The topographic state of fluvially conditioned mountain ranges

The topography of mountain ranges reflects the competition of constructive and destructive processes driven by tectonics and climate, respectively. There is a vital debate whether the topography of individual orogens reflects stages of growth, steady-state or decay that is fueled by the million-year time scales hampering direct observations on landscape evolution, the superposition of various process patterns and the complex interactions among different processes. Hence, there is a demand for sophisticated analysis tools to extract constraints on the long-term evolution of orogens from their topography. We review the field of orogen-scale landscape evolution from a numerical perspective, summarize the most prominent modelling concepts and their implications for the fluvially-driven development of mountain topography, and finally evaluate their applicability for understanding real-world orogens. Following the concept of equilibrium - a state where uplift rates are balanced by erosion rates and topography remains steady over time - the erosional long term response of rivers and hillslopes can be mathematically formalised by the stream power and mass diffusion equations, respectively. Based on a simple 1-dimensional model consisting of two rivers separated by a watershed we explain the influence of uplift rate and rock erodibility on steady-state channel profiles and show the time-dependent development of the channel - drainage divide system. Dynamic drainage network reorganisation adds additional complexity and its effect on topography is explored on a two-dimensional model. River capture events and drainage divide migrations as expression of the ongoing drainage network reorganisation cause changes in topography until a stable network topology, and hence full topographic steady-state is achieved. The long time spans needed for this drainage network optimization suggest that orogens on Earth may never reach full topographic steady-state. In real world orogens, we find evidence for premature, mature and decaying topography as well as relief rejuvenation by analysing slope-elevation distributions and trace the expression of crustal strain distorting drainage networks by applying the X transform. We conclude that modern concepts of landscape evolution allow sophisticated analyses of real-world orogens, but emphasize that unambiguous results are mostly derived from regions dominated by a limited number of interacting processes such as mountain ranges with limited or no glacial imprint. This results in a high demand for techniques to disentangle the complex topography of real-world orogens into the signals resulting from the individual land-shaping processes. (C) 2017 Elsevier B.V. All rights reserved.