A three-dimensional model of the Early Precambrian crust under the southeastern Fennoscandian Shield: Karelia craton and Belomorian tectonic province

The Early Precambrian structures of the Archaean Karelia craton and Belomorian tectonic province have been investigated by reflection seismology. Integration of the existing geological database for these two entities and adjacent tectonic units with the seismic data for the crust and upper mantle along the 1-EU geotraverse and cross-traverse 4B totaling c.1300 km allows development of a three-dimensional model of the deep structure of the southeastern Fennoscandian Shield. Interpretation of the seismic data provides evidence that the Early Precambrian crust is characterized by inclined layering. The crustal architecture is a combination of thrust–nappe and thrust–underthrust structural assemblages consisting of Neoarchaean and Palaeoproterozoic rocks that were subsequently further deformed during uplift and emplacement of granite-gneiss domes. The new model substantially modifies previous conceptions of this part of the shield as being constituted of blocks with sub-vertical margins and individual internal layering. It is apparent that the surface traces of gently dipping boundaries are defined by the erosion level and consequently these boundaries are not those of particular “tectonic blocks” in the traditional sense.