Topography, exhumation pathway, age uncertainties, and the interpretation of thermochronometer data

Abstract

The relationship between thermochronometer age and structural elevation is commonly used to infer long‐term exhumation histories. Previous studies suggest that inferred exhumation rates from the conventional (one‐dimensional, 1‐D) age‐elevation approach are sensitive to topography and variations in exhumation rate and pathway. Here we evaluate the magnitude of these effects by (1) using a 3‐D thermal‐kinematic model of the central Nepalese Himalaya to predict age‐elevation profiles for multiple thermochronometers as a function of exhumation rate and pathway (vertical, oblique, or thrust fault), and (2) calculating the probability that the true exhumation rate will be recovered from an age‐elevation profile for sample uncertainties of different magnitudes. Results suggest that profiles oriented orthogonal to long‐wavelength topography and the direction of lateral transport are relatively insensitive to their influence. For profiles oriented parallel to the transport direction, horizontal transport during exhumation partly counteracts topographic effects. The difference between model imposed and 1‐D exhumation rates from the slope of a best fit line through an age‐elevation plot is greatest when rocks are exhumed vertically and low‐temperature thermochronometers are used. The magnitude of error in 1‐D exhumation rate estimates varies dramatically as a function of sample uncertainty, particularly when exhumation is rapid. The nature of this variation can be used to design sampling strategies for which 1‐D interpretations of age‐elevation gradients are likely to be within error of the true exhumation rate. Alternatively, if sample uncertainties can be reduced, studies that combine thermal modeling with age‐elevation data can potentially provide important constraints on thermal and kinematic fields at depth.