Orogenic structures of the Precambrian basement of Estonia as revealed from the integrated modelling of the crust

Up-to-date gravity and magnetic field maps and deep seismic sounding data were used to study the orogenic structure of the Palaeoproterozoic (1.9-1.8 Ga) Svecofennian basement of Estonia. The structure of the crust was quantitatively modelled for two NNE-SSW trending profiles across the main - south (southwest) and north (northeast) - structural terrains and the tectonic transition zone between them. Lateral variability of the basement structure was revised using the available geological and geophysical maps and results of 3D modelling. Metavolcanic and metasedimentary rocks prevail in the orogenic basement of the Estonian mainland. The crust is overthickened, thus a remarkable gravity minimum is expected to occur here. In NNE Estonia, where the low-density (2680-2710 kg/m3) migmatized metamorphic rocks of the amphibolite facies prevail in the upper crust, the moderately overthickened (45-50 km) crust is coupled with gravity (-10 to -40 mGal) and magnetic (down to -800 nT) low. The deepest gravity minimum (-40 mGal) is associated with the thick and light upper crust at the Tallinn zone. In SSW Estonia and northern Latvia, the extremely overthickened (50-65 km) crust is composed of the dense (2750-2840 kg/m3) upper crust and overthickened, partially upthrusted lower crust. Here the mass deficit is compensated, and in many cases even overcompensated, by the upper crust composed of granulite facies rocks and blocks of the upthrusted lower crust. The Paldiski-Pskov zone is a transition between the two terrains, following deep crustal shear zones that dip to the SSW. The survived orogenic structure of the Estonian mainland formed due to the late Svecofennian SSW-NNE directed compression that resulted in crustal shortening and thickening, with the upthrusts along the Paldiski-Pskov zone and maximal crustal thickness in the SSW.