Thermomechanical lithospheric structure of the central Fennoscandian Shield

Abstract

The deep seismic sounding (DSS) profiles BALTIC, including its southern continuation, the Sovetsk–Kohtla–Järve (SKJ) profile, SVEKA, the northern part of BABEL, POLAR, FENNIA and Pechenga–Kovdor–Kostomuksha, were used in studying the present-day thermomechanical structure of the central Fennoscandian Shield. These profiles are located in different tectonic units, which represent different stages in Precambrian crustal and lithospheric growth. First, present-day geotherms were constructed for several points along the DSS profiles. Successively, strength envelopes were calculated using the obtained geotherms and rheological flow laws. Variations in strain rate were also considered in the computations of the strength envelopes. The integrated crustal and lithospheric strengths, the thicknesses of the mechanically strong crust (MSC) and mechanically strong lithosphere (MSL), and the rheological thickness of the lithosphere were derived from these strength envelopes. The obtained mechanical structures for different regions were analysed and compared with other geophysical data; e.g., seismicity-depth and isotherm-depth distributions. The rheological results show lateral variations in the lithospheric strength reflecting the geometry of the lithosphere and following roughly the same trend as the geochronological development of the Fennoscandian Shield. The mechanical structure shows distinct decoupling of the weak lower crust and the strong upper mantle, particularly with a wet rheology. This decoupling interrupts the transmission of the differential stress from the brittle upper crust to the ductile lower crust and through it to the mantle lithosphere. The weak lower crustal layer is also detected with a dry rheology in the Svecofennian area, whereas in the Archaean side, it is not distinct. The assumed frictional transition temperature of 350°C varies between the depths of 25 and 44 km with an average value of 35 km. This is in good agreement with the observed focal depth limit of 31 km. Consequently, it seems that the velocity weakening/velocity strengthening explains best the real lower boundary of seismicity.