Flexural Isostasy: Constraints From Gravity and Topography Power Spectra

We have used spherical harmonic coefficients that describe Earth's gravity anomaly and topography fields to quantify the role of isostasy in contributing to crustal and upper mantle structure. Power spectra reveal that the gravity effect of topography and its flexural compensation contribute significantly to the observed free-air gravity anomaly spectra for spherical harmonic degree 33<n<400, which corresponds to wavelength 100<<1200km. The best fit is for an elastic plate (flexure) model with an elastic thickness, T-e, of 34.04.0km. Smaller values underpredict the observed gravity spectra while higher values overpredict. The best fit T-e is a global average and so there will be regions where T-e is lower and higher. This is confirmed in studies of selected regions such as the Hawaiian-Emperor seamount chain and the Himalaya fold and thrust belt where we show that flexural isostatic anomalies are near zero in regions where T(e similar to)34.0km and of large amplitude in regions of lower and higher T-e. Plate flexure may also contribute at higher (n>400) and lower (n<33) degrees, but topography appears either uncompensated or fully compensated at these degrees, irrespective of the actual T-e. All isostatic models underpredict the spectra at 2<n<12 and so we interpret the low-order Earth's gravity field as caused, at least in part, by nonisostatic processes due to dynamic motions such as those associated with convective upwellings and downwellings in Earth's mantle.