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

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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.

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