Timing of Alpine fault gouges

K–Ar ages from clay-rich fault gouges in the European Alps are consistent internally, with established field constraints and with fission track ages, demonstrating the applicability of this method for direct dating of brittle deformation. Illite grown by retrograde hydration of granitic and/or high-grade metamorphic protoliths is the major K-bearing mineral in the fractions that have been separated (<0.1 to 6–10 μm). The ages obtained are bracketed by apatite and zircon fission ages from adjacent localities. This, together with the interpreted cooling curve for one well-constrained sample from the Simplon Fault, indicates that illite grew (or the K–Ar isotopic system in illite closed) at temperatures in the range 120–150 °C, consistent with the stability of illite alone rather than illite–smectite. Average ages obtained for fractions finer than 2 μm were: (1) 5.2±2.6 Ma (2σ) for 11 analyses from the detachment and footwall of the Simplon Fault; (2) 8.8±0.6 Ma for four analyses from the Centovalli Fault at Trontano; (3) 19.9±2.6 Ma for two analyses from a major Riedel fault related to the Periadriatic Fault, offsetting the Tertiary Bergell tonalite at Gesero; (4) 21.5±5.5 Ma for two analyses from a zone parallel to the Periadriatic Fault in the adjacent Southern Alps in Val Morobbia; and (5) 14.5±3.9 Ma for two analyses from a gouge zone on the northern rim of the Tertiary Mauls tonalite in the Eastern Alps. The ages from the Simplon, Centovalli and Periadriatic Faults are not identical and do not lend support to models linking these three faults as a single, coeval structure.