Climatic versus tectonic control on river incision at the margin of NE Tibet: 10Be exposure dating of river terraces at the mountain front of the Qilian Shan

Abstract

We document late Pleistocene–Holocene aggradation and incision processes at the mountain front of the Qilian Shan, an active intracontinental fold‐and‐thrust belt accommodating a significant portion of the India‐Asia convergence. The Shiyou River cuts through a NNE vergent fault propagation fold with Miocene red beds in the core and Pliocene–Quaternary growth strata on the northern forelimb. South of the anticline, Miocene strata dip 20° SSW, suggesting a similar orientation for the basal décollement. After aggradation of an ∼150‐m‐thick, late Pleistocene valley fill, the Shiyou River formed three terraces. The highest terrace, located 170 m above the river, constitutes the top of the fill. The other terraces are fill cut terraces: their treads are located 130–105 m and 37 m above the river, respectively. The 10Be exposure dating of the terraces suggests that river incision accelerated from 0.8 ± 0.2 mm yr−1 to ∼10 mm yr−1 at 10–15 kyr. We interpret fast Holocene river incision as largely unrelated to tectonic forcing. The late Pleistocene incision rate of 0.8 ± 0.2 mm yr−1 places an upper limit of 2.2 ± 0.5 mm yr−1 on the horizontal shortening rate, assuming that incision is solely caused by rock uplift above a décollement dipping 20°. However, the actual shortening rate may lie between ∼2.2 mm yr−1 and zero because deformation of the terraces and the valley fill cannot be unequivocally demonstrated. Our estimate is consistent with the bulk shortening rate of ∼5–10 mm yr−1 across several faults in NE Tibet derived from neotectonic and GPS data, although in case of the Shiyou River, Holocene deformation is barely discernible owing to intense climate‐induced river incision.