Combining FastScape χ Values and 10Be Erosion Rates to Evaluate Topographic Equilibrium in Evolving Landscapes: Examples from Namibia and the Central Himalaya

Abstract:

The FastScape landscape evolution model is a powerful and user-friendly tool that can be used in concert with catchment-wide cosmogenic ¹⁰Be erosion rates to assess states of dynamic equilibrium in landscapes with respect to their tectonic and climatic settings. FastScape was used to compute chi (χ), a proxy for steady-state river channel elevation (Willet et al., 2014), for model domains in Namibia (e.g., desert climate and passive continental margin) and the central Himalaya (e.g., active mountain building and a wet, monsoonal climate). Namibian and central Himalayan landscapes are eroding at widely different rates (e.g., 10¹ mm/ka and 10³ mm/ka, respectively). Chi values are sensitive to both DEM domain size and base level, cell resolution, and time, thus, chi values can only be evaluated and directly compared within a given domain. Chi values indicate areas or regions of equilibrium or disequilibrium within a given model domain, and not between domains in different geographic study areas. Chi can be used to ascertain if anomalously high ¹⁰Be erosion rates are affected by the addition of youthful sediment from landslides, debris flows, or glaciation of river catchments or tributary basins. In this study, glacial settings with high erosion rates show no relationship to chi values. For unglaciated tributary basins in a given catchment, chi values are related to cosmogenic ¹⁰Be erosion rates in the following ways: (1) basins in equilibrium have chi values that remain constant with increasing cosmogenic erosion rates; (2) basins in disequilibrium have an inverse relationship between chi values and erosion rates in a setting where erosion is driven predominantly by precipitation; and (3) basins in disequilibrium have a positive correlation between chi values and erosion rates in a setting where tectonic uplift is the dominant force driving erosion.