 400 km of continent-continent convergence before buoyancy related effects slowed convergence. Collision-related crustal thickening can be discerned using paleoaltimetry estimates from various basins from the Himalaya across Tibet. Estimates of paleoaltitude from the Lunpola basin in central Tibet, currently some 300 km N of the India-Asia suture indicate that this region had already achieved its current elevation of 4.7 km by the Late Eocene (~35±2 Ma). Approximately 950 km of continent-continent convergence had already taken place by that time suggesting that at least about 2/3 of the convergence was accommodated N of the suture. Coeval paleoaltimetry estimates from the Fenghuoshan, in northern Tibet, some 750 km N of the suture indicate that crustal thickening had not affected this region until after Eocene, and probably not until later in the Oligocene. Paleoaltimetry data from southern Tibet and Himalaya are restricted to younger than 20 Ma, but demonstrate that these regions have maintained their current high elevations (>4.6 km) for 15 or more million years. Together these data indicate crustal but not correlative mantle thickening have dominated the collision process, and that there are no data supporting lithospheric mantle delamination in this system.