TSK 11 Göttingen 2006 Glotzbach et al. Perturbation of isotherms below topography: con- straints from tunnel transects through the Alps, Gotthard road tunnel Poster Christoph Glotzbach1 Cornelia Spiegel1 Meinert Rahn2 John Reinecker1 Introduction For many years it has been known that near surface isotherms are influenced by the topography (Lees 1910). Re- cently, a number of studies were pur- sued to quantify the effect of topogra- phy on low temperature isotherms (e.g. Stüwe et al. 1994, Mancktelow & Grase- mann 1997). The magnitude of pertur- bation depends on several parameters: exhumation rate, geothermal gradient, wavelength and amplitude of topogra- phy, and finally by the age of surface relief change (Braun 2002). Modelled perturbation To obtain a rough impression of pertur- bation of near surface isotherms, a 2- D modelling approach following Stüwe & Hintermüller (2000) was applied to a profile intersecting the Aar and Got- thard massifs along the Gotthard road tunnel. Assuming steady state topog- raphy, wavelength of 20km, a tempo- rally and spatially constant exhumation rate of 1mmy−1, a geothermal gradient of 20°Ckm−1, and height, slope and as- pect dependent ground surface temper- atures, the modelling results reveals sig- nificant perturbation of the near-surface isotherms (Fig. 1). 1 Institut für Geowissenschaften, Eber- hard-Karls-Universität Tübingen 2 Haupt- abteilung für die Sicherheit der Kernanlagen, Villigen-HSK, CH Figure 1: Results of 2-D modelling along the Gotthard road tunnel, showing the to- pography of the tunnel-transect, and the modelled 60°C and 110°C isotherms. The modelled 60°C and 110°C isotherms are perturbed by 870 and 400m re- spectively, suggesting that the isotherm- perturbation effect significantly influ- ences the apatite fission track (AFT), and particularly the (U-Th)/He-system. To verify these modelled prediction, our study aims to directly measure perturbation of isotherms below to- pography by applying low-temperature thermochronology (zircon fission track, AFT and (U-Th)/He analysis). We therefore sampled three tunnel transects through the Alps (Gotthard and Mont Blanc road tunnels and Lötschberg rail- way tunnel), as well as their correspond- ing surface lines. The investigated re- gions are characterised by pronounced topography and rapid present-day sur- face uplift rates in the range of 1mmy−1 (Kahle et al. 1997). Measure perturbation AFT data from the literature (Schaer et al. 1975; Wagner et al. 1977) and own data were projected from within a corridor of 1 km along the tunnel-axis (Fig. 2). By linear interpolation of the AFT-ages, isochrones (here the 9, 8, 7 and 6.5My isochrones) can be esti- 1 Glotzbach et al. TSK 11 Göttingen 2006 mated. The modelled isotherms (Fig. 1) and estimated isochrones (Fig. 2) show comparable perturbations correlating with topography. For palaeotopo- graphic investigations the sample den- sity has been increased along the tunnel transect. Currently, the AFT-age den- sity is too low for palaeotopographic in- terpretations. Implications for age-elevation rela- tionships (AER) Exhumation rates are routinely de- duced from age-elevation relationships (i.e. from AFT-ages plotted vs. sam- ple elevation). This approach, however, is based on the assumption of flat-lying isotherms. For perturbed isotherms, exhumation rates deviated from AERs are overestimated (Stüwe et al. 1994). Fig. 3a shows conventional AERs from the Gotthard and Aar massifs, yielding exhumation rates of 0.45mmy−1 and 0.54mmy−1, respectively. Simplified 3-D modelling of the 110°C-isotherm, based on equations and input param- eters mentioned above, yields modified AERs, plotted against the distance from present elevation to the modelled 110°C- isotherm (Fig. 3b). In contrast to other correction ap- proaches (e.g. Reiners et al. 2003), this procedure allows to correct every AFT- sample separately, accounting for their specific spatial topographic location. The resulting ‘real’ exhumation rates are about 10% lower than the apparent exhumation rates revealed from conven- tional AER, yielding 0.42mmy−1 for the Gotthard massif and 0.46mmy−1 for the Aar massif. Future investigations Samples collected from three tunnel systems and their corresponding sur- face traces will be analysed by AFT, Figure 2: Sample pattern, available AFT-ages and interpolated isochrones along the Gotthard road tunnel transect. 2 TSK 11 Göttingen 2006 Glotzbach et al. Figure 3: AFT-ages for the Gotthard and Aar massifs, plotted against their sampling altitude, yielding a clear age elevation relationship (a) and the same AFT-ages plotted against the distance from present elevation to the modelled 110°C-isotherm (b). zircon fission track, and apatite (U- Th)/He thermochronology. This will al- low to estimate the effect of isotherm- perturbation on these thermochrono- logic systems. The existing thermo- erosive model will be refined, and dif- ferent kind of models shall be tested for their consistency with the grow- ing amount of low thermochronological data. References Braun J (2002) Quantifying the effect of recent relief changes on age-elevation relationships. Earth Planet Sci Lett 200:331–343 Kahle H-G, Geiger A, Bürki B, Gubler E, Marti U, Wirth B, Rothacher M, Gurtner W, Beut- ler G, Bauersima I & Pfiffner OA (1997) Re- cent crustal movements, geoid and density distribution: Contribution from integrated satellite and terrestrial measurements. In: Pfiffner et al (eds) Results of NRP 20; deep structure of the Swiss Alps. Birkhäuser Ver- lag, Basel, 251–259 Lees CH (1910) On the shape of the iso- geotherms under mountain ranges in radio- active districts. Proc R Soc London A 83, 295–317 Mancktelow NS & Grasemann B (1997) Time- dependent effects of heat advection and to- pography on cooling histories during erosion. Tectonophysics 270, 167–195 Reiners PW, Zhou Z, Ehlers TA, Xu C, Bran- don MT, Donelick RA & Nicolescu S (2003) Post-orogenic evolution of the Dabie Shan, eastern China, from (U-Th)/He and Fission- Track Thermochronology. American Journal of Science 303, 489–518 Schaer JP, Reimer GM & Wagner GA (1975) Actual and ancient uplift rate in the Got- thard region, Swiss Alps: A comparison be- tween precise levelling and Fission-Track Ap- atite age. Tectonophysics 29, 293–300 Stüwe K, White L& Brown R (1994) The influ- ence of eroding topography on steady-state isotherms. Application to fission track anal- ysis. Earth Planet Sci Lett 124, 63–74 Stüwe K & Hintermüller M (2000) Topogra- phy and isotherms revisited: the influence of laterally migrating drainage divides. Earth Planet Sci Lett184:287-303. Wagner GA, Reimer GM & Jäger E (1977) Cooling ages derived by apatite fission track, mica Rb-Sr and K-Ar dating: the uplift and cooling history of the Central Alps. Mem Ist Geol Mineral Univ Padova 30, 1–27 3