A tectonic model for Scandian terrane accretion in the northern Scandinavian Caledonides

Abstract

Muscovite and hornblende 40 Ar/ 39 Ar ages have been obtained from several structural levels within the Caledonian nappe stack of Troms, northern Norway. Meaningful ages range between c. 425 Ma and 370 Ma, suggesting protracted post-metamorphic cooling from Early Silurian to Late Devonian times. Whilst the possibility of earlier ‘Finnmarkian’ (Late Cambrian-Early Ordovician) histories is not precluded by these data, they provide further evidence of intense Scandian tectonothermal activity in this portion of the Scandinavian Caledonides. Three structural units within the nappe pile record hornblende 40 Ar/ 39 Ar ages ranging from 425 to 421 Ma. One of these records a similar muscovite cooling age, taken to indicate rapid nappe exhumation from peak Scandian metamorphism as Laurentia and Baltoscandia collided in the Late Llandovery and Wenlock. All other 40 Ar/ 39 Ar muscovite ages from the nappe pile range between 395 ± 1 and 373 ± 1 Ma. These are thought to track the progression of the 350°C crustal isotherm as it relaxed through the tectonically thickened Caledonian crust. Major internal thrusts appear to have been largely inactive at this time and acted as sites of enhanced fluid flow and retrogression. Further accretion of the orogenic wedge to the Baltoscandian margin was characterized by piggy-back style translation of the nappes on a low-angle basal decollement. This was accompanied by large-scale basement reactivation and out-of-sequence thrusting, resulting in further interleaving of the nappe stack in the Late Devonian (Frasnian).