Contrasts of seismic velocity, density and strength across the Moho

We provide an overview of contrast of elastomechanical parameters across the Moho, basically contrasts in seismic wave velocities, density and yield strength. These can be regarded as dynamic and quasi-static endmembers of elastomechanical parameters of the crust-mantle transition. With respect to practical applications they are closely related because seismic investigations play the role of exploring the earth structure and physical state forming the boundary conditions for determining density and rheological models. We address both average global Moho models and petrophysical and structural factors that cause deviations from the global averages, such as variations in Moho depth, temperature and rock composition, fabric (anisotropy) and macro-scale heterogeneity. Besides principle considerations these factors are put in a regional context in order to demonstrate how they are related to past and present tectonic processes. Seismic velocity and density contrasts are found between 0 and 25% whereas strength or viscosity contrasts may be orders of magnitude higher, but may vanish as well or show a discontinuity of higher order only. Especially, oceanic and old cratonic crust and near-Moho mantle may appear as a rheological unit although they are seismically well distinguishable. In terms of seismic velocities the Moho may "vanish" mainly under the influence of serpentinization or eclogitization. Originally defined as a first-order discontinuity in seismic velocity, the Moho has turned out to be an interface across which other seismic properties, such as seismic velocity gradients, anisotropy and heterogeneity scale parameters, can change strongly, too. However, knowledge of these parameters, as well as their relation to the rheology of the crust-mantle transition, is still restricted to local or regional examples so no global conclusions can be drawn. (C) 2013 Elsevier B.V. All rights reserved.