DENSITY OF THE CONTINENTAL ROOTS: COMPOSITIONAL AND THERMAL EFFECTS

We use gravity, thermal and seismic data to determine density of the continental roots and to estimate the effect of temperature and compositional variations in producing upper mantle density anomalies. Our analysis is based on the residual ‘mantle’ gravity and topography obtained by subtracting the effect of the crust from the observed values. We found that in situ lithospheric density differs notably between individual cratons, with the most dense lithosphere beneath Eurasia and the least dense - beneath South Africa. We calculate the effect of temperature in producing lithospheric density anomalies under cratons using temperature constrains based on interpretation of the heat flow data. We found that differences in the thermal regime of the lithosphere between the cratons are compensated only for about 40% by compositional differences. This shows that there is not a simple compensation of thermal and compositional effects. One possible explanation of this phenomena may be suggested if we assume different stratification of temperatures and densities with depth: the uppermost part of the lithosphere is less dense due to a domination of the compositional effects and vice versa for the lower part. Finally we determine density variations under cratons that are due to differences in lithospheric composition. On the average the subcrustal density under the cratons found to be 50 kg/m³ (1.3%) less than under the ocean in the same T-P conditions and its variations are small between the cratons, typically within ±10 kg/m³.