GLOBAL VARIATIONS IN DENSITY, COMPOSITION, AND THERMAL REGIME OF CONTINENTAL ROOTS

We use gravity, thermal, and seismic data to examine how the density and composition of lithospheric roots vary beneath the cratons. Our interpretation is based on the gravity anomalies calculated by subtracting the gravitational effects of bathymetry, topography, and the crust from the observed gravity field, and the residual topography that characterizes the isostatic state of the lithosphere. We distinguish the effects of temperature and compositional variations in producing lithospheric density anomalies using two independent temperature constrains: based on interpretation of the surface heat flow data (Artemieva and Mooney, JGR, 2001) and estimated from global seismic tomography data (Ekström and Dziewonski, JGR, 1998). We find that in situ lithospheric density differs significantly between individual cratons, with the most dense values found beneath Eurasia and the least dense values beneath South Africa. This demonstrates that there is not a simple compensation of thermal and composition effects. We present a new gravity anomaly map that was corrected for crustal density structure and lithospheric temperatures. This map reveals differences in lithospheric composition that are the result of the petrologic processes that have formed and modified the lithosphere. All significant negative gravity anomalies are found in cratonic regions. In contrast, positive gravity anomalies are found in two distinct regions: near ocean-continent and continent-continent subduction zones, and within some continental interiors. The origin of the latter positive anomalies is uncertain.