Brittle basement deformation during the Caledonian Orogeny

34 This study presents K-Ar geochronology ages of illite from fault gouge in the crystalline 35 basement in central-western Sweden. Samples of fault gouge were taken from two faults 36 localizing brittle deformation along and within mafic dikes that intrude Paleoproterozoic 37 granites. K-Ar ages from the ten dated fractions span from 823 Ma to 392 Ma. The older ages 38 obtained, spanning from 823 to 477 Ma, are influenced by a mixture of illite and K-feldspar, the 39 latter which likely formed during a hydrothermal event prior to faulting. The remaining ages are 40 obtained from fractions hosting only authigenically formed illite. The illite dominated gouge, 41 from both faults, show that illite crystallized during the Caledonian orogeny, with a range in ages 42 from 442.19.7 to 391.76.1 Ma. These results indicate that basement likely played a significant 43 role in continental contraction during the Caledonian Orogeny, influencing both the mode and 44 penetration of deformation into the continent. 45 46