Flexural Isostasy : Constraints From Gravity and Topography Power Spectra
Watts, A. B.
Moore, James Daniel Paul
Date of Issue2017
Earth Observatory of Singapore
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 < λ < 1200 km. The best fit is for an elastic plate (flexure) model with an elastic thickness, Te, of 34.0 ± 4.0 km. Smaller values underpredict the observed gravity spectra while higher values overpredict. The best fit Te is a global average and so there will be regions where Te 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 Te~34.0 km and of large amplitude in regions of lower and higher Te. 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 Te. 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.
Journal of Geophysical Research: Solid Earth
© 2017 American Geophysical Union (AGU). This paper was published in Journal of Geophysical Research : Solid Earth and is made available as an electronic reprint (preprint) with permission of American Geophysical Union (AGU). The published version is available at: [http://dx.doi.org/10.1002/2017JB014571]. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper is prohibited and is subject to penalties under law.