INTERACTION OF PALEOZOIC AND CENOZOIC DEFORMATION IN THE QILIAN SHAN AND NAN SHAN REGION, WESTERN CHINA: A TECTONIC MODEL

Previous reconstructions of the Qilian Shan and Nan Shan region in northern Tibet were emphasized either on its early Paleozoic history or its active north-south contraction. However, how the two events have interacted to produce the present map pattern has rarely been explored. Cenozoic deformation of the western Qilian Shan/Nan Shan is expressed by development of four major thrust systems which together form a thin-skinned imbricate thrust belt. Spatially associated with Cenozoic thrust systems are multiple melange belts that are commonly treated as early Paleozoic sutures. In contrast, the western Qilian Shan-Nan Shan region is characterized by thick-skinned deformation expressed by two major pop-up structures north and south of the Xining basin in the easternmost Nan Shan. Correspondingly only two "sutures" are present on the north and south edges of the thrust belt. We suggest a three-stage tectonic history for the present pattern of deformation in the Qilian Shan-Nan Shan region. In the early Paleozoic, the North China craton and its cover sequence were underthrust southward beneath the coherent Qilian arc at a low angle. The contact zone between the Paleozoic arc above and the subducted shelf sequence below lies a melange zone which is at least a few kilometers thick. In the Triassic, the melange zone and the North China shelf sequence were eroded away in the east due to the North China-South China collision. In Cenozoic time the western Qilian Shan/Nan Shan was imbricated by thrusting along a decollement at the base of the shelf sequence. As a result the melange zone was repeated by imbricate thrusts producing multiple sutures. In contrast, the eastern Qilian Shan and Nan Shan were shortened by basement-involved contraction.