Covariation of cross-divide differences in denudation rate and χ: Implications for drainage basin reorganization in the Qilian Shan, northeast Tibet

Abstract

Drainage divide migration is of wide interest because it drives changes in topography, aquatic species habitat, and fluxes of water and nutrients across Earth's surface. To date, divide migration rates have been measured in relatively few places, partly because of the rarity of denudation rate measurements on opposing sides of drainage divides. Here we report 54 basin-averaged denudation rates across the Qilian Shan, China, inferred from 10Be concentrations in stream-borne quartz. We combine these with 18 previously published basin-averaged denudation rates and new measurements of the topographic metric χ in the river networks. These data reveal divide migration rates of ∼3–1385 mm kyr−1 and area loss in the headwater reach of the Hei River basin. Cross-divide differences in denudation rate (ΔE) and channel-head χ values (Δχ) follow an exponential relationship ΔE=ΔE0e−bΔχ, providing empirical support for the hypothesis that Δχ values can reflect ΔE. Here, b has the same value within uncertainty as in ΔE-Δχ data in the southern Appalachians and the Ozark Dome in previous studies, consistent with a common set of processes controlling divide migration in all three regions. The value of ΔE0 is 1-2 orders of magnitude higher at Qilian Shan than at the two other sites, implying higher values of ΔE at a given Δχ. Our numerical simulations show that near-exponential ΔE-Δχ relationships can arise as topographic divides approach equilibrium under spatially uniform uplift rate, consistent with divide migration toward steady state in each of these regions. Further, they show that the magnitude of ΔE at a given Δχ increases linearly with rock uplift rate. Together, these results suggest that ln(ΔE) exhibits the same sensitivity to Δχ across mountain ranges, and that the magnitude of ΔE may be strongly sensitive to rock uplift rate.