Direct 40ar/39ar Dating of Late Ordovician and Silurian Brittle Faulting in the Southwestern Norwegian Caledonides

Structural data as well as U–Pb zircon and 40 Ar/ 39 Ar biotite and muscovite ages were collected from the Rolvsnes granodiorite in western Norway. The granodiorite intruded at c . 466 Ma, cooled quickly and escaped later viscous deformation. Brittle top‐to‐the‐ NNW thrust faults (Set I) and WNW – ESE striking dextral strike‐slip faults (Set II ) formed in a NNW – SSE transpressional regime. 40 Ar/ 39 Ar dating of synkinematic mica from both sets reveals a c . 450 Ma (Late Ordovician) age of faulting, which constrains early‐Caledonian brittle deformation. Set I and II faults are overprinted by a set of lower‐grade, variably oriented chlorite‐ and epidote‐coated faults (Set III ) constraining WNW – ESE shortening. A lamprophyric dyke oriented compatibly with this stress field intruded at c . 435 Ma (Silurian), indicating that Set III formed at the onset of the Scandian Baltica–Laurentia collision. The preservation of Caledonian brittle structures indicates that the Rolvsnes granodiorite occupied a high tectonic level throughout the Caledonian orogeny.