Timing and conditions of brittle faulting on the Silltal-Brenner Fault Zone, Eastern Alps (Austria)

The Silltal Fault is the northern brittle continuation of the Brenner Fault Zone, marked by a narrow zone of cataclasis and, in three locations, clay-rich fault gouges. The clay mineral composition of these gouges is dominated by higher temperature 2M1 polytype illite/muscovite, with no 1M/1Md illite or mixed layer illite/smectite detected. Smectite is limited to the northern samples from the Stephansbrucke location, whereas chlorite is present in all samples. The K–Ar ages from the different sample size fractions (<0.1, <0.4, <2, 2–6, 6–10 μm, “whole rock gouge”) show a wide spread, from ca. 115 to 12 Ma, with ages consistently decreasing with grain size. Although the ranges overlap, ages from the northern Stephansbrucke samples are generally older (115–36 Ma) than those from the south near Matrei (55–12 Ma), possibly reflecting increasing regional metamorphic temperatures to the south. The well-defined linear relationship between apparent age and hydrogen stable isotope (δD) values establishes a direct correlation between rejuvenation of the K–Ar system and increased interaction with meteoric water introduced and focussed within the fault zone. The dependence of both apparent age and δD on grain size also indicates that radiogenic and stable isotope exchange was controlled by grain size, reflecting new 2M1 illite growth, mechanical grinding of protolith muscovite during cataclastic faulting, or both. The results demonstrate the advantages of combining radiogenic and stable isotope analysis in interpretation of K–Ar ages from clay fault gouges. The combined approach was necessary to establish the crucial influence on apparent K–Ar ages of meteoric water influx focussed on the fault zone and its interaction with clay-size-fraction grains.