Subsurface loading and estimates of the flexural rigidity of continental lithosphere

Abstract

The flexural rigidity of the lithosphere is often estimated from the linear transfer function (admittance) between gravity anomalies and topography. Admittance estimates strongly weight provinces with large topographic relief, which will tend to be those provinces with low flexural rigidity if the cause of the topography is subsurface variations in density. If the observed admittance of continents is modeled in terms of surface loading of an elastic plate, there is a strong bias toward low flexural rigidities. Models incorporating both surface and subsurface loading yield much higher rigidities. Although the two models may match the observed admittance equally well, only those with both surface and subsurface loading are consistent with the observed pattern of coherence between gravity and topography as a function of wavelength. A new method of analysis shows that surface and subsurface loading are approximately equal in importance in the vicinity of the Kenya rift valley. The flexural rigidity of the East African lithosphere is about 2×1023 N m, or an effective elastic thickness of the plate of 25–30 km. Data from the conterminous United States are consistent with the presence of provinces with a wide range of flexural rigidities, averaging tens of kilometers in effective elastic thickness.