SOUTH-DIRECTED BACKTHRUSTING BENEATH THE EASTERN KUNLUN SHAN: IMPLICATIONS FOR MECHANISMS OF PLATEAU GROWTH

Field observations, seismic-reflection profiles, and inspection of ASTER images indicate the existence of a major active, south-directed backthrust system beneath the eastern Kunlun Shan directly south of the Qaidam basin. Observations made in the field along N-S transects of Golmud, Tuolahai, and Dazhaohuo drainages reveal a series of incised alluvial terraces that are sequentially tilted towards the Qaidam basin to the north (i.e., older terraces dip more steeply than younger terraces). The terraces converging to the north can be traced into the Qaidam basin and do not bear evidence for active north-directed thrusting along the northern mountain front. The range front between ~92°E and ~98°E exhibits a high degree of sinuosity and the presence of inselbergs as seen in the field and on ASTER images, consistent with the absence of an active north-directed thrust system between the Kunlun Shan and Qaidam basin. In contrast, the southern flank of the Kunlun Shan is marked by active south-directed thrust traces. This is best expressed along the north side of the eastward flowing Nalingele river where the Paleozoic Kunlun batholith thrusts directly over Quaternary sediments. A branch of the inferred north-directed backthrust system is also exposed ~70 km south of Golmud and ~ 16 km north of the left-slip Kunlun fault. There the fault places a Proterozoic marble unit over a Triassic flysch sequence. Seismic-reflection data from the southern Qaidam basin suggest that backthrusting beneath the Kunlun Shan may have initiated in the Neogene and that the Paleogene sediments may have been linked to the depositional systems in the Fenghuo Shan region to the south. That is, Paleogene strata in the Fenghuo Shan and Qaidam region were deposited in the same contiguous basin that was later partitioned by the development of the eastern Kunlun backthrust system. In the context of this interpretation, the Kunlun fault is permissible to be a lithospheric-scale structure. The existence of a Neogene backthrust system in the eastern Kunlun Shan, which carries low-elevation Qaidam crust over the mountainous region of central Tibet, has profound implications for dynamic theories of plateau growth and basin development.