Transient landscape dynamics across the Southeastern Australian Escarpment

Abstract

Passive margin escarpments provide some of the best examples for large-scale transient landscape evolution. Despite the relative simplicity of their geological setting, when compared with active orogenic systems, many open questions exist concerning their modes and rates of evolution. We use catchment wide denudation rates calculated from cosmogenic nuclides concentrations and high resolution topographic analysis to constrain landscape dynamics across the South Eastern Australian Escarpment. We determined denudation rates of ∼15 mm/ka in the lowlands at the foot of the escarpment and of ∼10 mm/ka in the highlands, while catchment draining the escarpment face display rates in the 20–60 mm/ka range. These denudation rates along a passive margin escarpment are among the highest in the world and show greater sensitivity to topographic gradients when compared to other passive margin settings. We interpret this situation as resulting from the intermediate precipitation regime of our study area, as opposed to drier or wetter settings, where hillslope processes can be inhibited due to water availability or deep weathering profiles and vegetation feedbacks, respectively. Combined with the extraction of topographic metrics across the escarpment, these rates allow us to constrain efficiency coefficients for fluvial incision and hillslope diffusion that are similar to other independent estimates in this region. These coefficients are used to calculate an escarpment retreat rate of 40 to 80 mm/ka over the last 100s of ka. Our analysis of high resolution hillslope morphological properties suggests widespread small-scale disequilibrium across this landscape, illustrating the pervasiveness of transience across all spatial scales in this geomorphological setting.