The impact of convective heat transfer on the interpretation of fission track data - numerical models and case studies
Final Report Abstract
The project uses a coupled hydraulic-thermal modeling approach to assess the combined effects of conductive and convective heat transport on the upper crustal temperature field and, hence, on apatite cooling ages and FT length distributions. Numerical modeling utilizes finite element techniques and describes a 2D cross-section through the upper crust in the vicinity of a steeply dipping fault zone. Various parameter studies were carried out to study the relative importance of hydraulic and material properties as well as denudation rate for perturbations in the thermal field and age data. High fault zone conductivities and high hydraulic head differences have the greatest effect on the distribution of the apatite cooling ages, but several other parameter combinations also result in variable cooling ages and FT length distributions in spite of a laterally uniform denudation rate. Models were further elaborated by incorporation of an evolving topography, i.e. enhanced erosion rates at the fault’s surface exposure and valley incision, respectively. Modeling results show that deep fluid circulation, both alone and in combination with an evolving topography, can significantly perturb the thermal regime of the upper crust. This in turn also affects the cooling age and time-temperature history recorded in the apatite FT length distribution. For example, deep fluid circulation can cause differences in apatite cooling ages of several Ma for samples exhumed at the same erosion rate from the same depth of a paleo-geothermal system. Evolving topography and topographically-driven fluid flow can also have a severe impact on the interpretation of thermochronological data in age-elevation plots. Firstly, the basic assumptions about closure temperature isotherms being horizontal or at a constant depth below the surface are not fulfilled. Secondly, topography-driven fluid flow can result in large convection cells in which downward fluid flow lowers the thermal gradient in the uppermost part of the crust and results in significantly older AFT and (U- Th)/He ages than in the case of purely conductive heat flow. In other areas, ascending hot fluids result in younger ages. These complexities strongly suggest that for a refined interpretation of lowtemperature thermochronometers with respect to denudation rates thermal models should be used which can incorporate the effects of both convective and conductive heat transport. The study clearly indicates the importance of spatial and temporal variations in crustal temperature fields for a quantitative understanding of FT data.
Publications
- (2006): The effect of convective heat transfer on fission-track data: 2D parameter studies using the finite element software FEFLOW. – International FEFLOW User Conference, September 10-15, 2006, Berlin, Germany
Timar-Geng, Z., Henk, A. & Wetzel, A.
- (2006): The influence of convective heat transport on the interpretation of fission-track data - numerical models and case studies. – Schriftenreihe der Deutschen Gesellschaft für Geowissenschaften, 49 (European Conference on Thermochronology, July 30 - August 04, 2006, Bremen): 147-149
Timar-Geng, Z., Henk, A. & Wetzel, A.
- (2006): Two-dimensional finite element models of convective heat transfer in the upper crust - implications for the interpretation of fission-track data. – 11. Symposium “Tektonik, Struktur- und Kristallingeologie”, 22. - 24.03.06, Göttingen: 228
Timar-Geng, Z., Henk, A. & Wetzel, A.
- (2007): Modelling the combined impact of eroding topography and fluid flow on apatite fission-track and (U-Th)/He data. – EGU General Assembly, 15. - 20.04.07, Wien.
Timar-Geng, Z., Henk, A. & Wetzel, A.
- (2009): Convective heat transfer in a steeply dipping fault zone and its impact on the interpretation of fission-track data - a modeling study. - In: Lisker, F., Ventura, B. & Glasmacher, U. (eds.): Thermochronological methods: from paleotemperature constraints to landscape evolution models". - Geological Society, London, Special Publications, 324: 87- 98
Timar-Geng, Z., Henk, A. & Wetzel, A.
