Abstract
Abstract. Many mountain ranges survive in a phase of erosional decay for millions of years following the cessation of tectonic activity. Landscape dynamics in these post-orogenic settings have long puzzled geologists due to the expectation that topographic relief should decline with time. Our understanding of how denudation rates, crustal dynamics, bedrock erodibility, climate, and mantle-driven processes interact to dictate the persistence of relief in the absence of ongoing tectonics is incomplete. Here we explore how lateral variations in rock type, ranging from resistant quartzites to less resistant schists and phyllites, and up to the least resistant gneisses and granitic rocks, have affected rates and patterns of denudation and topographic forms in a humid subtropical, high-relief post-orogenic landscape in Brazil where active tectonics ended hundreds of millions of years ago. We show that catchment-averaged denudation rates are negatively correlated with mean values of topographic relief, channel steepness and modern precipitation rates. Denudation instead correlates with inferred bedrock strength, with resistant rocks denuding more slowly relative to more erodible rock units, and the efficiency of fluvial erosion varies primarily due to these bedrock differences. Variations in erodibility continue to drive contrasts in rates of denudation in a tectonically inactive landscape evolving for hundreds of millions of years, suggesting that equilibrium is not a natural attractor state and that relief continues to grow through time. Over the long timescales of post-orogenic development, exposure at the surface of rock types with differential erodibility can become a dominant control on landscape dynamics by producing spatial variations in geomorphic processes and rates, promoting the survival of relief and determining spatial differences in erosional response timescales long after cessation of mountain building.
Highlights
The question of how landscapes evolve in the aftermath of mountain building has intrigued geomorphologists since the early stages of the discipline, and classic concepts such as the cycle of erosion (Davis, 1899) and dynamic equilibrium landforms (Hack, 1960) were defined in the context of these post-orogenic landscapes (Bishop, 2007)
Catchment-averaged denudation rates are negatively correlated with mean values of normalised channel steepness (Fig. 4b), a parameter often used to infer denudation rates based on the empirical positive correlation between denudation and normalised channel steepness in tectonically active landscapes (e.g. DiBiase et al, 2010; Kirby and Whipple, 2012; Harel et al, 2016)
We observe that catchment-averaged denudation rates may increase together with mean values of topographic metrics and precipitation rates for individual rock types, with such small sample sizes no such relationships are statistically significant at the α = 0.05 level except for catchments in phyllites (Figs. 4 and S6)
Summary
The question of how landscapes evolve in the aftermath of mountain building has intrigued geomorphologists since the early stages of the discipline, and classic concepts such as the cycle of erosion (Davis, 1899) and dynamic equilibrium landforms (Hack, 1960) were defined in the context of these post-orogenic landscapes (Bishop, 2007). We know with a reasonable degree of certainty that net erosion in these landscapes and the resulting rebound of the underlying lithosphere by isostasy are central mechanisms controlling the extended longevity of post-orogenic landforms (Gilchrist and Summerfield, 1990; Bishop and Brown, 1992; Bishop, 2007). A range of other factors and interactions play essential roles in the post-orogenic evolution of ancient landscapes, including variations in bedrock incision dynamics Peifer et al.: Growing topography due to contrasting rock types in a tectonically dead landscape ing reduction of the buoyancy of the lithosphere (e.g. Blackburn et al, 2018), and tectonic uplift in response to far-field stresses (e.g. Hack, 1982; Quigley et al, 2007)
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