Determining the effect of topography on isotherms in the shallow crust using low temperature thermochronometry in tunnels

Apatite (U-Th)/He and fission track age determinations of samples from a 20-km-long, near-horizontal tunnel in the Hochalm-Ankogel Dome (eastern Tauem Window, Austria) are presented in order to determine the role of (paleo)-topography in perturbing isotherms in the shallow crust. Apatite fission track ages (26-8 Ma) show no systematic correlation with distance along the tunnel or elevation. Two age components in the fission track data indicate cooling through ∼120°C at approximately 20 Ma and ∼80°C at approximately 6 Ma. Surface and tunnel (U-Th)/He ages (17-9 Ma and 13-5 Ma, respectively) are consistently younger than the equivalent fission track ages. (U-Th)/He ages vary systematically along the tunnel with older ages at the northern and southern tunnel portals and younger ages in the central section. Geological factors (faulting, lithology) appear to have had little effect on this age distribution. The (U-Th)/He age pattern is inconsistent with rock cooling underneath the present-day Hochalm-Ankogel Dome topography. The age minimum is interpreted to coincide with a paleotopographic maximum in the Hochalm-Ankogel Dome and suggests that the 40-60°C isotherms were warped sufficiently to affect apatite (U-Th)/He ages. The tunnel apatite (U-Th)/He ages match synthetic He ages derived from a two-dimensional numerical model of landscape evolution. Integrating the thermal model results with the data allows the relief development to be constrained and we calculate that the present-day Hochalm-Ankogel Dome topography was formed at 7 to 10 Ma. Copyright 2007 by the American Geophysical Union.