Quantifying relationships between erosion and uplift in tectonically active landscapes, Italy, using cosmogenic nuclides and channel morphometry

Andrew J Cyr, Purdue University

Abstract

Landforms reflect a competition between tectonic, climatic, and surficial processes. Many landscape evolution models that explore feedbacks between these competing processes, given steady forcing, predict a state of erosional equilibrium, where the rates of river incision and hillslope erosion balance rock uplift. It has been suggested that this may be the final constructive stage of orogenic systems. Assumptions of steady erosion and incision are used in the interpretation of exhumation and uplift rates from different geologic data, and in the formulation of fluvial incision and hillslope evolution models. However, it has been argued that steady-state is difficult to achieve, since landscape response time may be long compared to unsteady tectonics or Quaternary climatic fluctuations. This dissertation addresses this central question by comparing erosion rates, determined from cosmogenic nuclide concentrations in modern and ancient stream sediments, and morphometric indices, such as hillslope gradient and bedrock river channel steepness index, in different landscapes in Italy where the tectonic forcing is well constrained. These data are used to explore methods for identifying steady-state and quantitatively testing relationships between uplift rates, erosion rates, and landscape form. A steady-state landscape predicts that incision, erosion, and uplift rates are uniform in space and steady through time. However, it has not been demonstrated over what spatial and temporal scales steady-state can be achieved and maintained, and the functional relationships between incision, erosion, and uplift predicted by conceptual and analytical models of steady-state landscapes remain relatively untested. Erosion rate and morphometric data presented here demonstrate that (1) steady-state can be achieved and maintained in tectonically active landscapes for at least ∼1 Ma, (2) spatial patterns of bedrock channel steepness index may be able to identify steady-state and that in these landscapes erosion rates scale linearly with rock uplift rates, (3) along-channel patterns of erosion rate and channel steepness are a useful tool for determining whether landscape form is controlled by tectonic forcing or variable rock resistance, and (4) that although specific relationships between erosion and different morphometric indices may be unique to any particular location, the general relationships appear to be robust, and applicable to nearly any landscape.

Degree

Ph.D.

Advisors

Granger, Purdue University.

Subject Area

Physical geography|Geology|Geomorphology|Geochemistry

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