Abstract
Located at the transition between monsoon and westerly dominated climate systems, major rivers draining the western North Qilian Shan incise deep, narrow canyons into latest Quaternary foreland basin sediments of the Hexi Corridor. Field surveys show that the Beida River incised 125 m at the mountain front over the Late Pleistocene and Holocene at an average rate of 6 m/kyr. We hypothesize that a steep knickzone, with 3 % slope, initiated at the mountain front and has since retreated to its present position, 10 km upstream. Terrace dating results suggest this knickzone formed around the mid-Holocene, over a duration of less than 1.5 kyr, during which incision accelerated to at least 25 m/kyr. These incision rates are much larger than the uplift rate across the North Qilian fault, which suggests a climate-related increase in discharge drove rapid incision over the Holocene and formation of the knickzone. Using the relationship between incision rates and the amount of base level drop, we show the maximum duration of knickzone formation to be 700 yr and the minimum incision rate to be 50 m/kyr. This period of increased river incision is the result of increasing excess discharge, which likely corresponds to a pluvial lake-filling event at the terminus of the Beida River and correlates with a wet period driven by strengthening of the Southeast Asian Monsoon.
Highlights
An incising river responds to tectonic or climatic perturbation by adjusting its slope, expressed by formation of knickpoints or knickzones (Crosby and Whipple, 2006; Tucker and Whipple, 2002; Whittaker, 2012), and through changes of its channel width (Finnegan et al, 2005)
Using the relationship between incision rates and the amount of base level drop, we show the maximum duration of knickzone formation to be 700 yr and the minimum incision rate to be 50 m/kyr
Combining all evidence from river incision rates, glacial coverage, and local and regional climate records, we suggest that the brief, rapid incision event recorded by the Beida River knickzone is most likely linked to a pluvial event that resulted from an 360 increase of precipitation and possibly increased glacier melt during the mid-Holocene
Summary
An incising river responds to tectonic or climatic perturbation by adjusting its slope, expressed by formation of knickpoints or knickzones (Crosby and Whipple, 2006; Tucker and Whipple, 2002; Whittaker, 2012), and through changes of its channel width (Finnegan et al, 2005). Previous studies on headward migrating knickpoints focus on the role of tectonic uplift or a base level fall, and usually regard climate conditions and channel width as constant (e.g. Tucker and Whipple, 2002; Crosby and Whipple, 2006; Haviv et al, 2006; Wobus et al, 2006).
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