CREATING TIBETAN PLATEAU THE TIAN SHAN WAY: LARGE-SCALE OUT-OF-SEQUENCE THRUSTING AS A MECHANISM FOR TIBETAN PLATEAU DEVELOPMENT

Determining the growth history of the Tibetan plateau is central to understanding the dynamics of Cenozoic Indo-Asian collision. Tectonic models in this regard range from whole-sale uplift to progressive enlargement of the plateau either by continuous northward propagation or stepwise jumping of plateau front. To test these competing hypotheses, we conducted geologic mapping and analyses of satellite images and seismic reflection profiles across the Kunlun Range and the Qaidam basin in central Tibet. Our results indicate that the above models do not adequately explain the geologic history of the Kunlun Range, which has experienced significant out-of-sequence thrusting since the Neogene. Our field observations and reflection profiles from Qaidam basin suggest that Cenozoic tectonics of the Kunlun Range is dominated by south-directed back-thrusting initiated in the middle or late Miocene. The presence of thick Paleogene strata (7-2 km) on both sides of the Kunlun and the observation that the Qilian Shan-Nan Shan thrust (QNTB) had started developing since ~ 50 Ma require the existence of a single large basin trapped between the elevated southern Tibetan plateau (Lhasa block) and the high region of the QNTB, with a basin width of 600-800 km in the north-south direction. The development of the Neogene-present backthrusts in the eastern Kunlun Range requires the operation of flake tectonics in central Tibet, which could have been driven by westward propagation of the left-slip Kunlun fault. That is, the Qaidam upper crust thrusts over the Kunlun Shan while the Qaidam lower crust and mantle lithosphere thrust underneath the Kunlun Shan. In the context of this model, the present-day tectonics of the Tian Shan, Tarim basin, and the active Maza Tagh thrust belt in the central Tarim basin may be the modern analogues of Paleogene paleogeography of the Tibetan plateau.