The 1.79 Ga Särkilahti leucogranite – a horizontal magma layer below granulite-grade migmatites in SE Finland

We studied the basement-cover relations in North Karelia, eastern Finland by modelling (1) regionally the thickness of the cover sequence and (2) the geometry of the Sotkuma dome (basement inlier). We utilized geological and aeromagnetic maps, together with the abundant structural data, in compilation of the cross-sections. The sections were adjusted by gravimetric modelling of the regional data and detailed profiles around the Sotkuma dome. A schematic 3D block-model visualizing regionally the depth and overall geometry of the basement-cover interface in North Karelia is presented. Both the Archean basement complex and the Paleoproterozoic cover sequence display structures developed in response to collisional tectonic processes c. 1.92–1.82 Ga ago. Compared to the thickness of the Archean crust, the sedimentary cover sequence in North Karelia is rather thin; the modelling along the regional gravity profile indicates overall cover thicknesses around 4 to 6 km. Between the Sotkuma and Kontiolahti domes the cover rocks of the Höytiäinen Belt form an almost upright outlier between the basement blocks. Within the Outokumpu Area the basement-cover interface is interpreted as a very open synformal structure between the Sotkuma dome and the Maarianvaara basement inlier. According to the detailed gravity profile modelling around the Sotkuma dome, the basement-cover interface dips steeply (in the north and west) or moderately (in the south) outwards. The eastern margin of the dome is modelled as a step-like structure of west dipping thrust slices of basement gneiss. The modelled geometry, the structural features and the pattern of mafic dykes within the dome, are interpreted with a multistage genesis: (1) repeated rifting and normal faulting (N-margin ~2.2 Ga; E-margin ~2.1 Ga) during the extensional basin formation stage, and (2) crustal shortening (~1.9 Ga) and associated thrusting. In our modelling the Sotkuma basement window represents the northernmost part of a large ‘basement high’ block, spatially extending some 30 km south. Suitability of the term ‘dome’ – with genetic and geometric connotations – is questioned within the study area and usage of the neutral term ‘basement inlier’ is suggested instead.