TSK 11 Göttingen 2006 Glotzbach et al.

Perturbation of isothermsbelow topography: con-straints from tunnel transectsthrough the Alps, Gotthardroad tunnel Poster

Christoph Glotzbach1 CorneliaSpiegel1 Meinert Rahn2 JohnReinecker1

IntroductionFor many years it has been known thatnear surface isotherms are influencedby 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 surfacerelief change (Braun 2002).

Modelled perturbationTo obtain a rough impression of pertur-bation of near surface isotherms, a 2-D modelling approach following Stüwe& Hintermüller (2000) was applied toa profile intersecting the Aar and Got-thard massifs along the Gotthard roadtunnel. Assuming steady state topog-raphy, wavelength of 20 km, a tempo-rally and spatially constant exhumationrate of 1 mmy−1, a geothermal gradientof 20°C km−1, and height, slope and as-pect dependent ground surface temper-atures, the modelling results reveals sig-nificant perturbation of the near-surfaceisotherms (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 alongthe Gotthard road tunnel, showing the to-pography of the tunnel-transect, and themodelled 60°C and 110°C isotherms.

The modelled 60°C and 110°C isothermsare perturbed by 870 and 400 m 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 measureperturbation of isotherms below to-pography by applying low-temperaturethermochronology (zircon fission track,AFT and (U-Th)/He analysis). Wetherefore sampled three tunnel transectsthrough the Alps (Gotthard and MontBlanc road tunnels and Lötschberg rail-way tunnel), as well as their correspond-ing surface lines. The investigated re-gions are characterised by pronouncedtopography and rapid present-day sur-face uplift rates in the range of 1mm y−1

(Kahle et al. 1997).

Measure perturbation

AFT data from the literature (Schaeret al. 1975; Wagner et al. 1977) andown data were projected from within acorridor of 1 km along the tunnel-axis(Fig. 2). By linear interpolation of theAFT-ages, isochrones (here the 9, 8,7 and 6.5 My isochrones) can be esti-

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Glotzbach et al. TSK 11 Göttingen 2006

mated. The modelled isotherms (Fig. 1)and estimated isochrones (Fig. 2) showcomparable perturbations correlatingwith topography. For palaeotopo-graphic investigations the sample den-sity has been increased along the tunneltransect. 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-lyingisotherms. For perturbed isotherms,exhumation rates deviated from AERsare overestimated (Stüwe et al. 1994).Fig. 3a shows conventional AERs fromthe Gotthard and Aar massifs, yieldingexhumation rates of 0.45 mm y−1 and

0.54 mm y−1, respectively. Simplified3-D modelling of the 110°C-isotherm,based on equations and input param-eters mentioned above, yields modifiedAERs, plotted against the distance frompresent elevation to the modelled 110°C-isotherm (Fig. 3b).In contrast to other correction ap-proaches (e.g. Reiners et al. 2003), thisprocedure allows to correct every AFT-sample separately, accounting for theirspecific spatial topographic location.The resulting ‘real’ exhumation ratesare about 10% lower than the apparentexhumation rates revealed from conven-tional AER, yielding 0.42mm y−1 forthe Gotthard massif and 0.46mm y−1

for the Aar massif.

Future investigationsSamples collected from three tunnelsystems and their corresponding sur-face traces will be analysed by AFT,

Figure 2: Sample pattern, available AFT-ages and interpolated isochrones along theGotthard road tunnel transect.

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TSK 11 Göttingen 2006 Glotzbach et al.

Figure 3: AFT-ages for the Gotthard and Aar massifs, plotted against their samplingaltitude, yielding a clear age elevation relationship (a) and the same AFT-ages plottedagainst 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 testedfor their consistency with the grow-ing amount of low thermochronologicaldata.

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