Caledonian basement deformation related to oblique continent–continent collision in Scandinavia – insight gained from the interpretation of potential field data

The tectonic status, the mode of Caledonian deformation and the transport distance of the Precambrian units on the Fennoscandian Shield that constitute the so-called tectonic windows within the realm of the Scandinavian Caledonian nappe pile have long been a subject of dispute. It has been argued for several hundred kilometres of eastward transport for some of these tectonic units, whereas parautochthonous to autochthonous positions have also been suggested. In this contribution, geophysical data have been studied to shed light on this controversy. A dominant regional NNW–SSE to north–south structural grain of Precambrian origin extends beneath the Caledonian nappes. Indeed, similar structural patterns appear in several of the tectonic windows within the orogen. However, magnetic anomalies in southern Norway and the Nordland area show an apparent c. 90° counterclockwise rotation into the Caledonian trend along a line from Nordfjord in western Norway to Senja in northern Norway, whereas the anomalies in Finnmark (northernmost Norway), seem to be rotated clockwise along a line from Kvænangen to Porsangerfjorden. We suggest that this pattern extending through the Caledonides likely represents a boundary between two crustal domains with different Caledonian reworking. The eastern domain demonstrates less extensive modification during the Caledonian continent–continent collision. Some thrusts within basement windows in the eastern domain accommodated transport of a few tens of kilometres, revealing a parautochthonous status for most of these windows. Interpretations of aeromagnetic and gravity data have shown that the Caledonian nappes occurring along detachments extend to a maximum depth of c. 9 km below sea-level. The thickest nappe piles occur along the late Caledonian detachments. The eastern basement domain with minor Caledonian deformation has a thinner nappe stack (up to 4–5 km below). Our combined interpretations also show the need to modify the tectonostratigraphy of the northern Scandinavian Caledonides. The Lower Laksefjord Nappe in Finnmark, for instance, is interpreted as a parautochthonous window into the Precambrian basement and is not part of the Laksefjord Nappe Complex that has been transported hundreds of kilometres.