Bjarne Almqvist, Roelant van der Lelij, Karin Högdahl, Rodolphe Lescoutre, Jasmin Schönenberger, Haakon Fossen, Håkan Sjöström, Christopher Juhlin, Stefan Luth, Susanne Grigull, Giulio Viola

This study presents K-Ar geochronology ages of illite from fault gouge in the crystalline basement in central-western Sweden. Samples of fault gouge were taken from two faults localizing brittle deformation along and within mafic dikes that intrude Paleoproterozoic granites. K-Ar ages from the ten dated fractions span from 823 Ma to 392 Ma. The older ages obtained, spanning from 823 to 477 Ma, are influenced by a mixture of illite and K-feldspar, the latter which likely formed during a hydrothermal event prior to faulting. The remaining ages are obtained from fractions hosting only authigenically formed illite. The illite dominated gouge, from both faults, show that illite crystallized during the Caledonian orogeny, with a range in ages from 442.19.7 to 391.76.1 Ma. These results indicate that basement likely played a significant role in continental contraction during the Caledonian Orogeny, influencing both the mode and penetration of deformation into the continent.