Rapid Holocene bedrock canyon incision of Beida River, North Qilian Shan, China

[摘要] Located at the transition between monsoon- and westerly-dominated climate systems, major rivers draining the western North QilianShan incise deep, narrow canyons into latest Quaternary foreland basinsediments of the Hexi Corridor. Field surveys and previously publishedgeochronology show that the Beida River incised 130 m at the mountain frontover the Late Pleistocene and Holocene at an average rate of 6 m kyr −1 . Wehypothesize that a steep knickzone, with 3 % slope, initiated at themountain front and has since retreated to its present position, 10 kmupstream. Additional terrace dating suggests that this knickzone formedaround the mid-Holocene, over a duration of less than 1.5 kyr, during whichincision accelerated from 6 m kyr −1 to at least 25 m kyr −1 . These incision ratesare much faster than the uplift rate across the North Qilian fault, whichsuggests a climate-related increase in discharge drove rapid incision overthe Holocene and formation of the knickzone. Using the relationship betweenincision rates and the amount of base level drop, we show the maximumduration of knickzone formation to be ∼700  years and the minimumincision rate to be 50 m kyr −1 . We interpret that this period of increasedriver incision corresponds to a pluvial lake-filling event at the terminusof the Beida River and correlates with a wet period driven by strengtheningof the Southeast Asian Monsoon.

[学科分类] 土壤学