Methods for dating shallow faults in the Earth's crust are still evolving and their reliability remains controversial. Based on results from well-preserved fault gouge samples from the AlpTransit deep tunnel site in Switzerland, we propose a simplified and readily applicable approach to investigate and date clay-rich gouge zones. We collected 10 samples covering a north-south section of 15 km. All sampled faults are developed in orthogneiss host rock, have a rather consistent approximately northwest-southeast strike, are mainly normal, and are related to northeast-southwest orogen-parallel extension during Alpine exhumation. Fine-grained clay separates from the fault gouges consist of illite 1M/1Md and 2M1 polytypes, smectite and chlorite, with minor amounts of feldspar in some sample fractions. The K-Ar ages for <2 μm illite fractions range between 7.1 and 9.5 Ma (mean 8.5 ± 1.4 [2σ] Ma) and for <0.1 μm range between 3.9 and 7.2 Ma (mean 6.0 ± 2.1 [2σ] Ma); ages consistently decrease with grain size. These ages are close to published apatite fission track ages of ca. 6 Ma from the immediate vicinity, consistent with illite stability in fault gouges generally occurring at temperatures slightly above the partial annealing zone of apatite. The influence on authigenic illite gouge ages of potential contamination by fine-grained cataclastic protolith was evaluated by dating coarse K-feldspar fractions from host-rock clasts in two samples. The K-feldspar ages are significantly older, ca. 13.5 Ma. However, measured illite K-Ar ages are quite constant and do not correlate with amount of K-feldspar impurity, which suggests that fine-grained cataclastic feldspar grains have isotopically reequilibrated, presumably due to fluid-rock interaction within the fault zone.

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