Multiply deformed Paleoproterozoic foreland fold and thrust belt in northern Fennoscandia - The peripheral Kuusamo belt as a key example

The Central Lapland belt (CLB), Perapohja belt (PB), and Kuusamo belt (KB) form an important part of northern Fennoscandia and record ca. 700 Ma (2.5-1.8 Ga) of Precambrian tectonic processes. The 2.5-2.1 Ga sedimentary-volcanic successions in these belts form a basin-wide surfaces with a wide lateral continuum. Tidal influenced shallow epeiric sea setting dominated at 2.4-2.1 Ga followed by a continental breakup and subsequent formation of a passive margin at 2.1 Ga. The collisional evolution (1.93-1.76 Ga) started with an E-W, locally NW-SE and NE-SW, compressional phase (Dl; 1.93-1.90 Ga) that resulted in at least 600 km long and in part over 300 km wide E-vergent Lapland foreland fold and thrust belt (FTB). The KB is the best preserved example of this FTB and shows E-vergent imbricated structures in the east and E-vergent out of sequence thrusting in the west. The southern part of the KB has been affected by N-S shortening (D2) seen as upright to slightly inclined open to tight F2 folds and as oval-shaped dome and basin structures. The D3 overprinting causes folding with NW-SE trending spaced foliation in less competent rocks and parallel folds and faults in orthoquartzites. The D4 NE-SW trending overprinting deformation is characteristic for the high grade areas of the KB and is seen in outcrop to map scale tight folding. In general, the easternmost peripheral part of the E-vergent Lapland foreland FTB, formed upon non-stretched Archean basement, is only moderately affected by the later deformation events. In the stretched part of the Archean basement the earlier rift structures have facilitated the N-S shortening and the D2 structures dominate the map pattern. The D3, due to a SW-NE shortening, shows strong strain partitioning and localized deformation. After prolonged heating the Archean basement started to act ductile and long wavelength crustal-scale NE-SW trending D4 folds formed. The metamorphic zonation in the KB from greenschist facies in the south to medium pressure amphibolite facies in the north is explained by a major D4 anticlinorium (wavelength 70-100 km).