Structure and composition of the Precambrian crust along the POLAR Profile in the northern Baltic Shield

Abstract

Along the 500-km long POLAR Profile, the continental crust of the northern Baltic Shield consists of several Archaean and Early Proterozoic crustal segments.In the south, in the Karelian Province, the upper crust consists of a 3.1–2.6 Ga old Archaean substratum with a 0–6-km thick, low- to medium-grade volcanosedimentary cover. Here, shallow-basin facies rocks of the Central Lapland Complex have been thrust northwards onto the Karasjok-Kittilä Greenstone Belt. The repeatedly folded cover rocks form linear troughs between domes and culminations of granitoids. The 47–48-km thick crust of the Karelian Province is of a shield type. It is characterised by a low-velocity layer interpreted as a horizon of décollement below the supracrustal and granitoid rocks.In the Lapland Granulite Belt, Early Proterozoic high-grade metapelites and metapsammites with minor metaigneous intercalations have been thrust to the southwest. They are separated from the underlying Karelian Province by the tectonic mixture zone of the Tanaelv Belt. In the interpretations, two alternative models are given for the shape of the Lapland Granulite Belt: it is either a synformal nappe or a NE-dipping body pinching out at depth and overthrust from the north by the Archaean Inari Terrain. Both the Lapland Granulite Belt and the Inari Terrain have a 42–44-km thick crust with a relatively thin lower crust marked by lower seismic velocities.The Early Proterozoic Polmak-Pasvik-Pechenga Belt separates the Archaean Inari Terrain from the Archaean Sørvaranger Terrain in the north. This belt is interpreted as a suture zone which continues to depth. It may either dip steeply towards irregularities in the Moho or form a SW-dipping low-angle plane below the Lapland Granulite Belt. The northernmost part of the Baltic Shield is concealed beneath a 0–7-km thick, very low-grade metamorphic Late Proterozoic cover. This cover is divided into two parts by the Trollfjord-Komagelv strike-slip fault. Here, the crust is again of a normal shield type with a thickness of 42–46 km.