Present-day deformation of the Qaidam basin with implications for intra-continental tectonics

The Qaidam basin is a plateau of low relief, about 3 km in elevation and covering a large area of 1.2 x 10(5) km(2). Physiographically, the basin marks a transitional region between the northeastern Tibetan plateau (5 +/- 0.5 km in elevation) and the surrounding low-lying areas. The northeastern and southwestern edges of the basin are bounded by the Qilian Shan-Nan Shan and Qimantag-Eastern Kunlun Shan mountain belts, respectively. At present, the entire region is under northeast-southwest compression. Based on a synthesis of recent geological and geophysical results, we propose that each flanking mountain belt consists of a sequence of crustal nappes, originating from ramp thrusts at mid-crustal levels (>15 km deep). Over a distance range of 700 km, these two broad fold-and-thrust belts, operating in tandem and with some uniform shortening in the intervening Qaidam basin, successively raised the surface elevation from below 2 km to 5 km. At depth, the crust is correspondingly thickened from 40 km beneath the foreland of the Qilian Shan to approximately 55 km under the interior of the Qaidam basin, and then to nearly 70 km beneath Tibet. In contrast to the Himalayan front where earthquake faulting occurs down to a depth of 50 km in the foreland but the zone of active thrust faulting is less than 200 km in width, plateau building in the Qaidam region illustrates how a wide orogen is developing along northeastern Tibet in the interior of Eurasia. (C) 1999 Elsevier Science B.V. All rights reserved.