Spatial variations of river incision rate in the northern Chinese Tian Shan range derived from late Quaternary fluvial terraces

The rate of erosion is critical for characterizing landscape evolution and understanding the interactions between tectonic activity, climate, and surface processes in active orogenic belts. This work focuses on the spatial pattern of river incision rate in the northern Tian Shan range, an active orogenic belt in NW China, based on fluvial geomorphic investigations. Both remote and field-based observations have been used to define the terrace sequences of four selected rivers preserved at the mountain front. For each analyzed river, one of the defined terraces (i.e. terrace T-4 of the Kuitun River, terrace T-5 of the Jingou River, terrace T-5 of the Manas River, and terrace T-4 of the Urumqi River) is used as a reference surface from which to reconstruct the paleogeomorphic surface (i.e. the previous riverbed) which was abandoned as a terrace as a result of fluvial incision. By comparing the present-day topography with the reconstructed (pre-incision) landscape, and after a porosity correction, the volume of material eroded by river incision and the depth of incision have been estimated. When combined with the formation age of each reference terrace, the average rates of river incision in the mountain range have been estimated at similar to 4.1 +/- 1.5 mm/yr (Kuitun River), similar to 3.5 +/- 1.4 mm/yr (Jingou River), similar to 3.1 +/- 1.5 mm/yr (Manas River), and similar to 1.9 +/- 1.3 mm/yr (Urumqi River). The above rates display a notable decreasing trend from the Kuitun (west) to the Urumqi (east). Our analyses indicate that lithology and climate might not be the main factors controlling the spatial variation of incision rates in the northern Chinese Tian Shan range. Instead, the observed pattern is in close agreement with changes of slope, relief, and N-S crustal shortening across the range. We thus propose that tectonic activity is the primary factor controlling the erosion and landscape evolution of the northern Tian Shan range, NW China.