LITHOSPHERIC THICKNESS AND VELOCITY ANISOTROPY - SEISMOLOGICAL AND GEOTHERMAL ASPECTS

We compare inferences of regional variations in the lithospheric thickness in central Europe based on three types of observations. Seismologically determined thicknesses derived from representative P-wave residuals agree in most regions both with the magnetotelluric inferences of depths of the top of a layer of increased electrical conductivity and with the geothermally based calculations that allow inferences of the lithospheric thickness from surface heat-flow data. The seismological definition of the lithosphere as a high-velocity layer overlying the low-velocity zone in the upper mantle seems to be close to the geothermal concept in which the lithospheric base is defined by temperatures near partial melting, decreasing the seismic velocities and increasing the electrical conductivity. Interpretations of seismic data used for constraining geothermal models of the deep lithosphere assume velocity variations to reflect mainly changes in the mineral composition. However, recent observations of large-scale seismic anisotropy show clearly that, in the subcrustal lithosphere, velocity variations due to anisotropy caused by preferred orientations of olivine crystals can be much larger than the velocity variations caused by temperature or compositional changes.