Constraints on the rate of post-orogenic erosional decay from low-temperature thermochronological data: application to the Dabie Shan, China

We have investigated whether low temperature thermochronological datasets can be used to constrain the rate of surface evolution during the post-orogenic phase of a mountain belt. We use a numerical method to solve the heat transport equation in the Earth's crust, including the effects of a changing, finite-amplitude topography and the resulting flexural isostatic rebound. We demonstrate that accurate estimates of the amount of relief loss can be obtained by applying a recently developed spectral method that is based on estimates of the relationship between age and surface elevation as a function of topographic wavelength. We also show that the rate at which topography decays with time following cessation of tectonic activity can be constrained from estimates of exhumation rate derived from the slope of age-elevation profiles collected across short wavelength topography. Using the Neighbourhood Algorithm to perform a thorough search through parameter space, we are able to find a tectonomorphic scenario that predicts age distributions compatible with a thermochronological dataset collected in the Dabie Shan of eastern China by Reiners et al. (American Journal of Science 2003, vol. 303, pp. 489-518). We demonstrate that, in the Dabie Shan, the mean topographic relief has decreased by a factor of 2·5 to 4·5 during the last 60-80 Ma, while the mountain belt experienced a mean exhumation rate of 0·01 to 0·04 km Ma-1. We confirm the conclusions of Reiners et al. that there is no need to invoke a discrete Cenozoic tectonic event to explain the observed age distribution. The thermochronological dataset can also be used to put constraints on the effective elastic thickness of the lithosphere underlying the orogen (10 to 30 km). There is, however, a trade-off between elastic thickness, mean exhumation rate and amount of topographic relief loss. The most likely scenario also predicts that the topography has decreased at a constant rate since the end of orogenic activity about 100 Ma ago