Mechanical Anisotropy from Gravity/Topography Coherence: A Global Study

Princeton University, Princeton NJ

Investigators

Abstract

Despite decades of geophysical and geological research, first-order aspects of continental structure, mechanical behavior, and long-term evolution have remained enigmatic. Is Earth's continental lithosphere mechanically anisotropic? Evidence is emerging that gravity anomalies reflect the influence of topographic surface and interface loading via an elastic response that is azimuthally anisotropic. Scientists are investigating the cause of mechanical anisotropy in the lithosphere in an attempt to establish if it is characteristic for certain cratons or a more general phenomenon. They combine mechanical and seismic characterizations to define the lithosphere. Specifically, they are quantifying the control of fossil strain by comparing new measurements of directionally variable mechanical strength with published models of seismic anisotropy, and by alternatively comparing seismic anisotropy with geodetic observations and geodynamic models of active deformation. The depth dependence of the correlation between seismic and mechanical anisotropy is the diagnostic that constrains the depth to which the lithosphere has deformed as one unit under the action of tectonic processes. Mechanical anisotropy is estimated from the spatial dependence of the two-dimensional coherence between gravity anomalies and topography. For this, the scientists are developing a multitaper coherence analysis method that uses data windows sensitive to structure on the scale of individual geological units and beyond. The research combines geological, geodynamical, tectonic, and seismological information to arrive at an integrative characterization of the continental lithosphere. The methods being developed are also suitable for the study of oceanic and planetary lithospheres. The research crosses traditional boundaries of the Earth sciences, to applied mathematics for the development of dedicated spectral analysis methods, and to statistics for a query-based assessment of modeling uncertainties. A final by-product is a high-quality data base of regional continental gravity, topography, and geological data and a suite of publicly available modeling software.