Convective heat transfer in a steeply dipping fault zone and its impact on the interpretation of fission-track data - a modelling study

Abstract

AbstractThe effects of convective heat transfer by hydrothermal fluid flow on fission-track (FT) thermochronology are studied using numerical modelling techniques. Parameter studies are carried out on two-dimensional crustal segments with a steeply dipping fault zone exposed to constant denudation to evaluate the relative importance of different variables, including denudation rate as well as hydraulic and material properties. Time–temperature histories of particle points are calculated in the vicinity and also a few kilometres away of the fault zone. These time–temperature paths are then used in a forward-modelling approach to determine the expected FT cooling ages and track-length distributions.Modelling results indicate that hydrothermal fluid flow can significantly disturb the background conductive thermal state of the upper crust, and the interpretation of FT data using a steady-state geothermal gradient can result in erroneous denudation rates that overestimate the true erosion rates by more than 80%. A pattern of highly varied FT cooling ages from samples at the same elevation does not necessarily ask for differential tectonic movements, instead it can be generated by deep circulation of groundwater within a few million years (Ma). Denudation rates inferred from FT cooling age–elevation plots are likewise inaccurate in a hydrothermally active area because the important assumption about closure temperature isotherms being horizontal or at a constant depth below the surface is not met.