Timing of Deformation in the Scandinavian Caledonides

Abstract

Western Scandinavia is composed of a variety of Caledonian nappes transported eastwards on to the Baltoscandian Platform. Many of the higher tectonic units provide evidence of complex deformational histories that both preceded and accompanied nappe emplacement. Palaeontological evidence for the timing of emplacement of the nappes in the southern and central parts of the mountain belt requires that the main nappe translation occurred after Llandovery deposition. In the mountain front, nappe advance resulted in deposition of extramontane continental sandstones in the Wenlock, the sedimentation continuing into the latest Silurian in the Oslo area, prior to folding and thrusting. This deformational sequence is referred to as Scandian (or Scandinavian). In northern Norway, isotopic age—determination evidence on syntectonic plutons, intruded into the Baltoscandian miogeocline, indicates that deformation (530-490 Ma) commenced in the late Cambrian and continued into the early Ordovician. This early orogenesis, involving polyphase folding, metamorphism and nappe translation, is referred to as Finnmarkian. The youngest sedimentary rocks in the mountain front in Finnmark are of Tremadoc age requiring that the main nappe emplacement occurred during or after the Arenig.

In the higher nappes of the Scandian allochthon, a variety of isotope age-determination evidence is indicative of a complex pre-Silurian tectonic and metamorphic history. Major unconformaties locally separate late Ordovician from older successions. In several areas, the obduction of fragmented ophiolites can be shown to have occurred prior to the Ashgill and in one case to be of pre-mid Arenig age. Thus, in the higher allochthonous units, containing the transported island-arc, back and fore-arc and Iapetus oceanic environments, present evidence favours a history of Caledonian deformation that was concentrated to an early orogenic episode (probably reaching a climax in the earliest Ordovician) and later (mid Silurian to early Devonian) depression, metamorphism and thrusting of these and the younger Ordovician and Silurian rocks on to the Balto-scandian Platform. Late-orogenic erosion of the nappe pile was followed by deposition of an Old Red Sandstone intermontane molasse of early Devonian and, perhaps locally, of Pridoli (latest Silurian) age; these sedimentary rocks were also subject to Caledonian (probably late Devonian) deformation.