Effective Elastic Thickness of Planetary Lithospheres from Gravity and Topography

Abstract:

The effective elastic thickness (Te) of the lithosphere controls the flexural response to transverse loading and can be used in conjunction with rheological models to remotely estimate surface density and heat flux of terrestrial planets. In the vast majority of studies, Te estimation is carried out in a two-step process: (1) the joint spectra (admittance and/or coherency) of gravity anomalies (free air and/or Bouguer) and topography are calculated within finite-size windows, and (2) the spectra are inverted using a model for the loading of an infinite elastic plate or shell. In recent years, research in the spatio-spectral analysis of Cartesian grids and improvements in lithospheric loading models have allowed the mapping of Te on the Earth at unprecedented resolution. Nevertheless, the limitations imposed by working with Cartesian data and models are hampering further advances in terrestrial Te studies. The planetary community, on the other hand, has traditionally used spatio-spectral methods that work directly on the sphere, thereby avoiding the undesirable distortions and biases inherent in Cartesian studies. However, the models and methods developed for the Earth have never been applied to planetary data, therefore also limiting further advances in the mapping of planetary Te. Here we combine advances in both terrestrial and planetary studies to allow the mapping of Te of terrestrial planets using a spherical wavelet analysis of gravity and topography data. In particular, we invert the wavelet admittance and coherence, either separately or jointly, and using either Bouguer or free air gravity anomaly data. The method is applied to estimate Te on Earth’s continents and our results show that Te varies between <10 km in young orogenic provinces to >100 km in continental cores, in agreement with rheological models based on temperature-dependent rheology. Results obtained from the joint inversion of admittance and coherency show that simple lithospheric loading models fail to capture the complexity of the data, with adverse effects on the estimated parameters. Finally, we extend the analysis to the Moon and other terrestrial planets and discuss limitations and future applications of the fully spherical techniques.