Geomorphic constraints on active mountain growth by the lateral propagation of fault-related folding: A case study on Yumu Shan, NE Tibet

The mountain range of the Yumu Shan in NE Tibet is the topographic expression of actively growing fault-related folds developed in the hanging walls of a system of mostly northeast-vergent blind thrusts. Here we present a Tibetan example on the geomorphic response to lateral fold propagation using both qualitative analysis of the geomorphic characteristics of this region (e.g., wind and water gaps, drainage density, and along-strike topography of the range crest) and quantitative analysis based on cosmogenic ¹⁰Be surface exposure dating and optically stimulated luminescence dating on the rates of uplift and lateral propagation of the anticline. The elevations of the past positions of successively deflected paleo-rivers show a systematic decrease toward the eastern tip of the anticline, in tandem with an eastward decrease in drainage density. Combining the age data obtained by cosmogenic ¹⁰Be and OSL dating methods with the heights of abandoned terraces, we calculated average uplift and lateral propagation rates of ~378mm/ka over the past ~185ka and ~40mm/yr over the past ~150ka, respectively. Our single-site evaluation of the relation between tear faulting and channel deflection suggests that the tear faulting occurs along weakened bedrock already dissected by the former stream. This finding is inconsistent with the conventional hypothesis that the route of the deflecting stream is controlled by the orientation of the tear-faulting zone. Topographic vertical and lateral fold growth of the Yumu Shan were probably driven by the eastward propagation of a blind thrust since the late Neogene following the initial deposition of sediments in the Hexi Corridor, which were transported from the Qilian Shan.