The northward expansion of the Tibetan Plateau: Topographic evidence from the Bogda Mts. – southern Junggar Basin coupling system, northwest China

Abstract

The northward propagation of the Tibetan Plateau has led to the orthogonal N–S shortening in northwestern China, which influenced the topographic formation of the E–W trending Bogda Mountains–southern Junggar Basin coupling system. In this study, we focus on the landscape formation of this coupling system and investigate the effect of the propagating deformation in the periphery of the India-Asia collision zone using topographic analysis and optically stimulated luminescence (OSL) dating of river terraces. Together, the results of the topographic analysis show that the normalized steepness index (k_sn) value and the scale of knickpoints decrease from west to east, and there is no significant difference in χ values between two sides of the Bogda Mts. Compiled low-temperature thermochronological data for the Bogda Mts. shows a younging trend from west to east, suggesting that the uplift of the western Bogda Mts. began earlier. The OSL dating for the Dalongkou river terraces, on the northern slope of the Bogda Mts. resulted in ages of the T2, T3, T4 river terraces of 6.2 ± 1.3 ka, 13.1 ± 1.7 ka, and 14.2 ± 2.5 ka, respectively. The incision rate of the Dalongkou River increases upstream from about 1.22 mm/yr near to the southern Junggar Basin, to about 2.1 mm/yr, and to approximately 6.33 mm/yr at the base of Bogda Mts. and reveals an upstream increasing uplift rate. Based on these new data, we propose a model of ongoing late Quaternary folding leading to the upbending of central Bogda Mts., with an inflection point of the fold close to the southern Junggar Basin. Combined with the geomorphological comparison with the eastern North Tianshan landscape, we conclude that the lateral intensity of the tectonic uplift from the North Tianshan to Bogda Mts. has gradually weakened, and the uplift of Bogda Mts. has gradually accelerated since the Quaternary and is controlled by N–S orthogonal shortening caused by the India-Asia collision.