Fluvial evolution in a growing thrust-fold range of the Yumu Shan, NE Tibetan Plateau

Abstract

In an active thrust-fold belt, the drainage evolution, such as the alternation between transverse and longitudinal rivers, will influence the surface erosion within the range and the sedimentary sequence in the foreland basin. However, knowledge of the controls of changes in transverse or longitudinal drainage patterns in an uplifting range is limited. The Yumu Shan, on the northeastern margin of the Tibetan Plateau, is an example for a young and active thrust-fold range that shows evidence for drainage reversals since ∼3 Ma. The materials we used are the late Cenozoic depositional sequences around the Yumu Shan. Analyses of the lithology, grain size, roundness, and clast orientation of gravel deposits on the northern and southern flanks, combined with the results of cosmogenic nuclide burial dating, reveal that a transverse river, the Liyuan River, crossed the Yumu Shan range during 3.0–1.8 Ma by eroding weak Cenozoic sedimentary rocks. After 1.8 Ma, however, this transverse river was deflected and flowed along the edge of the range, and a longitudinal river developed along the back flank of the range. This process was synchronous with the exposure of resistant Paleozoic rocks by the continuous uplift of the range. After comparison with other potential controlling factors for drainage reversals, such as variations in rock uplift, climate or upstream aggradation, we find that the change in bedrock resistance was the most likely control on the change from a transverse to a longitudinal drainage pattern in this relatively young thrust-fold belt. Furthermore, through comparison with a series of active fold-thrust belts worldwide, we propose a simple conceptual model for the drainage evolution of active fold-thrust belts. In the earlier stage of the uplift, the exposed soft sedimentary rocks will cause transverse (antecedent) rivers to cross the range. After the continuous uplift of the range, resistant rocks cause the deflection of a transverse river and its change to a longitudinal river along the back flank of the range. In the final stage, after the several million years to around ten million years of uplift, with the development of a mature range, the local base level of the foreland will greatly fall, promoting a return to a transverse drainage pattern by headward erosion and capture of longitudinal rivers.