Instantaneous dynamics of the cratonic Congo basin

Understanding the formation mechanisms of cratonic basins provides an examination of the rheological, compositional and thermal properties of continental cratons. However these mechanisms are poorly understood because there are few currently-active cratonic basins. One cratonic basin thought to be active is the Congo basin located in equatorial Africa. The Congo basin is coincident with a large negative free-air gravity anomaly, an anomalous depression of the Earth’s surface and a large positive upper-mantle shear-wave velocity anomaly. Localized admittance models show that the gravity anomaly cannot be explained by a flexural support of the topographic depression at the Congo. We analyze these data and show that they can be explained by the depression of the Congo basin by the action of a downward dynamic force on the lithosphere resulting from a high-density object within the lithosphere. We formulate instantaneous dynamic models describing the action of this force on the lithosphere. These models show that the gravity and topography of the Congo basin is explained by viscous support of an anomalously dense region located at 100 km depth within the lithosphere. The density anomaly has a magnitude within the range of 27-60 kg/m3 and is most likely compositional in origin. Our models do not provide a constraint on the lithospheric viscosity of the Congo craton because the shallow location of the anomaly ensures strong coupling of the anomaly to the surface regardless of viscosity structure.