Brittle basement deformation during the Caledonian Orogeny observed by K‐Ar geochronology of illite‐bearing fault gouge in west‐central Sweden

This study presents K‐Ar ages of illite‐bearing fault gouges in crystalline basement in central‐western Sweden. Samples were taken from two faults that localized brittle deformation marginal to and within mafic dikes that intruded Paleoproterozoic granitoids. K‐Ar ages from 10 separated grain fractions span from 823 to 392 Ma. Older ages obtained (823–477 Ma) were influenced by a mixture of illite and K‐feldspar; the latter likely formed during a hydrothermal event prior to faulting. The remaining ages (442.1 ± 9.7 to 391.7 ± 6.1 Ma) were obtained from fractions from both faults hosting only authigenic illite and show that illite crystallized during the Scandian phase of the Caledonian orogeny. These results indicate that previously presumed autochthonous Caledonian basement was involved in continental contraction and subsequent collapse of the Caledonian orogen, influencing both the mode and depth of penetration of deformation into Baltica.