How Robust are Our Estimates for Mechanical Anisotropy in the Continental Lithosphere?
Abstract:Lithospheric strength variations modulate, and in turn are modulated by, many key tectonic processes, including rifting, orogeny, and the longevity of cratons. The anisotropy of many geological materials and processes might lead to a natural expectation of widespread anisotropy in lithospheric strength, perhaps especially in areas like cratons, which have long, complex geological histories, and their margins, which often record extremely anisotropic orogenic events. The observed coherence between gravity and topography remains the most popular metric for the analysis of flexural rigidity and its anisotropic directional variations, and, indeed, it is frequently anisotropic. Does this, however, correspond to anisotropy in the actual mechanical strength of the lithosphere? Using coherence, we should only reject the null hypothesis of isotropy when there is significant anisotropy in both the observed coherence and the resulting flexural strength. In addition, the anisotropy should not arise purely from marginal (in the statistical sense) anisotropy in the topography and gravity data themselves.
In the last decade or two, new methods to estimate coherence between two-dimensional bivariate fields have been developed, most notably those using multitapers or wavelets. Using synthetic tests on wholly isotropic models, we find widespread spurious anisotropy using both these methods. Using a series of statistical and geophysical tests developed to identify and remove such spurious directionality, our global reanalysis shows sparse evidence for meaningful anisotropy in the mechanical strength of the lithosphere. Although the geological argument for anisotropy in these regions and its role in tectonic cycles remains highly plausible, this anisotropy has not yet been convincingly verified by any cross-spectral method.