ONTOGENIES OF OROGENIES AND LANDSCAPE ERODIBILITY

Understanding how erosion rates are influenced by tectonic activity, climate and lithology is crucial for interpreting the evolution of Earth’s topography. Here we investigate the relationship between ¹⁰Be erosion rates and topographic gradient across tectonic provinces of the continental U.S. using morphometric methods and landscape evolution modeling. Regressions of log erosion rate vs log average basin gradient are positively correlated for each of the tectonic provinces but are significantly different among each other, suggesting that erosion rates are partially determined by tectonic province and partially determined by topographic gradient. For any given average basin gradient, average erosion rate might vary by up to four orders of magnitude. Average erosion rates are highest in the Pacific ranges and lowest in the Appalachians. Landscape evolution models show how different rock erodibilities, uplift rates and topographic gradient are interrelated. We conducted three modelling experiments, in each of which we consider landscape evolution models for three spatially independent geologic provinces. If erodibility (k) is fixed in each model province but rock uplift rate (U) is different, all the points plot a linear log gradient versus log rate but mostly separate on gradient because higher rock uplift rates result in higher topographic gradient. If erodibility differs among provinces with the same rate of uplift, different provinces exhibit the same gradient versus rate slope, but are laterally separated such that lower gradients occur across provinces with greater erodibility. Only the experiment that incorporates different rates of uplift and erodibility yields results similar to those observed in the 10Be database.

We interpret the data to show that rock erodibility, as inferred from relations between topographic gradient and erosion rate broadly decreases with the increasing age of deformation and with distance from the tectonically active western margin. We suggest that surficial rocks in younger (western) orogens likely exhibit greater brittle deformation and erode more rapidly, while less-fractured and less-erodible units are progressively exhumed over the ontogeny of an orogen.