AbstractsEarth & Environmental Science

The Rombaken Tectonic Window (RTW) is a basement culminations within the Caledonian thrust nappes in northern Norway. It appear as an inlier of Paleoproterozoic supracrustal rocks on Paleoproterozoic basement rocks. The RTW consist of several metasedimentary and metavolcanic N-S striking belts enclosed within large granitic bodies. The RTW marks the boundary between Archean basement rocks to the east and north and are an important link between similar-aged juvenile type rocks in Norway, Sweden and Finland. A multi phase, regional-scale transpressive ductile oblique-slip shear zone was identified in the RTW, termed the Rombaken-Skjomen Shear Zone (RSSZ). Four events of progressive deformation were documented; two east directed fold-thrust events with upright folds (D1 and D2) followed by a sinistral fold limb parallel oblique-slip (D3) shearing and a later (D4) diagonally crosscutting dextral oblique-slip steep ductile shear zone development. A granite crosscutting the D4 shear zone gives an approximate age of the deformation with SHRIMP Zircon U-Pb ages of ca, 1790 Ma. Gold and sulphide mineralisation The RSSZ is intinately associated with sulphide and gold mineralisation as evidenced by remobilisation of epigenetic SEDEX deposits, deposition of shear zone parallel metasomatic deposits or orogenic gold. The RSSZ can be traced E-NE into Sweden and is associated with similar mineral deposits. The extensive similarities of the Paleoproterozoic rocks within the inliers and outliers in Norway and the metasedimentary melts in the northern Fennoscandian shield in Sweden and Finland suggest a similar tectonic evolution consisting of breakup of the Archean continent into several basins, developing into island arcs and back arc basins, an active margin turning into accretion and finally, the Svecofennian orogeny. The orogeny consisted of accretion on to the Baltic Shield and involved crustal-scale structures with fold-thrust belts locally developing into steep oblique-slip shear zones. These structures are not only found in the RTW but in all similar domains in the whole northern Fennoscandian shield. The arcuate shape of the orogen, from E-W striking in Sweden to N-S striking in Norway, and the thinning and dissappearanv eof the Svecofennian rocks towards the NW are explained by the development of a secondary orocline in the western part of the Fennoscandian shield . The late dextral shear zones associated with D4 are interpreted as accommodation structures developing along the Archean-Proterozoic boundary when convergence shifted from N-S to a more E-W orientation. This major structure is critical to the understanding of the metallogenic deposits and may have acted as giant traps for juvenile fluids, especially when the known deposits are either remobilised or formed during the Svecofennian orogeny and its network of structures across the whole of the Fennoscandian shield.