Fault gouge and mylonite samples of prominent faults (AU 126 m and AU 96 m) occur at the Grimsel Test Site (underground research laboratory) in the Aar Massif. Fractions < 40, 5–20 and < 1 μm were separated, and the minerals investigated and compared with the Grimsel ‘granodiorite’. Size fractions were separated and their mineralogy was examined by SEM, XRD, DTG and IR-spectroscopy techniques.

The phengite, green biotite, plagioclase and K-feldspar of the penetratively deformed host Grimsel ‘granodiorite’ form a Rb-Sr isochron of 19 ± 3.9 Ma. The deformation has a similar K-Ar age. The Rb-Sr data from the phengite and albitic plagioclase of the gouges and the mylonite samples suggest an earlier isotopic exchange 26 Ma years ago. The fault gouge and mylonite seems to have been formed during an early hydrothermal-tectonic pulse of the Alpine metamorphism. However, there may be small amounts of inherited 87Sr, and a mixing line cannot be excluded.

The apparent K-Ar ages of the <40 size fractions of the Grimsel ‘granodiorite’ decrease from 10.4 Ma to 6.6 Ma, suggesting recrystallization processes during continuous fault activity. Small amounts of chlorite/smectite (vermiculite?), developed from biotite or chlorite, indicate continuous activity at low temperatures. Radium 226 mobilization and tritium, 18O, 2H values from percolating waters in a parallel fault confirm the existence of recent fluid activities from <8000 years BP to the present day.

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