Folded fluvial terraces in a young, actively deforming intramontane basin between the Yumu Shan and the Qilian Shan mountains, NE Tibet

Intramontane basins in actively deforming regions contain significant information about the evolution of orogenic belts. We explored the tectonic characteristics and evolution of an intramontane basin between the Qilian Shan and Yumu Shan mountains on the NE Tibetan Plateau. We utilized the deformation of fluvial terraces along the Dahe River to constrain the rate and pattern of Quaternary deformation across the basin. Fluvial landforms include a widespread Mid-Pleistocene alluvial-fluvial fan surface and five terrace levels inset below this surface. We dated deposits associated with these landforms by optically stimulated luminescence (OSL) analyses on eolian loess and fluvial sediments. Our results yield ages of 142.8 ± 11.3 ka (Fs), 103–125 ka (T5), 96–115 ka (T4), 87–104 ka (T3), and 13–15 ka (T2), which we interpret to reflect the abandonment of fluvial terrace surfaces. Elevation surveys indicate that the terrace surfaces are folded along the Dahe anticline and are gently tilted northward across the basin. Analysis of terrace deformation suggests that the Dahe anticline grew by limb rotation and accommodated upper-crustal shortening at a rate of 0.14 +0.14/–0.03 mm/a. We determined the onset of deformation to have occurred between 0.3 and 0.9 Ma, based on the rotation rate of the southern limb of the anticline. This age is significantly younger than the onset ages of the thrust faults along the Qilian Shan (ca. 10 Ma) and the Yumu Shan (ca. 3.7 Ma). The character of the terrace deformation suggests that the Dahe anticline was growing from a south-dipping decollement, which also induced surface tilting across the entire basin. The Late Quaternary activation of the Dahe anticline folding and the detachment slipping provide evidence that the deformation of the intramontane basin occurred much later than that of the surrounding mountain ranges and accommodated part of the crustal shortening through basin narrowing.