Spatial patterns of Late Quaternary river incision along the northern Tian Shan foreland

Abstract River-incision rate is widely used to track changes in tectonics or climate over time and space. However, the feasibility of utilizing the spatial variations of river incision to reconstruct past tectonic and climatic processes remains unclear. Here, we focus on the spatial patterns of river incision along the northern Chinese Tian Shan foreland. Three alluvial fans FP, FeH, and FlH are determined as the alluvial fan context of river incision and terrace classification providing the geomorphological framework across the foreland region. Four rivers (i.e. the Kuitun, Jingou, Manas, and Urumqi Rivers from the western, central, and eastern part of the foreland) are used to reconstruct the paleogeomorphology from the reference, which is the best-preserved terrace of each river system with ages clustering in different parts of the Latest Pleistocene-Early Holocene. The depth of incision constrained by the reference terrace of each river is obtained by comparing the present-day topography and the reconstructed one. The resulting profile of channel incision (depth and rate) of each analyzed river displays an overall decreasing-downstream trend from the maximum where the river exits from the mountain range, to zero. Such a trend has been attributed to the progressive lowering of the river gradient that was caused by the adjusted ratio of sediment input versus water discharge induced by climate change. Superimposing on the decreasing-downstream trend, an obvious step can also easily be observed onto the profile of channel incision of each river, downstream of which channel incision is significantly less. The step occurs near the thrust fault controlling growth of the outermost anticline through which each river cuts, thereby implying the key role of local rock uplift in forming the river-incision step. The former observation implies that, at the same timescale, more river incision at the exit from the mountain range does not necessarily mean stronger climatic forcing of incision there. We thus propose that, in a foreland setting, it should be the temporal pattern of river incision rather than its spatial variation that is helpful for unraveling the change in the forcing factor of downcutting.