![]() ![]() By "final attempt" we mean that while we are now in the position to analyze gravity/topography with the theoretically "best", geologically sensitive, method of spectral analysis, the quality of the results remains strongly influenced by the marriage (of convenience and of popular choice) to coherence or admittance, as we will show. This in contrast to any other Fourier based method. For this we developed a Cartesian technique of spatiospectral localization in the sense of Slepian, with which regions of arbitrary geometry can be handled, and directionally sensitive (or agnostic) analyses carried out. The second innovation is that we have definitely thrown off the yoke of needing to analyze rectangular regions when Fourier-based methods are involved. For North America, the results are not unambiguous: lithospheric elastic anisotropy may be weakly expressed but is certainly hard to measure with confidence. The first is that we are investigating a possible anisotropy in the coherence or isostatic response, based on advanced spectral analysis methods and sound statistical judgment. Two key innovations distinguish our approach. We have conducted a global study, on each of the worlds' continents, and will use this global scope to guide our presentation of the results for the North American region. While we are have become aware of the difficulties and pitfalls of estimating the effective elastic strength of the lithosphere via the spectral analysis and inversion of the coherence between gravity anomalies and topography - and will provide a brief summary on what we have discovered in this regard - we have made one final attempt at characterizing the elastic lithosphere via the coherence of Bouguer anomalies with topography. On the Elastic Strength (and Its Anisotropy) of the North American Continental Lithosphere (in a Global Perspective) We test several different models of laterally-varying lithosphere and asthenosphere Although we do not observe significant variations in the direction of predicted anisotropy with depth, we do find that the inclusion of deep continental roots pushes the depth of the anisotropy layer deeper into the upper mantle. Thus, we hypothesize that at least some continental anisotropy is associated with sub- lithospheric viscous shear, although fossil anisotropy in the lithospheric layer may also contribute significantly. We evaluate both the orientation of the predicted azimuthal anisotropy and the depth dependence of radial anisotropy for this downwelling flow and find that the inclusion of a strong continental root provides an improved fit to observed SWS observations beneath the North American craton. For the North American continent, the Farallon slab descends beneath a deep cratonic root, producing downwelling flow in the upper mantle and convergent flow beneath the cratonic lithosphere. To examine the influence of a continental lithosphere with variable thickness on predictions of continental seismic anisotropy, we impose lateral variations in lithospheric viscosity in global models of mantle flow driven by plate motions and mantle density heterogeneity. In contrast, beneath the continents both the lithospheric ceiling and asthenospheric thickness may vary considerably across cratonic regions and ocean-continent boundaries. In the ocean basins, where the asthenosphere has a relatively uniform thickness and lithospheric anisotropy appears to be small, observed azimuthal anisotropy is well fit by asthenospheric shear flow in global flow models driven by a combination of plate motions and mantle density heterogeneity. Shear flow in the asthenosphere tends to align olivine crystals in the direction of shear, producing a seismically anisotropic asthenosphere that can be detected using a number of seismic techniques (e.g., shear-wave splitting (SWS) and surface waves). Interaction Between Downwelling Flow and the Laterally-Varying Thickness of the North American Lithosphere Inferred from Seismic Anisotropyīehn, M. The upper mantle temperatures are initially e. ![]() We do this via an iterative adjustment of the model. ![]() The temperature distribution in the lithosphere is estimated considering for the first time the effect of composition as a result of the integrative approach based on a joint analysis of seismic and gravity data. We estimate the integrated strength and elastic thickness (Te) of the North American lithosphere based on thermal, density and structural (seismic) models of the crust and upper mantle. Lithospheric structure and deformation of the North American continent ![]()
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